Generator power consumption experiment - for estimating battery bank size

minisolar

Hey all,

Been a long time. I spent quite some time here a year ago or so planning my power system with the incredible help of this forum. But then Corona happened and I had to finish other things in my off grid cabin before getting to the power system. 

I am now on my last project (indoor shower) and I hope to be able to get to the power system before winter. I will reread all my threads and comment before starting up the plan for that here.

What I did realize is that since my family now spends about 3 weekends (2 nights - 3 days each time) at the cabin - I can get a better idea of my power needs. Currently my cabin is powered by my 2000w honda. We have lights, freezer fridge conversion, rv water pump, and a ductless 12,000 btu ac... we also have a small microwave and toaster that are used for a few min a day max. Honda has been able to handle it but we do not run the ac and microwave or toaster at the same time.

I bought this: 

https://www.amazon.com/gp/product/B07DPJ3RGB/

I have an extension cord running from Honda to an inlet that then feeds my breaker box for all my supply. I was thinking to have this device plugged into my generator and then let it run for the time we are at the cabin and then report our usage here and see if the planned power system is good or too large too small... 

The power system we came up with last year was 6x AGM 225ah batteries making a 12v system with 675ah. No solar charging. The plan was for me to just plug in the generator at night, when we run the ac continuously (the biggest power hog), to power cabin and charge batteries at the same time so then I can then run cabin off the batteries during the day. 

Any notes on this experiment with the power monitor on the generator? Will that work in terms of giving us good information or is it a waste of time? 

BB.

Starting will a Watt*Hour meter and measuring your energy usage per day (the more data you collect, the more accurate your power usage estimates will be).

At that size of battery bank (approaching, I suggest, 800 AH or more)--You are certainly looking at a 24 volt (or possibly 48 volt) system as being a good solution--Especially if you think you will be using more power down the road. 12 volt systems are pretty difficult to expand beyond 800 AH or 1,800 Watt (AC inverter) because of the heavy DC current and cabling required. 24 (and 48) VDC cuts the current by 1/2 (48v, to 1/4) and gives you more voltage drop allowance (0.5 volt max suggested for 12 volt, 1.0 for 24 volt, and 2.0 volt system load drop)...

All in all, making the DC side of the wiring much easier/cheaper. Plus, for Solar Charge Controllers... A 60 Amp MPPT controller (for example) will support a 2x larger 24 VDC or 4x larger 48 VDC array (same hardware--Power=Voltage*Current -- 2x voltage=2x power at same current, 4x voltage=4x power)...

Sticking to a 12 volt system only makes sense if you have 12 volt native loads you want to run... If all your loads are 120 VAC, or have 120 VAC "wall transformers"---Then I suggest that 120 VAC to charge cell phones (and slightly more panels/battery bank capacity to support the slightly less efficient  AC loads), laptops, tablets, etc. is easier and fewer issues that running from 120 VAC. As a suggestion, keep these small/vampire loads on power strips or switched outlets so you don't have small loads running 24x7 and slowly draining your system (possibly without your express knowledge).

-Bill

minisolar

Great to see you are still so prolific here Bill! I admire your knowledge and willingness to help.

I do not foresee me needing more power since this is a weekend cabin (cabin is a cute term... it is 144sqft shed with 13ft ceiling haha). 

This will be a typical weekend:

Assuming batteries are fully / almost fully charged when we arrive (since we charged them before leaving the cabin last trip):

Day 1: We arrive around 11am-12pm. Turn on freezer conversion and 2-3 lights. Then until about 8pm we use those and 2-3 min of microwave (800w) and toaster (700w). Plus a total of maybe 30-40min of rv water pump and maybe one phone charge.. Then at about 8pm we will turn on genset (and AC) so then the genset will both charge the batteries and power the cabin during the night.

Day 2:

Batteries are fully charged from the night and genset will be turned off. So from now (6-7am) until 8pm (so a longer day vs day 1) we will have the same kind of usage as described in day 1 except the freezer conversion is now on batteries from earlier in the day plus maybe a few more min on rv water pump. Then again genset will be switched on at 8pm for the night and batteries will be charging all night 

Day 3:

We get up to fully charged batteries at 6-7am. Switch generator off. So main usage is freezer conversion again until we leave at 9-10am. So batteries should be fairly full when we leave. 

So a 675AH 12V bank, fully charged, should be able to carry a fairly small freezer conversion fridge, 2-3 led lights, 4-5min of 800/700watt microwave or toaster, and maybe 40m in total of rv water pump between 6-7am until 8pm? If the answer is yes then I am good. 

I will test the entire cabin usage next time we go and then just the fridge to see how much it takes in a full day. 

Marc Kurth

I like your methodical approach!

Quick questions:

How long between visits? (You said that you are assuming full charge because you charged before leaving)

How do you know that your battery bank is fully charged when you turn off the generator?

minisolar

Hey Marc,

We usually go 2-3 weekends a month then in winter we don't go from November until march. (hopefully in the future we will). 

So it will be left alone without any draw from a week to two weeks then for those few months in winter. I will have them fully charged when I leave them for those winter months. 

I will have a battery monitor to keep an eye on the batteries state of charge 

BB.

Thank you very much Minisolar for your kinds words....

This is where the details matter...

• Type of batteries (flooded cell, AGM, Li Ion, etc.)...
• Lead Acid batteries, roughly 1 month between charges (~75F/25C)
• AGM batteries, roughly 6 months between charges (~75F/25C)
• Li Ion batteries, years of standby no specific state of charge (although 40% or so is usually best)

FLA and AGM batteries... for every 10C/18F increase, self discharge is 2x faster--I.e., 93F, 2 weeks for FLA, 3 months for AGM between charges. Too long between charging, lead acid batteries sulfate faster (roughly, below 75% State of Charge--Not good).

As Marc asks--Lead Acid batteries need a fairly long time on charge to get near 100% SoC... Say 5% rate of charge and 75% SoC... Roughly 5 Hours of charging (bulk/absorb) and 2 hours of "absorb setpoint voltage--Or approaching ~7 hours of "genset charging".

Not a bad idea... 1-2% rate of charge (solar panels) on a simple/reliable charge controller to "float your batteries" when you are not there. Some folks have mounted the panels on the side of cabin to allow free of snow (unless it drifts). But even drifting is not usually a problem as batteries are sub freezing, so very slow self discharge--Ride through the thaw cycle.

• 675 AH * 14.5 volts * 1/0.77 panel+controller derating * 0.02 rate of charge = 196 Watts of "float" panels (between visits).
• 675 AH * 14.5 volts * 1/0.77 panel+controller derating * 0.05 rate of charge = 489 Watts of panels slowly recharge (between visits).

Choose 5% rate of charge--And just use genset to "bulk" bank when needed (basically between 50% and 80% state of charge). The last 20% charge can be handled by the solar panels (5% rate of charge from 80-100% SoC~4 hours or 1-2 days) to fully charge (or even from below 80%--4-5 days to bring bank back to 100%).

Genset will pump "full" charging current (upwards of 10-20% rate of charge) or "bulk" up to ~80% SoC -- Let solar finish when you are not there...

More or less, batteries being actively cycled between 50% and 80% do not (as I understand) by loads+generator (+panels) do not suffer, badly, from sulfation when active cycling). Most efficient use of generator fuel--When loaded... Worst fuel efficiency is charging (especially) from 90% to 100% SoC (genset is lightly loaded, worst "fuel economy").

Numbers above are very rough estimates... "Close enough" for FLA and Lead Acid to work out some ideas.

Some other ideas.

-Bill "like a bad penny" B. 

minisolar

I am going with AGM for 100%. I believe they are a good fit for my setup.

That is a good idea to have a small array to just slowly charge when I am away. I will put in my notes.

In my still evolving power plan I have a transfer switch. So when I plug my genset it takes over and charges batteries and powers cabin at the same time. The issue is that if, by chance, while it is bulk charging (I believe IOTA mentioned 750watt draw when bulk charging) both the ductless AC and freezer kick in the same time for their compressors - it can over load the genset causing it to shut off

The main issue with that is that I will be sleeping when it happens so the transfer switch will then switch over to the inverter/batteries for power. If AC doesn't shut off because of it (it has voltage protection) - then it might empty the batteries over night since now both ac and freezer conversion will be running non stop all night while batteries are already low in power from day usage...this will ruin the batteries. 

A solution maybe - not have a transfer switch. Essentially have the batteries get charged and used at the same time... I think that is not an issue since most people here have an array that charges their batteries during the day while they use their batteries for power at the same time. So in the evening when I plug in the genset it will start charging the batteries and the cabin power will run by inverter from battery bank at the same time. Any issues with that? 

BB.

Yes, you can go with AC inverter powering the loads while the genset runs the battery charger... A bit of "extra" conversion loss (AC to DC  back to AC).

You can also look at AC inverters that offer generator assist/power assist. For example, this 2,000 Watt Victron inverter will take a limited AC input current (say 8 amps from genset), and use the AC input to both charge the battery bank and run your 120 VAC loads--And surge upwards of 30 Amps AC to load, while limiting AC input to programmed input max current (say 8 amps).

https://www.solar-electric.com/victron-energy-multiplus-inverter-12-2000-80.html

May be more expensive than you planned--But Generator Assist has been a very nice option used by several major inverter mfg. No extra hardware (such as a manual or automatic transfer switch)--And the (Victron's) VE.Net networking options, integration with solar charge controllers & battery monitors, Li Ion support, etc. give you some neat features and a little bit of "future proofing".

Gives you options such as--Let the genset run a tank of gas (1 gallon) and "auto shutdown" and the AC loads continue automatically (UPS like function).

Anyway--Some other ways of designing//configuring a system.

-Bill

minisolar

If I understand correctly - this inverter will not only replace my IOTA battery charger, transfer switch, and inverter - it will also be able to limit the amount of current used to prevent my generator from overloading. That is phenomenal. 

This is a beautiful and elegant solution that simplifies my plan significantly.. The more I am reading about this I am blown away. Very expensive but it replaces 3 items in one seems like. It was meant for my application of generator and battery bank. Wow. 

Do you know of similar inverters that might be less expensive? Btw this Victron doesn't say if it is pure or modified sine.. I am sure it is pure right? 

I will also try to search myself. 

I just got that power meter thing in the mail. I will charge it over night so it is ready to go for the cabin. I will report back. I hope it doesn't fry... My generator is in a noise box (with ventilation) but it does get a little hot in there. It is what it is. 

BB.

I believe the Victron inverters are all PSW/TSW type... No MSW (at least as sold through our host NAWS).

Here is a thread from 2013 about generator support:

https://forum.solar-electric.com/discussion/17098/demonstration-of-generator-support/p1

Looking for less expensive Generator Support units? First, look for combination inverter-charger units. Those have the basic hardware to possibly have generator support function.

I just picked a random Cotec 1,200 Watt inverter-charger @ 12 VDC...

https://www.solar-electric.com/cotek-sc-1200-112-1200-watt-12-volt-inverter-charger.html

Nothing in the main specifications about generator support... But if you look at page 23 in the manual, section 4-2-3, it seems that it may have generator support function:

https://www.solar-electric.com/cotek-sc-1200-112-1200-watt-12-volt-inverter-charger.html

4-2-3. Generator function If the demand for AC power still increases, the external AC circuit breaker may still trip if nothing is done. This problem can be solved by the Generator function. If the total demand for energy exceeds the maximum external power supply, energy can be added to the AC outputs load by means of the inverter. This appliance can be automatically connected in parallel with the external power supply.

See Fig. 15. Here the AC-input is still limited to 20 Amps.

This is not enough to supply the total load (27A) connected to the AC output load. The inverter will supply the remaining 27 – 20 = 7A. This means that the e restricted amount of external AC power will be compensated by energy which is stored in the batteries.

Not sure if this means "generator support"--But it sounds like it (the maximum AC input current is programmable with DIP switches).

The 2,000 Watt @ 12 VDC Cotek inverter-charger--Around $970--So not that much less than the Victron unit which is software programmable--The Cotek is less configurable(?):

https://www.solar-electric.com/cotek-sc-2000-112-2000-watt-12-volt-inverter-charger.html

Marketing blurb...

https://www.solar-electric.com/lib/wind-sun/Cotek_SC_Series_Competitive_Analysis.pdf

Outback as a 2,000 Watt @ 12 VDC inverter-charger with generator support (more expensive than Victron/etc.):

https://www.solar-electric.com/outback-power-fxr2012a-hybrid-inverter.html

https://www.solar-electric.com/lib/wind-sun/fxr_install.pdf

Support: This mode is intended for systems using utility grid or a generator. In some cases the amount of current available from the source is limited due to size, wiring, or other reasons. If large loads need to be run, the FXR inverter augments (supports) the AC source. The inverter uses battery power and additional sources to ensure that the loads receive the power they demand.

Not saying these are the only 12 VDC units NAWS has... But just an example of reading the manuals to figure out what they can do...

You might look for other overseas brands--Don't know anything about Must -- But they seem to have some pretty inovative specifications. I have no idea if they offer generator support (their model numbers are less than obvious with a quick look):

https://www.mustpower.com/

Anyway--NAWS has engineers and folks that know their product lines better than I (I don't work for NAWS--I am just a volunteer here on the forum).

While looking around--Check the rest of the specifications... For example the Victron above takes just 9 watts "powered on/no AC loads"--Whereas the Outback unit is 35 Watts Power On/No Loads (Tare) losses... The Cotex 2,000 Watt seems to be 4 amps @ 12.5 amps or 50 Watts... They do have "search or standby modes" that can reduce power usage when not supplying AC loads--If you run the inverter 24 hours per day--50 Watts * 24 hours = 1,200 WH -- Enough to run a full size refrigerator (wasted inverter energy)--All of sudden, the 9 Watts of the Victron at $200 more--Not to bad.

Generator Support seems like an obvious idea--But the hardware and software needed to support the function is probably not trivial. Whomever you choose, make sure you have support available (from dealer or factory)... For example, I think Victron expects their dealers to provide customer support.

And that is about the limit of what I can help you with--I do not have much more to offer about brands/products/usage/etc...

And double check my numbers above--I am just skimming the documents and I can easily mix stuff up. Trust but Verify!

-Bill

minisolar

Thank you for the info Bill. I also wasn't able to find a comparable inverter so it seems like this victron is the one to go with. 

And honestly this idea blew my mind... It simplifies my set up, reduces the amount of parts, and removes the issue of overload. Can't ask for much more. I'm hoping I can sell my unused brand new parts (iota charger, aims power inverter, and go power transfer switch) for about $500 total so then the upgrade to victron doesn't bite as much.

In Amazon reviews a person complained that he had an issue and victron refused to help stating the dealer, Amazon, needs to provide support... So that's not good. 

I'll buy it from a dealer who has good support. One that I came across (haven't fully researched yet) is inverters are us. Started in 2003 they offer free programming help and full support plus for another $115 you get an additional 5 year warranty... For a total of 10 year warranty. Which also eases the fear of spending that much on a part. I just hope no one steals it from my land. Might put a small GPS tracker in it just in case haha. 

I've seen a short video from victron showing how you connect it with a $60 usb part to your pc and then program it... A wealth of settings in there. 

You think you'll be able to assist with settings for it? I'm hoping to set and forget it and I'm not aiming for some super efficient and complex settings or something. Just reliable setting that will do its work. 

BTW - I just got that power meter. Charged it overnight. I just tested it with my toaster at home. It worked well and it tracks KWH among other things. So the next few weeks I will test and record my usage so we can see if my 675ah battery bank plan is good. 

BB.

Just post your configuration questions here. I will help if I can (I have no special knowledge).

It looks like there are at least two versions of the Victron inverter-charger. It sounds like you read the same reviews I did on Amazon--And the Amazon version is (possibly) an older version that has slightly larger dimensions vs what NAWS has listed.

The links above point to our host, Northern Arizona Wind & Sun (solar-electric.com), and I suggest you talk with them too. They have competitive pricing (same as Amazon for this unit?) and are there for support--And they provide this forum free for use by everyone.

In either case, you are very welcome to post your questions here and I/we will do our best.

-Bill

Dave Angelini

Hey Bill, I did not read any of this but if it is inspected or going to be, the last time I was involved with getting a permit for Offgrid,
 Victron did not have the credentials for US/Canada housing.

minisolar

It isn't being inspected.

Is this fire related? I am in upstate NY so no fires. 

Photowhit

Just to throw out another possibility of an Inverter/Charger for your place, Samlex has a 1200 watt inverter/charger (UL458 NOT 1741 if you need inspection) I didn't look for it this time in NAWS, but they can likely get it. It's $600! but only 1200 watts, likely enoough fr mst things, but turning off Air for microwave...

https://www.donrowe.com/samlex-evo-1212f-pure-sine-inverter-charger-p/evo-1212f.htm

minisolar

Thank you so much Photowhit !

That is much less money! Does it have the same features in terms of overload protection via dividing ac output ? 

This idea really blew my mind... (THANKS BILL!) To be honest I do not see any reason why I won't do this. For an additional $650 I get 10 years warranty, overload protection, safer for ductless and freezer - no brown out, simplified setup... I think it is a win win all around. 

I am curious to see if after I post my kwh for my weekend if my 675ah will be too big... I feel like I am not using much power. A gallon of gas keeps my cabin running for a day and a 1/4 about and a lot of waste while the generator is idling (in eco mode of course but still... waste). 

BB.

Remember that your total load (all loads, starting, etc.)--Need to be supported by the battery bank. AGM do support much higher surge currents than FLA batteries (flooded cell lead acid)... If this was an FLA bank, would be suggesting at least an 800 AH (250 Watt inverter per 100 AH @ 12 VDC).

So from a "simple" balanced system design, a 675 AH AGM @ 12 volt battery bank is not oversized for a 2,000 Watt rated AC inverter.

Cotek and Samlex are "sister companies" (as I understand)--So there can be a lot similarities between the two product families.

In the case of the Samlex inverter-charger above, I see programmable maximum AC input current--But nothing else that seems to indicate a generator support/assist function.

As I understand, MiniSolar wanted a 2,000 Watt inverter, powered by (something like a Honda eu2x00i or similar genset). His loads can be over the genset's capability (small inverter-genset)--So the reason for larger inverter.

Wanted 12 VDC battery bus--Because of existing hardware (Iota charger, and an existing inverter--Both 12 volts).

He does have the option of using existing system--And, for example, full time generator charging via the Iota (as needed), and let the inverter+battery do all AC output (and have 2,000 Watt rated output vs smaller genset).

The Victron option is that inverter-charger manages AC input (to something like 6-8 amps @ 120 VAC) and the inverter-charger will send "genset" 120 VAC out (6-8 Amps max), charge the battery bank as needed, and if the 120 VAC loads exceed ~6-8 amps, the Victron inverter-charger will "support/assist" the AC generator and have a 2,000 Watt (or more) 120 AC output (960 Watts from genset + upto 2,000 Watts from inverter-charger).

And thinking of adding a smallish 2% to 5% solar array for float charging during "winter" and assist charging during sunny weekend usage (and bring to full charge during week).

No external AC transfer switch needed. No external battery charger needed. Replace existing inverter-charger with Victron inverter-charger w/ generator assist/support.

Which does beg the question... Is there a reason to stick with 12 VDC now? Understand the cabin is small, and no additional loads are expected... But 24 VDC (or larger) systems have more expansion capacity vs 12 volts (which is pretty much limited to 1,800 Watt max output +/- due to cabling/voltage drop limits of 12 VDC systems). Generally the larger/higher bus voltage 24/48 products have "more features" (for better, for worse).

-Bill

minisolar

Bill you nailed it on the head. You broke it down perfectly. 

Also, now that I want to go with the Victron, and still haven't bought it nor did I buy batteries yet.. I can go 24v since I am already pretty much committed to Victron and will sell my unused 12v anyway even if I go with 12v Victron or 24V. 

The question is, do I really need 24V? 

The only other big things I might have in the future... pellet stove (which) doesn't use much power and if it runs in winter that means ac isn't running so a decrease in power usage vs ductless AC. And a very low chance of needing a 2nd ductless ac... Really like almost no chance. Cabin will go to 384sqft - 448sqft max size. Which current 12k btu ac can easily support. 

Other things in future? led tv? Router... really small things. Tools are all battery operated. And if I need a big thing.. I can run it on the honda by itself. 

So any other worthwhile benefits? 

I see that Victron has a 24v version for same price? but wouldn't that keep me at the same 2000w max ac loads? A 3000w version is $250 bump. So going with 2000w 24V makes no difference. I can even bump to 3000w for $250 just to really future proof. 

I can then go with 4x 200ah 12v agm batteries and get a 400ah 24v system giving me more watt vs the 12v system. Bank is about the same price as the 675ah 12v. 

Then price is very much the same almost? between 2000w Vic 12v + 675ah vs 2000w Vic 24V + 400ah.... 

Question is - does it future proof my ac loads for the future better ? If I need to buy a new Vic it will really kill the whole thing. I can go now for the 3000w Vic for another $250. 

Thoughts? 

mike95490

Even if you have just light loads, the 24v system will be more efficient, use smaller copper wires, and is easier to add battery capacity with series batteries and not parallel strings @ 12V

minisolar

Yeah really at this stage I have no issue going with 24v. Seems like it is the same price (if getting a 2000w Vic) or a little less once all wires are calculated as well. I am just wondering if it makes any sense to go for the 3000w VIC just to really proof my future.

BB.

A 12 volt @ 2,000 Watt inverter and upwards of 800 AH battery bank is "pushing the limits" of a 12 volt battery bus capabilities (in my humble opinion).

If you need 2,000 watts (or more) and plan on a >600 AH battery bank, then the 3,000 Watt Victron @ 24 volts is probably a good solution.

If you need 12 VDC (a lot), then staying with 12 VDC bus may make more sense for you. If your 12 VDC loads were small, a DC to DC converter or even a 120 VAC to xx volt adapter can be a good solution. And in some cases, there are 24 VDC native loads that are handy for cabins and such (marine, 18 wheel trucks, etc.)... LED lighting can be found in 24 VDC, and so can "RV Type" Water pumps. Everything else, run from 120 VAC usually works well. (lets you run lighting and water without inverter on 24x7--Saving a bit on the energy usage).

When you get to solar charge controllers--They are generally rated for maximum output current at all voltages 12/24/48 (a few do derated output at 24&48 volts by a bit). For example a single 60 amp MPPT controller can manage (the numbers below are approximate "cost effective maximum array"--Always refer to controller specifications/manuals for exact answers):

• 60 amps * 14.5 volts charging * 1/0.77 panel+controller derating = 1,130 Watt max array
• 60 amps * 29.0 volts charging * 1/0.77 panel+controller derating = 2,260 Watt max array
• 60 amps * 58.0 volts charging * 1/0.77 panel+controller derating = 4,520 Watt max array

So, for charge controllers, 2x increase in battery voltage, means a 2x larger solar array support--With the same charge controller.

Other issues such as DC wiring becomes "easier" (1/2 the current, much smaller wire costs, frequently fewer DC breakers and fuses needed, easier to send 24 VDC farther than 12 VDC, etc.).

One other option in the 3 kWatt Victron is (for a bit more $ and the 3kW inverter uses ~20 Watts tare losses):

https://www.solar-electric.com/victron-energy-multiplus-inverter-12-3000-120-ul-1741.html

Victron Energy MultiPlus 3000 Watt 12 Volt Inverter & 120 Amp UL 1741 Certified Battery Charger

There is a 3kWatt UL 1741 12 volt Victron inverter-charger too (same price for 12 volt or 24 volt units)... I still suggest that 3,000 Watts @ 12 VDC is something to be avoided unless you have good reasons otherwise (such as an RV coach with 12 VDC alternators/house&vehicle batteries).

While not needed for your cabin (no inspections)--It does meet higher standards for home use.

But this is your money--Not mine. I am not trying to empty your wallet. Your original 12 volt / iota charger / 2,000 watt inverter / eu2000i should work OK for your needs too. Just another way to slice and dice the problem and solutions.

-Bill

m151

Under your plan it looks like your using the Honda for about 100 hours/month x 8 months. With an extended run tank you could run continuous at about 200 hrs/month or 1600hrs per year. The Honda eu2000 is very durable and will adjust its RPM downward under light load. The difference in fuel may not be too different. This way you spend no extra money and it may outlast a set of batteries. You could take it with you for the winter (theft?) and use it as a back-up during the winter at your home. Other members will have an opinion on how long the Honda will last, but it's a long time.

BB.

From what little I know--Expect around 2,000 to 6,000 hours from an eu**** family genset... With normal maintenance and oil changes (2,000 hours being an OK life for a portable genset... 6,000 Hours and, if still running, harder to start, may smoke a bit, use oil).

For less expensive gensets--Not unusual to hit problems at 100-500 hours (broken parts to worse).

An old rule of thumb for equipment vs truck "life"--Assume 35 mph to 1 hour conversion:

• 100 hours * 35 mph = 3,500 mile vehicle life/major repair equivalent
• 500 hours * 35 mph = 17,500 vehicle life/major repair equivalent
• 2,000 hours * 35 mph = 70,000 mile vehicle life
• 6,000 hours * 35 mph = 210,000 mile vehicle life

On an older GPS for my car--I took miles driven/hours when moving and hit pretty much exactly 35 mph long term average (commuting, city/freeway traffic/trips/etc.).

And hours per year... about 2,000 hours per year "work time" (8 hours a day, 5 days a week, 50 weeks a year).

24 hours per day * 365 days a year = 8,760 hours per year...

3 days a week * 8 hours per day * 24 weeks (about 1/2 year) = 576 hours of "weekend getaway usage" (your numbers may vary).

Some more ways to slice and dice the data to get a better understanding.

-Bill

minisolar

M151 - you are correct and I did think about that and I currently am doing just that and can continue to without the need for battery bank and the whole hoopla. 

I built a sound proofing box for the generator and it sits in it about 40ft away from the cabin and it is really minimal noise.. But....

When I shut that generator down and I'm surrounded by woods and it is so serene and quiet you realize that it is just so much nicer to be without the generator in the background.. I'm willing to pay a few thousand dollars to have that. 

At night I don't care since I don't hear it. Maybe I'll consider solar panels in the future but not now. 

Bill - 

I am committed to 24v. I'll sell my 12v unused equipment and then go 24v route. It just makes more sense. I'm also committed to the victron. I'm still debating 3000w 24v or 2000w 24v... But for $150 more I might just go with 3k just to be safe unless it requires me to have a substantially larger battery bank and then I might stay in the 2000w victron. 

I just got back from a small trip to the cabin. We arrived at 2pm yesterday (later than normal) and we left around 10am today. The entire time the freezer conversion was on and the ac was turned on around 7pm. It isn't very hot so it probably didn't work that much overnight. It was shut off at 7am. So the ac ran for 12 hours about. I used that device and it recorded 3.457 kwh for the entire tripe duration so about 19 hours. Also it gave lowest and highest watt pulls. Lowest it recorded was 9.8w while the highest was 1145 watt. I remember that the iota was going to pull about 750 watt, if I'm not mistaken, when bulk charging so 750w and 1145w = 1900watt... That is right about the max for the Honda.. Yet another reason to go with victron since it will prevent overloading the generator and we can see that I'm pretty close to overloading with charging... And if I ever add another ac - I'll be over the top for sure. 

I'll now record the usage every time I go and post it so we can decide on appropriate battery bank size. 

BB.

More or less--If you only pull 2,000 Watts from either the 2kW or 3kW unit--The DC battery loading is the same.

The 3kW unit does have a higher "tare current/loss current" from being "on" (11 Watts for 2kW vs 20 Watts for 3kW)... An extra (9w*24h= ) 216 WH per day losses (if you ran the inverter 24x7) may not make a huge difference.

There are two 3kW units--One that does not have UL 1714 approvals, the other does (for $1xx more). It is probably worth the extra money for the UL Approved unit. It does not only include (possibly) extra testing for approvals and "better components" (don't know for sure), it also included traceability for mfg processes and traceability (to ensure no unapproved materials/components are substituted) with UL/NRTL random inspections every 1 year or so.

Certainly, using the larger inverter will draw more current is you load it to 2,000+ Watts... But an overloaded 2 kW inverter vs a 3 kW on a "too small" battery bank won't really be a difference to you (2kW inverter may shut down quickly--An "overloaded" battery bank may work when fully charged but "battery cutoff voltage" when at 50% discharge).

At a start--Get the 3kW UL1714 unit--Wire the DC wiring from inverter to battery bus for the larger current draw--And use the smaller battery bank for now (don't draw more than 2kW)--The worst that may happen--A random excessive load (pump, microwave, hair dryer, etc. on at the same time) may cause a low battery voltage cutoff (much less likely with AGM as they surge much better than FLA battery bank of the same AH capacity).

Even if you planning on AGM batteries, I would still design using the FLA design rules--They work well and are conservative... For FLA, 500 Watt AC inverter per 100 AH @ 24 volt battery battery (these are very roughly estimates--But they do seem to work reliably):

• 2,000 Watt inverter * 100 AH * 1/500 Watt = 400 AH @ 24 volt minimum suggested battery bank (2kW load FLA rule)
• 3,000 Watt inverter * 100 AH * 1/500 Watt = 600 AH @ 24 volt minimum suggested battery bank (3kW load FLA rule)

Maximum power is one way to define the minimum battery bank sizing... And we also have the percentage discharge per day... Typically we use 1-3 days of storage, and 50% maximum planned discharge (for longer battery life). In your case, you can either plan (for example) 2 days of no sun and zero genset usage and last day sun and/or genset usage bulk charging before you leave):

• 3,300 WH per day * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 max discharge * 1/24 VDC batt = 647 AH @ 24 VDC battery bank (2 days storage + 1 day sun+genset)

Or you can say--Run genset during "day time" and only run inverter "only" 12 hours evening/overnight:

• 3,300 WH per day * 1/2 load (evening/night) * 2 days of storage * 1/0.50 max discharge * 1/24 VDC batt = 324 AH @ 24 VDC (just overnight power, genset+solar bulk charging during day)

Note that we now have two sets of answer... A 2kW inverter needs around 400 AH minimum (for FLA surge support). A 3 kW inverter (using 3kW max power) needs 600 AH @ 24 volt battery bank. This is a conservative number (for AGM--As you can use a smaller bank and still get very good surge support).

And we have "storage" using the 2 day & 50% discharge rule... For FLA batteries--I suggest that using 25% discharge per day (2 days storage, 50% max planned discharge) is another FLA rule. More or less, on a pure solar charged system--We have a limited amount of hours of sun per day (especially during winter). The hours the sun is in the sky (6-10 hours per day) will fully recharge a 25% discharge (to 75% state of charge) using pure solar.

Discharging to 50% takes a good size array (10% or more) to fully recharge over the next 2-3 days of full sun on FLA batteries (and AGM to a degree too). If you are planning on using a genset anyway--You can "bulk" during the day (50% to 80% SoC) when you are there--And even though you are deeper cycling the battery bank--Active cycling (50% to 80% discharge/charge cycle) is not really hard on the lead acid battery bank. It is sitting at ~75% SoC, or less... Again why the 2 days&50% discharge rule of thumb for lead acid batteries.

If, for example you plan on using larger loads (power saws, larger water pump, etc.) only during the day with a Honda eu2x00i genset--Then you are not even looking at a needing the 2 kWatt loads (and up to 4 kW surge)--So a smaller battery bank may be OK for your needs too (and AGM can surge 2-5x more than FLA).

For now, you could even look at a string of 4x 6 volt @ 200 AH FLA batteries in series (for 24 volts). And 2x parallel strings (400AH) or 3x parallel strings for 600 AH as needed... Use this bank for 3-5 years (with monthly check of electrolyte levels while cycling) and see how things work out... Then switch to AGM (or even LiFePO4) after you see how things work a few years down the road... Low out of pocket for battery bank ($100 per battery, 2x4=8 or 3x4=12, or $800-$1,200 "starter" battery bank).

For the "permanent" bank, you probably want to find cells/batteries that match your AH needs (i.e., a 400 or 600 AH battery/cell block, vs paralleling strings together). Paralled strings of batteries do work OK... I suggest, you can avoid doing more than 2-3 parallel strings, you life would be easier (less wiring, less cells to check water/voltage on, etc.).

But as a cheap way to get started--1/2/3 strings of "golf cart" batteries are hard to beat (wallet wise) as a starter/learner set of batteries (may people do "murder" their first set or two until the work out the issues/get used to the required maintenance).

Whatever you choose--Has to work for you.

-Bill

minisolar

Thank you for the great info Bill. 

The 3.54 KWH I measured last time I was in the cabin was misleading since it was day time and night time together. But night time is not relevant for battery bank sizing since this is when the cabin will run off generator and charge batteries at the same time. 

The real question is: what size battery bank I need that can support me from 7am until 7pm. Just for that one day. Since from 7pm I will be charging with generator... 

Therefore, next time I will be there: I will only measure ac loads from 7am until 7pm - which is when normally the genset will be off and cabin ac loads will run exclusively on batteries. 

For now, if I estimate 1 KWH usage between 7am and 7pm (but will find out soon exactly how much I use during that time).

That means a battery bank of 100ah 24V? 

1khw x 2 (50% charge of battery) x 1.2 inefficient factor = 2.4 kwh

2400 / 24 (volt) = 100ah. 

But I believe that for a 2000watt 24v inverter the min is 300ah bank, right? 

Another thing is that the 2k 24V vic will use 1400 watt when bulk charging which means that for almost 100% it will need to throttle down input from generator to prevent it from overloading quite frequently during the night... 

Will that hurt batteries? have them start and stop bulk charging? 

If it can hurt them, then maybe I am better off with a 12v system after all? since with 12v the bulk charge of Victoran will only be 750 watt so I am almost never going to overload generator when supporting ac loads and bulk charging at the same time during the night... 

Also, I spoke with a rap from wind & sun and he mentioned that the Victron can start and turn off the generator automatically if I wish... so it will turn it on to charge and off when it is full. I do not think my old honda genset can do that without modifications. Any idea what they are talking about? If that is true and I can modify the genset fairly easily - then wow that is good. 

BB.

More or less, a 400 AH @ 12 volt battery bank stores identical power as a 200 AH @ 24 volt bank (P=V*I... 1/2 the capacity and 2x the voltage = same stored energy).

Ideally, you are looking for 5% to 10% rate of charge (or higher, if you need for faster charging/deeper discharge & recharge).

• 1,400 Watts * 1/29 volts charging = 48 Amps @ 24 volt bank
• 1,400 Watts * 1/14.5 volts charging = 97 Amps @ 12 volt bank
• 48 Amps * 1/0.10 (10%) rate of charge = 480 AH @ 24 volt bank
• 97 Amps * 1/0.10 (10%) rate of charge = 970 AH @ 12 volt bank

Then you start looking at the inverter-charger charging output... 

For example, the two 2,000 Watt @ 12 and @ 24 VDC units -- 12 volt @ 80 amps or 24 volt @ 50 Amps

While the 12 volt unit has higher current charging--The 24 volt unit actually outputs more power into the battery bank:

• 14.5 volts * 80 Amps = 1,160 Watts into 12 volt bank
• 29.0 Volts * 50 Amps = 1,450 Watt into 24 volt bank

So the 24 VDC version will support a 25% larger (stored energy) bank vs 12 volt 2kw model...

Of course, the maximum AC input current and (I think) the battery bank charging current is programmable to what you need.

It is interesting--Most folks run the genset during the day (or morning) for daytime loads (cooking, washing machine, water pumping, laptop computer, cell phone charging, etc...), fueling genset, and use the inverter only at night for quiet time.

Also note, refrigerators and freezers may use more power when loaded with fresh (warm) food... For example, the compressor may run 120 Watts @ 50% duty cycle when "stable" (food already cold, no "extra loads" like cooling drinks/making ice). For "new" / warm food, making ice, cooling soda and beer, the compressor may run 100% duty cycle until all is stable:

• 120 Watts * 0.50 duty cycle * 12 hours = 720 WH for 1/2 day fridge/freezer when "normal operating conditions"
• 120 Watts * 1.00 duty cycle * 12 hours = 1,440 WH for 1/2 day fridge/freezer cooling "warm food"/making ice

As you can see, 1st day into cabin may use more energy (fridge in this example) than 2nd day in cabin (after food has cooled to storage temperature).

To run the eu2x000i at/near full power, that is around 4 hours on 0.95 gallon tank of fuel (to 9 hours or so @ 25% load)... There are real simple conversion that let you feed from an external fuel tank. Basically the eu2x000i has an internal fuel pump that can draw a slight vacuum on the full tank--Replace the vented fuel cap with a "sealed" cap and hose--And connect the other end of the hose to a marine fuel tank/dip tube in 5 gallon (or larger) fuel tank / etc. (obviously, be careful--Want to make sure no leaks or spilled fuel to start a fire):

https://www.amazon.com/s?k=eu2200i+extended+run+kit

It looks like you have the math correct... For a flooded Cell Lead Acid Deep Cycle battery, suggest a 100 AH per 500 Watt of inverter @ 24 volt battery bus:

• 2,000 Watt inverter (max load, surge to 4,000 Watts for a few seconds) * 100 AH * 1/500 Watt inverter = 400 AH @ 24 volts

AGM batteries probably surge 2-2.5x better (at least) than FLA batteries, so a 200 AH or smaller bank @ 24 volts of AGM would probably support your 2,000 Watt maximum loads (on a 2,000 Watt or 3,000 Watt inverter only running 2kW max).

To be honest, I try to be "conservative" with system design. The idea being that 5-7 years down the road (as batteries age) and in middle of day/night with 1/2 charged bank--The system still work as expected/as you need.

Off grid systems are usually over designed to a degree to account for these issues (and variable sun when you get into solar). I don't suggest designing to "exactly" what you need. You may end up with a system you have to "fiddle with" / watch weather / watch family / have to fill the 0.95 gallon tank at 2am, and watching that friends don't plug in a hairdryer (you would be very surprised at how many times we are informed that a hairdryer is mandatory for the spouse).

For the genset, I suggest that you only run the battery charger at a maximum of 80% of generator capacity:
http://cdn.powerequipment.honda.com/pe/pdf/manuals/00X31Z396030.pdf

• Rated max continuous output 1,600 Watts @ 120 VAC
• 1,600 Watts * 0.80 deratring = 1,280 Watts continuous (for battery charging suggested--Battery charging can draw max programmed current for hours--Most "home loads" are variable--Fridge on/off, washer on/off, water pump for home water pressure on/off, etc. for 10-30 minutes at a time)
• 1,280 Watts derated / 120 VAC = 10.7 Amps @ 120 VAC suggested maximum draw for eu2000i
• 1,280 Watts * 0.90 inverter-charger charging efficiency * 1/29.0 volts charging = 40 Amps maximum charging current @ 24 volt bus
• 40 amps * 1/0.10 (10% rate of charge) = 400 Amps @ 24 volt bank "nominal"
• 40 amps * 1/0.05 (5% rate of charge) = 800 Amps @ 24 volt bank "maximum suggested"

FLA battery mfg. suggest 10% minimum rate of charge... 5% rate of charge is what I would suggest is absolute minimum rate of charge... You can charge at 13%-20%+ rate of charge--But that can stress the lead acid battery bank (batteries can overheat with high charging current at >~80% State of Charge.

By keeping "moderate" / middle of the road parameters and expectations--Least amount of active management and best life for your systems.

For charging (and loads)--As an example, we use 6 volt @ 200 AH golf cart batteries. And say it is decided that we want 400 AH @ 12 volts.

• 2 x 6 volt in series = 24 volts
• 2 x "12 volt" @ 200 AH strings in parallel = 12 volt @ 400 AH battery bank
• 12 volts * 400 AH = 4,800 WH of stored energy (full to dead)
• For a 4 x GC battery bank
• OR:
• 4 x 6 volt batteries in series = 24 volts
• 1 x string = 200 AH @ 24 volts
• 24 volts * 200 AH = 4,800 WH of stored energy (full to dead)
• For a 4 GC battery bank

Either configuration gives you the same stored energy--And a 10% rate of charge is the same wattage in either case draws same wattage from genset:

• 14.5 volts charging * 400 AH * 0.10 rate of charge = 580 Watts from genset
• 29.0 volts charging * 200 AH * 0.10 rate of charge = 580 Watts from genset

The major advantage with higher bus voltage is you can use smaller AWG wiring and circuit breakers/etc... 2x bus voltage, 1/2 wiring current.

If you notice--I am not trying to "give you the answers"... Because I do not know your exact needs and expectations. Giving you the modeling math so you can make your own decisions with your data and your assumptions. Again--I am after a "balanced" system design.

Load->battery bank. Battery bank->charging needs (solar, genset, etc.). Battery Bank size -> minimum rate of charge (5-10-13%+ rate of charge for genset and solar). Location+Loads->size of solar array (hours of sun per day by season) & amount of genset energy/fuel needed.

-Bill

minisolar

Fascinating stuff Bill. Much appreciated.

Making sure I understand you - 

1. Going with either 12v 400ah bank or 24v 200ah bank and a 2000w inverter is fine and up to me. 

2. The max Watts I should be pulling from generator for charging is 1280watts.

Question - if I have a 24v 200ah bank, at 7am it is at 100% charge, then by 7pm I used 1 kwh, which means that I used about 20% of my total battery capacity right? (200*24 =4800. 1000/4800= 20.8%?). Then at 1280 Watts input charging - how long will it take to fully charge batteirs back? Seems like quite quickly... I believe it can't even bulk so much since batteris will be 80% charged. Right? 

If that is true, then by running generator at night at this scanario the gensey will mostly just idle... Wasteful. 

Now what if instead I have a 24v 400ah bank.. That means that maybe I do it differently: First day I arrive around 1130-2pm aka "short day". Now I know that I used 3.5 kwh from 2pm until 11am the next day this weekend when I measured it. So that is 36.4% of my battery bank. Therefore, I might be able to not charge at all the first short day, night, and then the full day after.. So by the end of my 2nd day at the cabin I'll get pretty close to the 50% mark by night time. Essentially I'll have from 11am untill 7pm another 1.3 kwh before I hit that 50%. No ac during that time so I think that's very doable... So that means only one night charging - 2nd night. I think that is much better. (BTW I have an extended tank system so genset can run for a long time) 

So... 

After this weekend I'll have a better numbers that will help make a decision - kwh during short day, night kwh, long day kwh.

If I can only charge overnight on night number two.. That will be amazing and much more efficient. And if in the future I find that I need more - I can bump up to 600ah 24v. 

Best price so far is home depot renogy AGM 12v 200ah battery for $370. I can probably get it out the door for about $320.. So a 24v 200ah bank will be $640... And twice that for 400ah. Hard to beat. And either one series or two series and one parallel. Which I believe is good? 

*edit - read specs for the AGM renogy battery. It says -4f max.... My batteries might be in -20 in upstate NY. I looked up other agm and many say temp around there. So will I not be able to use and charge batteries if it is too cold? Only battery I saw that specs showed a much lower temp (-40f!) is full river agm. But they are twice the price of this Renogy. 

BB.

Minisolar,

1. Going with either 12v 400ah bank or 24v 200ah bank and a 2000w inverter is fine and up to me. 

I was pulling numbers out of the air to say that (at this bank size)--Either would be a near "identical" source of power/energy. I was not saying "this is the bank" I recommend for you.

To figure out the "right bank" for you--Need the daily loads (for discharging) and your peak Wattage (or current @ xx volt battery bus) you expect to draw.

AGM batteries "surge" batteries than FLA--So a smaller battery bank can work (UPS use AGM/GEL batteries and are expected to only provide 15-30 minutes of backup power to the computer system. Just enough time to save your work and shutdown the computer--15-30 minutes is not enough power for the average home/cabin). So that is the first calculation--Expected loads for 12/24 hours per day.

Also, you can choose 2 days of storage and 50% discharge (4x daily usage) as "optimum" for full time off grid. If, however, you are using a genset (later with some solar supplement) for weekend use--Then 1 day of storage and 50% discharge (2x daily usage) can work fine for you. 1/2 the cost of batteries (remote cabin, theft is always an issue), and for weekend use--Batteries may age out faster than cycling out (2x more batteries may last 2x longer--Cost power AH/WH used over time is about the same between 1x and 2x bank). AGMs typically last 5-7 years (good quality, treated well).

2. The max Watts I should be pulling from generator for charging is 1280 watts

If you have a Honda eu2000i, they are rated for 1,600 Watts continuous... I always suggest to derate a "non-commercial" genset by 80% for longer life (engine, alternator, electronics):

• 1,600 Watts max rated continuous load * 0.80 derate for life = 1,280 Watts

Not saying that @ 1,500 Watts for an hour will "kill" the genset--Just some points to aim at... And you may end up with programming limits for the inverter-charger:

1,280 Watts / 120 VAC output = 10.7 Amps, but inverter-charge may only accept "whole numbers:

• 10 amps * 120 VAC = 1,200 Watts
• 11 amps * 120 VAC = 1,320 Watts
• 1,200 Watts (example) * 0.9 inverter-charger eff * 1/29 volts charging = 37 Amps (approximate)
• 37 Amps / 0.10 (10% rate of charge) = 370 AH @ 24 volts (10% rate of charge--Nominal bank)
• 37 Amps / 0.05 rate of charge = 740 AH @ 24 volts (5% rate of charge--Suggest no large bank)

Note that "bank sizing numbers" are approximate--Again in solar anything within 10% is pertty much lost in the noise.

I am not going to "quibble" over the those two numbers. +/- 10% (+/- 128 Watts) is not going to make an issue that you would ever see (life of genset, fuel efficiency, etc.).

Question - if I have a 24v 200ah bank, at 7am it is at 100% charge, then by 7pm I used 1 kwh, which means that I used about 20% of my total battery capacity right? (200*24 =4800. 1000/4800= 20.8%?). Then at 1280 Watts input charging - how long will it take to fully charge batteries back? Seems like quite quickly... I believe it can't even bulk so much since batteries will be 80% charged. Right? 

If that is true, then by running generator at night at this scenario the gensey will mostly just idle... Wasteful. 

The exact charging time varies with "major" stuff like charging below 80% SoC (batteries accept very high charging currents-->20% rate of charge). 80% to 90% SoC the batteries slowly reduce charging current from >20% to probably less than 5%). And 90% to 100% from less than 5% to 0.1% or even 0.01% rate of charge (i.e., at 100% SoC, just self discharge or higher current level, your are damaging the battery bank).

More or less, just to put a little math (all models are inaccurate--Some are more inaccurate than others) to see what we are looking at. An eu2000i is "reasonably" fuel efficient at 25% load:

• 1,600 Watts * 0.25 loading = 400 Watts output
• 400 Watts * 0.9 inverter losses * 1/29 volts charging = 12.4 Amps into 24 VDC battery bank

One could argue that (assuming no other loads on genset), that fuel efficiency drops to undesirable levels when falling much below 12.4 amps (for talk--Say 1/2 that level, or below ~6 amps). At that point, turning off the genset will probably save you fuel costs and genset life.

If this was my cabin... And I was using (mostly) a genset for power (plus battery bank & inverter)--And fuel was a big expense--My usage would be:

• Run battery bank from 100% SoC to 50% SoC
• Start genset and recharge from 50% to 80 SoC while I am there.
• Last day, recharge to >90% SoC if I have a 1-2% solar array--Or >95% SoC if no solar array for floating.
• AGM batteries are typically rated for 6 months between recharging if fully charged (~100% Soc to 75% SoC) then they need recharging again (below ~75% SoC in storage--No cycling--Lead Acid/AGM will sulfate and die sooner).
• Lead Acid/AGM batteries will not sulfate (as I understand) if actively cycled between 50% and 80% (or even down to 20% to 80%).
• Deeper cycling, Lead Acid batteries wear out sooner... But as above... Spend ~2x more money on batteries to last ~2x longer--Pretty close to a wash in overall costs.

Now what if instead I have a 24v 400ah bank.. That means that maybe I do it differently: First day I arrive around 1130-2pm aka "short day". Now I know that I used 3.5 kwh from 2pm until 11am the next day this weekend when I measured it. So that is 36.4% of my battery bank. Therefore, I might be able to not charge at all the first short day, night, and then the full day after.. So by the end of my 2nd day at the cabin I'll get pretty close to the 50% mark by night time. Essentially I'll have from 11am until 7pm another 1.3 kwh before I hit that 50%. No ac during that time so I think that's very doable... So that means only one night charging - 2nd night. I think that is much better. (BTW I have an extended tank system so genset can run for a long time) 

I think I have answered with my suggestion of genset charging 50%-80% SoC while there (and if you drop down to 40% to 85%--Not going to quible).

Note that Lead Acid, and especially AGM batteries are near 100% efficient when measuring Amp*Hour draw vs charging. The AH becomes less efficient when >90% SoC (for flooded cell batteries the "gassing" and heating losses dominate for EQ charging--AGMs are not "gassed" or elevated EQ charged--At least not on purpose).

You can use a voltmeter (or panel volt meter--And even a current meter) to monitor your bank and "get a feel" for when to charge and turn off the genset.

Or you can get a BMS (battery monitor system). They run from estimating state of charge from votlage profile of bank:

https://www.solar-electric.com/midnite-solar-mnbcms.html (simple, cheap)

To active meters that measure voltage/current profiles and model battery characteristics to give better estimates of battery bank state of charge (even "nice" BMS can "drift"--Need to understand how they work and their limitations):

https://www.solar-electric.com/victron-energy-bmv-712-smart-battery-monitor.html (BMS--Even with Bluetooth)

Or the large number of Battery Monitors on Amazon/EBay/etc.. This one is cheap enough to try and toss in the garbage if it does not work (I have not used any of these--Links are for your reference/research starting point):

https://www.amazon.com/AiLi-Voltmeter-Ammeter-Voltage-Motorhome/dp/B07FGFFHC6

If I can only charge overnight on night number two.. That will be amazing and much more efficient. And if in the future I find that I need more - I can bump up to 600ah 24v. 

Be careful when tossing numbers around... A 600 AH @ 24 volt battery bank is not small (it is enough to run a cabin with fridge/water pump/washing machine/LED Lighting, laptop computer, etc) very nicely (around 3.3 kWH per day) with the appropriately sized solar array.

A large bank may last 5-7 years (AGM) and a "balance" bank design may last 5-7 years (spending 2x more money for same "aging life"). However "in the middle" while you are using it--Hopefully less time spent managing your energy and more time enjoying the cabin and out doors.

And watch your genset run time expectations... using the 50%-80% charging:

• 300 AH  * (.80-.50) "daily genset" cycle * 1/37 Amps charging = 2.4 Hours of runtime to "recharge" 50-80% range on "smaller" bank
• 600 AH  * (.80-.50) "daily genset" cycle * 1/37 Amps charging = 4.9 Hours of runtime to "recharge" 50-80 range on "larger" bank
  

Of course this assumes no other loads on genset/battery battery bank. If you have other loads at the "same time", those will reduce charging current to battery bank and extend runtimes.

So 2.4 hours "every day" genset runtime (1/2 tank of fuel). Or every "two days" on full tank of fuel (very rough numbers again)... Really your choice.

Best price so far is home depot renogy AGM 12v 200ah battery for $370. I can probably get it out the door for about $320.. So a 24v 200ah bank will be $640... And twice that for 400ah. Hard to beat. And either one series or two series and one parallel. Which I believe is good? 

*edit - read specs for the AGM renogy battery. It says -4f max.... My batteries might be in -20 in upstate NY. I looked up other agm and many say temp around there. So will I not be able to use and charge batteries if it is too cold? Only battery I saw that specs showed a much lower temp (-40f!) is full river agm. But they are twice the price of this Renogy. 

Sounds nice... I don't recommend brands per say... I am the guy that does a bunch of research and the thing I buy dies in a week. Go an impulse buy--And it lasts for years (at least it seems that way  ).

I am on grid and don't use off grid power (just a genset for the rare "California went stupid" and I need backup power for a few days/weeks (in case really stupid). Others here have more "real life" experience with specific hardware. I am just trying to get you "close" so you don't spin your wheels in the back 40.

Lead Acid batteries are great for cold applications. When fully charged, they go well below -60F without issue. If fully discharged, they freeze not much below 32F--So for winter storage, fully charged, and possibly a float solar array--You are good to go.

Cold is also good for lead acid batteries. They age slower (every 18F colder, they age 1/2 as fast). Also they (roughly) self discharge 1/2 as fast for every 18F below "room temperature".

If you plan on winter adventures with your cabin--An insulated box with some sort of heat to bring batteries up to temperature may be needed (genset running battery heater):

Again, I do not live in cold country (cold here is random frost every few years). FLA batteries, once they are warmed, tend to stay "warm" (say >40F) in an insulated box. AGM batteries are more efficient and may not "self heat" during cycling without some warmth (insulated box, ground mounted in cellar, etc.).

AGM batteries are more "freeze tolerant". FLA batteries tend to crack the plates/cases if hard freeze. AGM (no "liquid" electrolyte) tend not to be damaged by hard freeze. Neither battery type should be cycled when frozen. You want batteries >75% SoC throughout winter until you reopen (charge with genset)... So neither battery is going to freeze if 75%-100% SoC in the lower 48.

This is your money, and your needs. There is always the choice of brands/costs/review/your needs. Nobody expected a VW Beetle (classic bug) to last near as long as a Mercedes... Even though both were made in Germany. And you could probably rebuild the betle multiple times vs the cost of maintenance for the Mercedes.

Larger bank, more expensive (and better) hardware, more up front costs. Hopefully less time and maintenance down the road and let you simply enjoy the place.

By the way MiniSolar. Are these posts helpful for you and interesting--Or are they approaching frustrating?

Without face to face feedback--I am guessing how deep to with answers, and where you want the discussion to go...

Take care,
-Bill "as wife kicks me under the table for talking too much as an engineer" B.

minisolar

Bill you are amazing. This is super interesting for me! Thank you!

After this weekend I will have a really good input about ac loads. But I think in general I agree with you - much more efficient to have a slightly larger bank and let them go to 50% SoC so then genset will do bulk charging. Then a small solar array for winter.

I am leaning very much toward 4x 12V 200ah AGM batteries to make a 24V 400ah bank. but final call will be when I have better readings. 

Your "if it was my cabin" is spot on and exactly what I plan basically. You mention charging from 50% SoC to 80% SoC I cannot fully grasp the math but I believe that  you say that it will take 2.4hr about to bring a 300ah bank from 50%-80% SoC. then from 50% to 100% how long will it take about (and bank will be 400ah) to full charge? Seems like Genset will work in high watts for 3 hours then go down for charge stages so if it has 12 hours to work (7pm - 7am) I will get up to 100% SoC likely, right? even if 400ah bank. 

The Victroan will also act as a transfer switch right? So when genset is running it will use power from it for ac loads and charge at the same time? 

I think I will make a super insulated box for this equipment right next to generator. And in winter I will start by running cabin on generator and maybe have the exhaust from it heat the batteries before I switch over to using them? I can also just heat with with a device of course. 

I started to adjust my diagram from the previous plan of 12v bank with ATS, Inverter, and Battery charger. I left some things in there which now need to be adjusted (breaker size, wire sizes, and so on. I will also need a device (step down it is called?) to bring down the 24V to 12V for my 12V loads. I attached the file... Any chance you can open it and let me know about the adjustments I need to make (wire size, breaker, and so on) ? 

btw the battery monitor I got is: 

https://www.amazon.com/gp/product/B07FGFFHC6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Was reading Victron manual a little bit. Most is flying over my head. 


Some things: 

1. I can set a 3 way switch to remote shut off inverter. I will need to do that. 
2. Seems like there are 3 features related to generator... Ac input current limit - Which is the one to limit overloading genset? UPS Feature - which switches to battery power when geenrator is raving up when something is calling for more power to prevent damage to the requesting appliance. Dynamic current limiter - confusing since it seems to be the same as ups?
3. search more is factory set to off. It says that it will switch off if sensing less than 30watt.... seems odd so if I am only running lights and pulling 15watts - it will shut off the lights? strange. they have another search option called AES? 

BB.

You are very welcome Minisolar. I have fun thinking these problems out too (never like games/chess/etc.). The "real problems of life" always were more interesting for me.

Just some quick answers (I have to head out in a bit):

More or less, lead acid batteries will accept large amounts of current below ~80% state of charge (20-25%) very efficiently. So, 100 AH out, 100 AH in, 20 Amps => 100 AH / 20 Amps = 5 hours to recharge.

From 80% to 90% the batteries are now being held at "set point" by the charge controller. Amperes into the battery are being limited by the battery bank itself. And this limit is a bit complicated... You hit the limit at 20% rate of charge lower (closer to 80% or so)... Whereas if you are charging at 5% rate of charge, you hit the battery current limit closer to 90% state of charge (sort of still in "bulk" charge because of lower charging current).

Just randomly... I estimate using this rule of thumb (for FLA batteries... AGM I don't know enough to do better). For shallow discharge less than 75% State of Charge)--Call it 2 hours in absorb (set pont voltage charging). If your bank is 50% or lower, use 6 hours of "set point" charging--To get to 100% SoC.:

• 400 AH battery bank; 37 Amp charge
• 20%-80% SoC--37 Amps in (1,200 Watt charge), Vbatt at or below 14.4 volts (AGM set point)
• 400 AH * (0.8-0.5= 0.3) of battery capacity / 37a charge = 3.2 Hours of charging (bulk)
• 80% to 90% SoC--37 Amps (max) to 5% (or 20 amps) rate of charge. Holding at 14.4 volts
• 400 AH * 0.10 capacity / 20 amps least amount of current in absorb = 2 hours (absorb)
• 90% to 100% SoC--20 amps/5% to 0.4 amps/0.1% rate of charge. Holding at 14.4 volts
• Call it another 6 hours (assuming 50% or deeper cycling) (extended absorb)
• 3.2 hours + 2 hours + 6 hours = 11.2 hours for 100% state of charge (50% to 100% SoC in this example)

My short math:

• 400 AH * 0.50 (1/2 of battery capacity) * 1/37 Amps = 5.4 hours
• + 6 hours of "extended absorb of 50% or less SoC" to get to actual 100% SoC
• 5.4+6=11.4 hours quick estimate for time

Note--All models are inaccurate. And these may be "overly conservative" for AGM batteries (weighted toward FLA batteries). But--The short answer is Yes, you can run the genset for 12 hours to get fully charged (and run other loads as needed).

Note the 1% or 0.1% "ending tail" charge rate--This is how to "know" battery is full (AGM being sealed, you cannot measure Specific Gravity). Most AGM have much less than 0.1% self discharge--So this is a "good stop" for new AGM batteries and bank in "good condition". FLA batteries especially, as they get old/failing, self discharge can get to 1% 2%--So using 0.1% is "not a reliable" charging ending point (FLA batteries that are at 2% rate of self discharge need to be replaced ASAP--There are conditions where the batteries will overheat and even catch fire).

When you get your bank, you can use the 1% or 0.1% (0.1% * 400 AH bank = 0.4 Amps "ending" current) as your charge ending point (current clamp meter, battery monitor, etc.)... Then you have the "real time" based on your bank.

And somebody like Mark Kurth who knows 100x more than I about batteries and AGM batteries specifically can give better answers.

My above, good enough to "bracket" a solution--And should be on the conservative side.

Normally, you are aiming for 90%-95% SoC as ending charge--Going to 100% SoC is kind of "hard" on many battery types (gassing, plate erosion, gasing, venting for AGM, high battery temperatures, faster internal corrosion, etc... I would aim for 90-95% as "full" for most "end of weekend" charging. And only go for 95%-100% on day before winter shutdown.

Remember, we are playing a lot with your genset runtime... 2.4 hours per day to 50%-80% SoC is different than the orginal plan to run genset 12 hours per day and batteries 12 hours per day... Your time on battery may be much longer (22 hours per day) vs the original 12 hour battery cycle time. So, more draw on batteries than first cut planning.

Batteries (in general) like to be cool (50F-75F for lead acid)--Longer battery life. And during non-use, well below 32F actually extends the life of Lead Acid batteries. You may want to run uninsulated/well vented battery box for 9 months of the year... And super insulate only in deep winter when using cabin (where bank is located--Heated space in winter, ground contact, crawl space, etc.).

Batteries are consumables... They will eventually die from age/cycling/etc. Just avoid "killing them" early (over discharging, under charging are the two major causes of early battery death).

Let us know how that battery monitor works for you (do a review here)--Always interested to learn new stuff.

You are going to make me read the Victron Manual--Aren't you?

If so, there are something like 4-6 different Victron models of inverter-chargers we have talked about here. Can you give you a link to the model/manual of the system you are thinking about?

With "small system"--The standby/search mode (inverter is in low power AC off until an AC load is detected) can save significant energy. If you have a larger (battery, genset, solar) system with 100% on loads (say frost free fridge/freezer)--Then standby/search mode is not really useful (many frost free appliances need 100% on AC power for timers/sensors to work correctly).

For 24 to 12 VDC... Look for DC to DC buck converters in Amazon/EBay--There are tons of them:

https://www.amazon.com/s?k=dc+to+dc+buck+converter&ref=nb_sb_noss

Need to know your 12 VDC loads (average and peak/surge loads).

And you can get sealed/ruggized units with different options of DC to DC power supplies:

https://www.solar-electric.com/residential/voltage-converters-switches.html

Terminology:

Buck--Voltage dropping (input voltage needs to be, at least, a few volts above output voltage) (24 VDC input to 12 VDC output)
Boost--Output voltage needs to be a few volts above input voltage (12 VDC input to 24 VDC output)
Buck-Boost -- Any voltage in to regulated voltage out. (10.5 VDC to 30+ VDC input to 12 or 24 VDC / etc. regulated output)

-Bill