Seeking help with off grid cabin

znate

Hey everyone,

I'm moving to a cabin and would like to run it on solar. I'm looking for some advice on what I need.
I'd like to eventually be able to run a fridge, chest freezer, lights, laptop, washing machine, toaster would be nice, rv water pump and a few other small items like phone chargers and whatnot. I don't currently have the big items like the fridge, freezer or washing machine so I can't use a killawatt meter to measure my needs. I can buy low power models when I get that far along. What size system do I need? I ran some numbers and came up with 250 amp hours a day at 12v. Does that seem about right?

I was originally planning on a 12volt sytem because I wanted to use 12v lights and other 12v items like an rv water pump but after reading about the issues with uneven battery charging I'm open to other ideas. Perhaps a 24v system so if I decided on a dc fridge I'll still be able to run it direct.

I found someone selling some used Crown CR325 batteries. He told me their date stamped 2011 with lots of life still in them. Voltages range from 6.16-6.28. Under a load test they're in the medium to low part of the ok range. He has 8 and wants $500 for them. Does this seem like a good buy? From what I've read I'm going to kill my first set of batteries so I don't want to start of with Surretts. It would be nice if these would last a couple years or so.

I'm stumped when it comes to choosing a charge controller. Can they be used with wind generators as well as solar? I may be interested in small wind down the road. Do I need a seperate controller for that or can I use the same one for both solar and wind?

Since I'm on a budget I was thinking of getting the used batteries, a couple 235 watt panels, and a charge controller for now. I'll use a crappy inverter for now to run ac loads. I dont need any of the big items till the summer so If I get a system to run some lights and a computer for now and upgrade in the spring I'll be happy.

I guess I'm looking for advice on a charge controller and batteries. Do the used batts make sense or should I just get a couple T-105s? What about a controller, any recomendations? Am I forgetting anything else?

Thanks

bill von novak

Re: Seeking help with off grid cabin

znate wrote: »

I'm moving to a cabin and would like to run it on solar. I'm looking for some advice on what I need.
I'd like to eventually be able to run a fridge, chest freezer, lights, laptop, washing machine, toaster would be nice, rv water pump and a few other small items like phone chargers and whatnot.

Quick numbers: (in kwhr/day)
Refrig 1.4
Freezer 1.2
Laptop w/accessories (8 hrs/day) .5
washing machine (1 hr/day) .5
toaster (.5 hr/day) .5 (note - toasters don't eat into your energy budget much but they do significantly increase the size of your inverter)
Lighting (LED, 8 hrs/day) - 1
water pump .25

5.4kwhr/day, call it 6 for some margin

So 500 amp-hours at 12 volts with an 85% efficient inverter. Which means a 3000 amp hour battery bank so you don't go below 50% on a three-day-without-sun run. (Or a 750 amp hour at 48 volts.) If you have a generator you run regularly you can reduce this since you won't need to run for more than a day without charging.

Also be aware that this is a small amount of power. Even efficient small homes take 10-12kwhr a day.

Since I'm on a budget I was thinking of getting the used batteries, a couple 235 watt panels, and a charge controller for now. I'll use a crappy inverter for now to run ac loads. I dont need any of the big items till the summer so If I get a system to run some lights and a computer for now and upgrade in the spring I'll be happy.

Probably a good idea. The system above is going to be expensive, so starting with a smaller/cheaper system will give you some experience. Naturally with only ~480 watts of charging you're not going to be able to run much more than a few lights and a laptop. You'd be looking at about 1500 watt-hours per day available depending on location. (Not every day; on a cloudy day you'd have to turn almost everything off.)

Also going super cheap early on will make it less painful later when you have to scrap everything except the panels to upgrade.

I may be interested in small wind down the road.

Don't bother. People who say "small wind" generally have in mind something they can mount on their roof, maybe on a short pole, and generate a little extra power when it's stormy. Such turbines generate almost nothing. (But they do make a lot of noise and require maintenance.)

Cariboocoot

Re: Seeking help with off grid cabin

Welcome to the forum.

What you need is lots and lots of money. Why? Because "fridge, chest freezer, lights, laptop, washing machine, toaster" all use power. Refrigeration equipment: 1-2 kW hours per day each. Toaster: nasty 1,200-1,500 Watts while on. Washing machine: big start-up surge, heavy running demand.

Don't even think about trying this on a 12 Volt system. 12 VDC equipment won't save you anything, and probably will cost you more.

Avoid used batteries if you want the system to work. They may function today and drop dead tomorrow.

If you buy those 235 Watt panels you will have to have an MPPT type controller as they are 99% certain to be GT panels with a Vmp around 30 and thus will not match any system Voltage directly. They also will be inadequate for charging anything but a very small system.

A "crappy inverter" is the same as throwing money away.

So ... How about setting up a small "lights & computer" system on 12 Volts for now, and installing a full-power system for everything else later?

znate

Re: Seeking help with off grid cabin

Wow thanks for the quick replys.

So If I want to build a 6kwhr/day system how much money are we talking about? Could it be done for 10 grand? I have two to work with for now so I was hoping to get the used batteries and a charge controller that I wont need to replace when I upgrade. How do I figure out what size controller I need? I was looking at the Conext XW MPPT 60 Amp but for a system that large will I need multiple controllers or is there one that's large enough on it's own?

I'm thinking I'll run the generator any time I have to run the washing machine. I figure a load every three days. I can use that time to recharge that batteries. If I have to do without a toaster that's no big deal. I don't have one now so I can live without it. I'm open to alternatives to a fridge. If I make ice in the chest freezer and use a cooler for a fridge it should cut down on power I'd think. I've run a small propane fridge in the past but found it was rather expensive. $20 bucks every three weeks.

Cariboocoot, what are you running off the system in your sig?

Cariboocoot

Re: Seeking help with off grid cabin

If 6kW hours is the number you're shooting for you should be going with a 48 Volt system. As such you'd be looking at roughly 500 Amp hours of battery @ 48 Volts, a 60 to 80 Amp MPPT controller, and about 3200 Watt array. It might be possible to do this for $10,000 US.

Don't discount the toaster and especially not the refrigerator; you just have to take them into account when doing the design. My system was basically built to handle the full-size refrigerator plus 'extras' and can actually produce around 2.4 kW hours per day. I have certain advantage of climate and elevation combined with seasonal-only use; it would be impractical to try and keep it going in the Winter up here because the available daylight shrinks to 6 hours a day total.

I also have the satellite Internet set-up, water pumps, and even a microwave that can be run midday if wanted. The overnight storage is slim, so when the sun goes down stuff gets turned off. Everything costs too much up here, but if I could I would expand the solar array and battery capacity both to make it easier to get through the marginal days.

bill von novak

Re: Seeking help with off grid cabin

znate wrote: »

So If I want to build a 6kwhr/day system how much money are we talking about? Could it be done for 10 grand?

Maybe with some very good deal-getting. Probably not. It's very hard to do a battery backed system for $3/watt (and you'd need about 3500 watts.)

I have two to work with for now so I was hoping to get the used batteries and a charge controller that I wont need to replace when I upgrade. How do I figure out what size controller I need? I was looking at the Conext XW MPPT 60 Amp but for a system that large will I need multiple controllers or is there one that's large enough on it's own?

You will definitely have to replace the batteries, so go cheap for now.

For the controller choose a good MPPT device like a Flexmax 80 or a Midnite Classic. You probably won't have to replace that if you get a good one now since output voltages are programmable.

I'm thinking I'll run the generator any time I have to run the washing machine. I figure a load every three days. I can use that time to recharge that batteries. If I have to do without a toaster that's no big deal. I don't have one now so I can live without it. I'm open to alternatives to a fridge. If I make ice in the chest freezer and use a cooler for a fridge it should cut down on power I'd think.

More power taken making ice and opening/closing door, and more energy pumping water - I wouldn't go that route. Can you live with one very efficient fridge?

znate

Re: Seeking help with off grid cabin

I think I'll go with a chest freezer to fidge conversion. From what I've found online it looks like I can expect it to use 500wh/day. Forget the freezer and washing machine. I think 1.5kwh/day should be enough. Sound okay?

So what voltage should I go with for a system that size?

To figure out the size of controller do I take my power produced per day and divide by my system voltage? For example, If I have 1500 watts into 12 volt bank. 1500/12 = 125 or

1500/24 = 62.5

So if I go with a 24v system I need a controller that's rated at or above 62.5 amps? Is that correct?

Cariboocoot

Re: Seeking help with off grid cabin

You do it this way:

Watt hours per day (DC) divided by nominal system Voltage = Amp hours used.
Multiply by 4 to get battery bank size at 25% averaged depth of discharge.
Peak charge current should be 10% of that, possibly more if you have less-than-ideal conditions.
Then you can multiply that by 'ideal' panel Vmp for the system to get Watts of array on a PWM type controller or multiply by nominal system Voltage and divide by 0.77 typical efficiency to get Watts of array on an MPPT type controller.

Let's assume 2.0 kW hours DC (you need to include inverter efficiency and consumption in determining this).

So 2000 / 24 Volts = 83.3 Amp hours. Times 4 = 333 Amp hour 24 Volt battery bank.
First adjustment: get this to align with batteries that are actually available. You might be able to shave some load or depend on panel supply or deepen average discharge and make use of 320 Amp hour batteries. Otherwise you have to round up to nearest available size.

Then you would need to supply 32 Amps peak charge current:
PWM type controller, multiply by 35 Vmp to get 1120 Watt array.
MPPT type controller, multiply by 24 and divide by 0.77 to get 997 Watt array.
Second adjustment: increase array size to meet available panels. For example four 250 Watt panels on an MPPT controller.

Check it 'backwards' this way:
Array size * minimum hours of equivalent good sun (usually 4) * 0.52 over-all system efficiency.

Applying that to a 1kW array and 4 hours of sun you get 2080 Watt hours which is just above the 2000 Watt hour target, so it's good.

Starting to make sense?

SolInvictus

Re: Seeking help with off grid cabin

A crappy inverter can also be backup for when the expensive one breaks.

Written by bill von novak:<br>
Refrig 1.4
Freezer 1.2
Lighting (LED, 8 hrs/day) - 1

He can do better than this with a standard refrigerator/freezer with an Energy Star rating of 365 kWh/year or less (1 kWh/day). With a 26 W CFL running 8 hours and a few other lamps operated intermittently, I use maybe .3 kWh/day/person for lights.

If your inverter can run a washing machine, then it can run a toaster but maybe not simultaneously.

znate, unless you are in a very cloudy place or high latitude, you do not need a 3,200 W PV array to run your items. Your original calculation of 250 amp hours a day at 12v is 3,000 Wh/day, so your PV system should generate double, about 6,000 Wh, on a sunny day which requires a 1,400 rated watt PV array if you have normal sunlight. Use NREL's PV Watts to verify the PV array size for your location. I do more than you contemplate using a 900 rated watt PV array, not using a generator and still have surplus power after several consecutive sunny days.

Cariboocoot

Re: Seeking help with off grid cabin

Nope. That's not how off-grid systems work.

You must consider that all of the power may need to be stored in and drawn from the batteries. This is why over-all efficiency of an off-grid system is a lousy 52%. There are some steps that can improve this such as making use of opportunity loads when the sun is shining and the batteries are full.

PV Watts is designed for grid-tie systems where there is always some place for the power to go. Whereas it can give you some relative information about insolation in your area throughout the year and how different angles will affect production it does not apply directly to off-grid systems. Thus a 1400 Watt array will not result in 6kW hours per day but rather only 3kW hours that you can truly count on (if the sun shines).

Power is stored in the batteries, the PV is there to recharge them. Trying to count on full production from the array the way you would with a grid-tie system is asking for trouble.

SolInvictus

Re: Seeking help with off grid cabin

I think your advice is based on squandering half the power charging the batteries in a way that does not significantly increase their lifespan. You advise a 4 times overbuild of the PV array while I advise a minimum of a 2 times overbuild for off-grid systems. My advise is based on my experience. Certainly a 4 times overbuild will yield a well functioning PV system, but it is not the most cost efficient. I think we differ in the weight we apply to the cost analysis.

I vaguely remember reading that a 3 times overbuild is the most cost efficient using prices from last year. znate plans to expand his system incrementally allowing him to experiment to find the right system for him.

BB.

Re: Seeking help with off grid cabin

I am not sure I understand your points:

SolInvictus wrote: »

I think your advice is based on squandering half the power charging the batteries in a way that does not significantly increase their lifespan.

The 0.52 system efficiency does not really take into battery bank size...:

• 0.77 array derating (panels running at normal temperatures) * 0.80 battery efficiency (can be from 0.80 to 0.98 depending on type of battery and charging profiles) * 0.85 AC inverter efficiency.

The sizing of the battery bank is recommended from 1 to 3 days of storage and 50% maximum discharge. When you "run the numbers", for most systems 2 days of storage and 50% maximum discharge is a pretty good range.

Yes, you can run (true deep cycle batteries) below 50% state of charge (even down towards 20% state of charge), but the batteries will (typically) fail sooner.

Interestingly, somebody posted a chart here a few months ago that showed batteries discharging between 80% to 20% state of charge for a "typical" battery will, roughly cycle the same amount of kWH's until death.

I.e., if you cycle a battery bank to 70% and it lasts 5 years, cycling to 40% SOC will only last ~2.5 years... Basically you buy a 2x larger bank and it lasts 2x longer--No real savings in costs.

You advise a 4 times overbuild of the PV array while I advise a minimum of a 2 times overbuild for off-grid systems.

We have two suggested array sizing tasks... One is the A,AAA watt array with B hours of sun per day and C efficiency will give you D Watt*Hours per day.

The other is sizing the battery bank. In general with lead acid batteries, you need around 5% to 13% rate of charge for the battery bank to provide enough energy to charge the battery bank, quickly recharge the battery bank after a run of bad weather, give enough energy to account for battery self discharge, and support loads the run during the day while the sun is up.

My advise is based on my experience. Certainly a 4 times overbuild will yield a well functioning PV system, but it is not the most cost efficient. I think we differ in the weight we apply to the cost analysis.

I vaguely remember reading that a 3 times overbuild is the most cost efficient using prices from last year. znate plans to expand his system incrementally allowing him to experiment to find the right system for him.

So, for battery sizing, 2 days of storage and 50% maximum discharge gives us a 4x daily loads for bank sizing (typical recommendation). 1 day of storage is a pretty stiff discharge (high charging and discharging currents can be difficult to get enough "hours in a sunny day" to fully recharge a daily cycling bank). And going deeper than 50% on a daily discharge can cause problems with battery life...

We try to be conservative for off grid home designs... If this were for an RV where you may run it 10-15 weekends a year--Deeper cycling and less than optimum charging is usually OK--Batteries either age out (rather than cycle out) or you simply do not have the space/weight capacity to support a larger battery bank.

-Bill

vtmaps

Re: Seeking help with off grid cabin

BB. wrote: »

if you cycle a battery bank to 70% and it lasts 5 years, cycling to 40% SOC will only last ~2.5 years... Basically you buy a 2x larger bank and it lasts 2x longer--No real savings in costs.

That's been my thinking, and I have also given that advice on this forum. But sometimes a larger bank works out better because of peak power concerns. Example: my batteries (which are cold and old) have enough voltage sag that they cannot run the washer unless they are fully charged. The washer uses only 0.4 kwh for a load, and my batteries have adequate capacity. The problem is that the battery voltage will sag to my low voltage cut off (24 volts) after a few minutes of supplying 500 watts. If I had a larger bank I wouldn't have so much sag.

Nonetheless, I chose to keep my battery investment low. Work them hard, replace them sooner (because I work them hard), and have less money invested in batteries at any one time. Part of living off-grid (for me) is not living the "on-grid" lifestyle. That means I don't discharge my batteries to do my laundry... I do laundry when the sun is shining or the generator is running.

Batteries are the heart of an off-grid system. Choosing the right battery is the most difficult part of designing a system. It's all about loads... the average load, the peak loads, the length of the peak loads, the time of day (or night) that the loads occur.

--vtMaps

BB.

Re: Seeking help with off grid cabin

Yep, that is another check for battery sizing... Maximum surge current (for flooded cell) of C/2.5 rate of discharge and maximum continuous discharge current of ~C/8 to (maybe) C/5.

AGM can support higher discharge/surge current--But it is usually other rules of thumbs that size the battery bank larger (overall depth of discharge/speed of recharging). Similar for recharging--A high recharging current can take a smaller/100% charged battery out of regulation too (over 72 volts for a 48 volt battery bank) with some charge controllers.

If you don't choose wisely for the batteries--You may be chasing your tail with system issues for the life of the battery bank.

-Bill

Cariboocoot

Re: Seeking help with off grid cabin

I am not talking about a 3X overbuild or a 4X overbuild: I'm talking about sizing a battery bank so that it doesn't go dead quickly. For 90% of the batteries used this means you automatically lose 1/2 the calculated capacity because regularly discharging to less than 50% capacity is murder on batteries. That gives you 50% capacity to work with, and if you divide the average consumption over two days you get 25% DOD. The remaining 25% capacity provides one "no sun day" of energy and on the third day of clouds you start the generator. This also allows the most efficient use of generator capacity.

PV array has to be sized to charge the whole battery, regardless of SOC, and the shortcut of basing peak current on 10% capacity works most of the time. In some areas you'd want even more, due to less-than-perfect insolation. Cutting it down to the minimum battery manufacturer's recommended 5% is asking for trouble as it doesn't account for concurrent loads and will not charge the batteries sufficiently in the hours of daylight available (which should be based on minimum sun, not maximum).

My advice is based on my experience and includes a lot of repairing of systems designed by people who didn't do the calculation right in the first place.

I have seen people (including Bill, who does not have an off-grid system) advise on the basis of "three days with no sun" which is impractical because you end up with more panel and battery than you need.
I have seen people advise on the basis of panels producing 100% of their rated power, which they don't.
I have seen people advise on the basis of 100% system efficiency, which doesn't exist in the real world.
I have in fact seen people give out a lot of really bad advise which results in system with no margin for error or variation in operating conditions.

And then I have to fix their messes. Some of these are "professional installers" who in fact have only a vague understanding of what they are doing based on being able to connect wires together and have it work momentarily. I wouldn't trust them to change a light bulb. Whereas my success rate is 100%, even with seemingly hopeless situations.

You can improve over-all efficiency with certain management techniques, but you must be able to adapt your usage to them. Until you know whether or not this can be done on an individual system it is better to err on the side of caution.

The two biggest mistakes repeated over and over are insufficient panels for charging the amount of batteries and insufficient batteries for supplying power for the loads.

The sun does not shine brightly all the time.

P.S.: I'm getting sick of people derailing threads of newcomers with bad advise and off-topic discussions.

znate

Re: Seeking help with off grid cabin

Cariboocoot wrote: »

You do it this way:

Watt hours per day (DC) divided by nominal system Voltage = Amp hours used.
Multiply by 4 to get battery bank size at 25% averaged depth of discharge.
Peak charge current should be 10% of that, possibly more if you have less-than-ideal conditions.
Then you can multiply that by 'ideal' panel Vmp for the system to get Watts of array on a PWM type controller or multiply by nominal system Voltage and divide by 0.77 typical efficiency to get Watts of array on an MPPT type controller.

Let's assume 2.0 kW hours DC (you need to include inverter efficiency and consumption in determining this).

So 2000 / 24 Volts = 83.3 Amp hours. Times 4 = 333 Amp hour 24 Volt battery bank.
First adjustment: get this to align with batteries that are actually available. You might be able to shave some load or depend on panel supply or deepen average discharge and make use of 320 Amp hour batteries. Otherwise you have to round up to nearest available size.

Then you would need to supply 32 Amps peak charge current:
PWM type controller, multiply by 35 Vmp to get 1120 Watt array.
MPPT type controller, multiply by 24 and divide by 0.77 to get 997 Watt array.
Second adjustment: increase array size to meet available panels. For example four 250 Watt panels on an MPPT controller.

Check it 'backwards' this way:
Array size * minimum hours of equivalent good sun (usually 4) * 0.52 over-all system efficiency.

Applying that to a 1kW array and 4 hours of sun you get 2080 Watt hours which is just above the 2000 Watt hour target, so it's good.

Starting to make sense?

Ok I think I'm starting to catch on.

So if I want to use those used batteries I could use 4 of them in a 24 volt system which would give me 325 ah which is slightly less than what you said above. Does it make sense to do it that way? I could use the best 4 and keep the rest for another project or try to sell them.

Or should I go with a higher sytem voltage, say 48v? Whats the advantage? slightly higher efficiency? Smaller guage wires? Is it easier to get components for a 48v system?

Thanks for all the help everyone. Some of it's over my head but I appreciate it anyway.

Cariboocoot

Re: Seeking help with off grid cabin

znate wrote: »

Ok I think I'm starting to catch on.

So if I want to use those used batteries I could use 4 of them in a 24 volt system which would give me 325 ah which is slightly less than what you said above. Does it make sense to do it that way? I could use the best 4 and keep the rest for another project or try to sell them.

Or should I go with a higher sytem voltage, say 48v? Whats the advantage? slightly higher efficiency? Smaller guage wires? Is it easier to get components for a 48v system?

Thanks for all the help everyone. Some of it's over my head but I appreciate it anyway.

What the application is will make a difference. Particularly the big question of "what happens if the batteries fail?" Is it critical? If so, avoid used batteries. Sometimes the new ones are problem enough. If it's not critical you can gamble on used batteries.

24 Volts works for most applications. It is more difficult to get fuses/breakers rated to handle 48 Volt systems because the system actually functions at ~60 Volts when charging. So if you see those Square D breakers rated for "48 VDC" know that they do not guarantee performance at the higher Voltages such a system actually sees. On the other hand they work fine at 24 Volts.

There is slight improvement in efficiency in 48 Volts over 24, and it is the way to go if you need more power (either stored or on-demand). The 'threshold' for this is debatable, as a lower Voltage system can take an occasional, short-term 'hit' of high current. As with so many things the decision should be made based on the continuous average power need.

I seem to have posted this link several times in the past few days, but here it is again: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

znate

Re: Seeking help with off grid cabin

Ok I'm going with the used batteries. It's not a huge deal if they crap out and my system is down for a bit.

So It looks like my only option is a 24 volt system based on the battery size and my power needs. If I hooked them all up but didn't have larger PV it would only kill the batteries sooner, correct?

I plan to use 4 Heliene 235W poly solar panels,
Outback Power FLEXmax 80 MPPT Solar Charge Controller (is there something better for my application?) Larger than I need but it leaves room to upsize

One question I have, can I use a dc-dc converter to bring the 24 volts down to 12 volts so I can use a suresine inverter? This is what I was thinking http://www.ebay.com/itm/24V-to-12V-DC-DC-Car-Power-Supply-Converter-30A-New-/201000771965

Cariboocoot

Re: Seeking help with off grid cabin

If you can't recharge them they go dead once the stored power is used up. If they sit around under 70% SOC for days they will sulphate and become unable to be recharged. This is why charging is important.

So, 325 Amp hours @ 24 Volts? About 3.9 kW hours stored. If they are aged don't expect them to have full capacity; the actual Amp hours go down over time no matter how well-treated the batteries are.

Your proposed array should be sufficient to charge them (at about 39 Amps peak current). Do you need an FM80? The FM60 is slightly less money and would still handle the current without problem.

I would not try to run an inverter from a DC-to-DC converter. True, the Suresine should pull less than the 30 Amp rating of that converter but only for the constant power rating; one surge and the converter will pop. A lot of us wish Morningstar would make a 24 Volt version of this inverter!

Have you looked at the Samlex inverters? Pretty good value for smaller size equipment and they have 24 Volt versions down to 300 Watts. These PST models are not good for motors, though.

BB.

Re: Seeking help with off grid cabin

Cariboocoot wrote: »

So, 325 Amp hours @ 24 Volts? About 3.9 kW hours stored.

I think Marc intended to say that you have 3.9 kWH of "useful" stored energy (to 50% of battery capacity).

However, the total stored energy is ~325AH*24Volts=7,800 WH=7.8 kWH of stored energy.

We size the array to charge 10% of the battery bank's total capacity. The extra 50% in reserve is to allow for aging, surge capacity, and those occasional emergency situations. The Batteries will simply last longer (more years) too if you avoid regular excursions below 50% state of charge (try to never go below 20% SOC--That will risk permanent damage to cells/battery elements).

So, the array size needed to "optimally" recharge such a battery bank (5% to 13%, with 10-13% usually being "optimum"):

• 325 AH * 29 volts charging * 1/0.77 panel+charge controller deratings * 0.10 rate of charge = 1,224 Watt array for 10% rate of charge

-Bill

znate

Re: Seeking help with off grid cabin

BB. wrote: »

The Batteries will simply last longer (more years) too if you avoid regular excursions below 50% state of charge (try to never go below 20% SOC--That will risk permanent damage to cells/battery elements).

So, the array size needed to "optimally" recharge such a battery bank (5% to 13%, with 10-13% usually being "optimum"):

• 325 AH * 29 volts charging * 1/0.77 panel+charge controller deratings * 0.10 rate of charge = 1,224 Watt array for 10% rate of charge

-Bill

Is there built in protection on an inverter to kill power if Batteries are getting low? I imagine that depends on the quality of inverter purchased. If it doesn't have protection is there any other way to add it?
How does one usually tell the state of charge? Does the charge controller display that?

I guess I should add a 5th panel based on what you're saying Bill. That would put me at 1175 watts.

Rybren

Re: Seeking help with off grid cabin

Cariboocoot wrote: »

These PST models are not good for motors, though.

'Coot I'm curious why you think that the Samlex PST inverters are not good for motors. They're true sine wave inverters with less than 5% THD (don't recall the exact numbers)

What, in your opinion, makes them unsuitable?

Cariboocoot

Re: Seeking help with off grid cabin

They're not all that efficient especially when it comes to 'real' surge power. They don't work that well with inductive loads. This is not to say it absolutely will not work for such use, but that there is the possibility it won't or won't work well.

On the other hand the SA series do, and they aren't exactly expensive either.

BB.

Re: Seeking help with off grid cabin

Inverters typically have 10.5 volts set as their cutoff voltage. And this is done to "save" the AC inverter (keep maximum current from getting too high, output voltage browning out your AC appliances).

But this is way too low for a battery bank... Normally, setting a cutoff voltage of ~11.5 volts when the battery is under load, and perhaps as high as 12.0 volts when lightly loaded--And you can see--You see that you really need a variable cutoff voltage based on current and time (i.e., 11.5 volts for 5 minute average, 10.5 volts for 10 second average, etc.). And then there is temperature dependance too (cold batteries run higher output/charging voltages).

There is no really good way of monitoring voltage and accurately determining battery state of charge.

You can get Battery Monitors that have a remote output (Victron) that you can program to turn on an alarm at 50% SOC and turn off the alarm at 80% SOC (etc.)... But that is not cheap and requires you to interface a low voltage on/off signal with your inverter/input DC relay/etc.

Some inverters do have programmable low voltage cutoff points--It is better than nothing--But it is difficult to automate "fail safe" battery monitoring.

Get your system running, monitor voltages/power levels/specific gravity--And you can probably figure out some voltage setpoints that will work well enough for your needs.

-Bill

bill von novak

Re: Seeking help with off grid cabin

znate wrote: »

One question I have, can I use a dc-dc converter to bring the 24 volts down to 12 volts so I can use a suresine inverter?

Short answer: no.

Longer answer: if you want to have two battery banks, and live within the power limitations of the DC/DC converter, and ensure that the output of the DC/DC works for battery charging, and charge the 24V bank and use the 12V bank for the inverter - then yes, you can do that. I can not imagine why you would want to, though.

Cariboocoot

Re: Seeking help with off grid cabin

bill von novak wrote: »

Short answer: no.

Longer answer: if you want to have two battery banks, and live within the power limitations of the DC/DC converter, and ensure that the output of the DC/DC works for battery charging, and charge the 24V bank and use the 12V bank for the inverter - then yes, you can do that. I can not imagine why you would want to, though.

Because of the desire to use a Morningstar 300 Watt inverter, which only come in 12 Volt versions.

Rybren

Re: Seeking help with off grid cabin

Interesting. I'm not sure what you mean by real surge power. I do know that the 1500W version runs my 10A circular saw without an issue. I haven't tried any other motor type loads, but I'll watch for issues and will report back.

Thanks.

Cariboocoot

Re: Seeking help with off grid cabin

Rybren wrote: »

Interesting. I'm not sure what you mean by real surge power. I do know that the 1500W version runs my 10A circular saw without an issue. I haven't tried any other motor type loads, but I'll watch for issues and will report back.

Thanks.

That's just it: sometimes it will work fine.
Starting a circular saw isn't as hard as you might think. Certainly not as hard as starting the same size motor on a water pump for example, because it isn't starting against a load. That's when you get the sudden big leap in current.

I've tested some other low-end PSW inverters that do the same, but otherwise are perfectly fine.

Rybren

Re: Seeking help with off grid cabin

Thanks 'Coot, I appreciate the insight.

znate

Re: Seeking help with off grid cabin

bill von novak wrote: »

Short answer: no.

Longer answer: if you want to have two battery banks, and live within the power limitations of the DC/DC converter, and ensure that the output of the DC/DC works for battery charging, and charge the 24V bank and use the 12V bank for the inverter - then yes, you can do that. I can not imagine why you would want to, though.

I like that idea. Could I set it up so that when the 24v batteries are full the extra power would feed another charge controller that would charge the 12 volt bank?
Is there a dump load type circuit built into charge controllers that could be used to feed the second controller?

Cariboocoot

Re: Seeking help with off grid cabin

znate wrote: »

I like that idea. Could I set it up so that when the 24v batteries are full the extra power would feed another charge controller that would charge the 12 volt bank?

Yes. It requires an MPPT controller to down convert the Voltage and a second battery.

Is there a dump load type circuit built into charge controllers that could be used to feed the second controller?

Not needed. Although you can arrange to have the second controller switch on only when the main bank is full by utilizing the AUX control of an Outback or MidNite for example. Otherwise it could be operated by a Voltage controlled switch. Normally you'd just include the power usage of the secondary bank in the over-all design and allow it to run all the time.