Battery Backup with a twist

RichardU

I am on the grid and I have an 18kw generator. I'm looking to build a battery backup system for a few circuits so that in an extended grid-down situation, I don't need to run my generator 24/7. I also have room for a few (2-6) solar panels which would be for an ultra-critical circuit (medical equipment) in a grid-down-generator-down situation. In the meanwhile, it would nice to use the output from the panels to offset my power bill.

I would appreciate any suggested recommendations. Thanks.

BB.

Re: Battery Backup with a twist

Welcome to the forum Richard.

The place to start is usually with a Kill-a-Watt type meter to measure your loads (average watts, kWatt*Hours per day, etc.)...

Without that basic information, we are all guessing.

In general, even with an off grid power system--Just wear on the batteries from cycling can cost more than simply using the Grid Power.

To same money, Grid Tied Inverter + Solar panels is usually the best choice. However, there is no backup power available if the grid goes down (there is a new solution that includes some backup AC power during the middle of a sunny day).

Short term outages, the genset is usually a better option (I like smaller gensets if the AC loads are small--Fuel costs/storage/availability become an issue--An 18 kW genset may use 1-3 gallons per hour. A 1,600 watt Honda eu2000i will run 4-9+ hours per gallon of gasoline...

One person here had natural gas in his sub-division for emergency pumping (near river). Relatively cheap, no storage issues--Was fine until the utility had to turn of natural gas to the neighborhood. All depends on your needs.

If you need longer term backup power (medical), we can work with you to define your needs--And go from there.

Just to give you an idea--A "smallish system" can supply around 1 kWH per day. 3.3 kWH per day will run a small home with a refrigerator. 10+ kWH per day is getting pretty large for an off grid system.

3.3 kWH per day is only:

• 3,300 WH per day * 1/24 hours per day = 137.5 Watt average load (running 24 hours per day)

So, depending on the nature of your loads--Those that require power 24x7 can really drive the size of the solar array+battery bank up.

-Bill

RichardU

Re: Battery Backup with a twist

Thanks for the response. I believe I will need 100 watts for 16 hours a day and 200 watts for 8 hours which is a total of 3.2 kWH. I'm in Atlanta, GA. What else do I need to know?

westbranch

Re: Battery Backup with a twist

so for 8 hours a day you need 300W/hr and for the other 8 hrs you need 100W/hr? What type of equipment are you powering?

BB.

Re: Battery Backup with a twist

3.2 kWH per day--That is a "medium" sized system. Using our rules of thumbs for a "typical" off grid, balanced system design.

First the battery bank, 1-3 days of "no sun storage" (pick two days) and 50% maximum discharge. Use a 24 volt battery bank suggested minimum battery bus voltage (keep DC current at "reasonable" levels):

• 3,200 WH per day * 1/0.85 inverter efficiency * 1/24 volt battery * 2 days storage * 1/0.50 max discharge = 627 AH battery bank @ 24 volts

You can use three parallel strings of 6 volt @ 220 AH golf cart batteries (4 batteries per string). That is 12 batteries total (I would suggest other batteries, but this gives you a "size" of bank and some of your lowest cost battery bank configurations).

To recharge a 24 volt @ 660 AH battery bank at 5% to 13% rate of charge (from solar):

• 660 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 1,243 Watt array minimum
• 660 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 2,486 Watt array nominal
• 660 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 3,231 Watt array "cost effective maximum"

Next, we also need to recharge the 3.2 kWH per day of 120 VAC power you use per day... For Atlanta Georgia, fixed array, tilted to latitude, using PV Watts, we get:

Month    Solar Radiation (kWh/m 2/day)
1      3.86     
2      4.67     
3      5.21     
4      6.17     
5      5.95     
6      5.81     
7      5.82     
8      5.83     
9      5.21     
10      5.51     
11      4.42     
12      3.72     
Year      5.18      

Tossing out the bottom three months (assume using backup genset), we get the minimum average usable sun per day as 4.67 hours of sun per day for February (break even month for solar/possible genset use):

• 3,200 WH per day * 1/0.52 overall system eff * 1/4.67 hours of sun per day = 1,318 Watt array minimum

So the "useful" solar array would range from 1,318 Watt minimum to 3,231 Watt cost effective maximum with ~2,486 Watt array as being a "healthy" nominal array for an off grid system in daily use.

With a 2,486 Watt array, the minimum hours of sum per day to recharge the battery bank:

• 3,200 WH per day * 1/0.52 overall system eff * 1/2,486 Watt array = 2.5 hours of sun per day minimum

If this was an emergency backup system, perhaps you would use the minimum array+genset for emergency backup power.

If you have a medical need/looking for many months off grid and need to minimize your fuel usage, then you would look at the 2,486 to 3,231 Watt array (note: numbers are +/- ~10% -- I am just copying all the digits so you can reproduce my math and follow where I am pulling the numbers from).

Now--What size AC inverter would you use... For a 660 AH @ 24 volt battery bank using "rule of thumb" for flooded cell battery bank:

• 660 AH * 24 volts * 0.85 inverter eff * 1/20 hour discharge = 673 watt average load (call it 5 hours a night for two nights with reserve)
• 660 AH * 24 volts * 0.85 inverter eff * 1/8 hour discharge = 1,683 Watt max continuous load
• 660 AH * 24 volts * 0.85 inverter eff * 1/2.5 hour discharge = 5,386 Watt max surge (start well pump, table saw, etc.)

So, the average good quality AC inverter will support about 2x rated load for surge power... So, an inverter would range from ~673 to 1,683 watt average, and ~(5,386/2=) 2,693 Watt unit...

Or, in round numbers, around a 700 to 1,800 watt inverter, and as large as ~2,700-3,000 watt inverter.

AC Battery charger--Again around 5% to 13% or even 20-25%--Pick 10% and 20%, you would be looking at a 24 volt AC charger capable of:

• 660 AH * 10% = 66 amps nominal
• 660 AH * 20% = 132 amps as a "good sized" AC battery charger (need to monitor battery bank temperature)

And a good sized generator (or AC outlet) to power them.

Does this make sense to you? Any questions/changes?

Obviously, we need to pick hardware and batteries--But this gives you a good back of the envelope design/capabilities.

RichardU

Re: Battery Backup with a twist

westbranch wrote: »

so for 8 hours a day you need 300W/hr and for the other 8 hrs you need 100W/hr? What type of equipment are you powering?

I'm not quite sure what you're saying about "8 hours ... the other 8 hours"? It's 3,200w per day. Is there a reason for re-expressing that in terms of a 16 hour day?

It's medical equipment and a variety of other stuff. Peak draw would be 1000w but only for a minute or so.

RichardU

Re: Battery Backup with a twist

BB. That's a pretty good sized envelope you're using for your back-of-the-envelope response. Thank you so much for such a thorough answer. It's exactly what I need. I'll digest it shortly and respond.

Richard

westbranch

Re: Battery Backup with a twist

Peak draw would be 1000w but only for a minute or so.

that is why I re-expressed your numbers, you have to plan for the largest momentary load when deciding on which equipment to use,
for example you have to have a battery that can 'produce' that instantaneous 1000W (or more) for start-up of a motor.
BTW you should expect a motor to use 3 - 5x its stated Amperage, possibly more on start-up. When running it can be less. This is why a K-A-Watt meter is so handy.

hth

solar_dave

Re: Battery Backup with a twist

To offset your power bill when not in backup duty you will need something like a Sunny island setup.

RichardU

Re: Battery Backup with a twist

Fantastic. So I need a 660AH battery bank, a 1,300 to 3,000 watt array, a 1,500 to 3,000 watt inverter. And a charger, assuming it's not included as part of the inverter. And I'm assuming a solar charge controller.

My big question is, how much more complication or money is it to be able to use the array when not in backup duty? I think Outback calls that Grid Interactive. They make the GTFX3048, a 3000 W Grid interactive inverter for $1,633. Except for the fact it's 48v, would that be recommended, or do I need a Sunny Island setup as solar_dave suggested, or anything else?

I guess I'm ready for specific equipment suggestions so I can start reading some manuals. I've already been trying, but there are many many choices out there.

Thanks again for all your help.

BB.

Re: Battery Backup with a twist

In itself, you don't need to pay (much) more for a Grid Tied Hybrid Inverter system vs a pure off grid system.

For a Hybrid inverter system (off grid+GT capable+AC Battery Charger) you can look at the Schneider and Outback systems:

http://www.solar-electric.com/nexaxwseinan.html
http://www.solar-electric.com/oupogrsiwain.html

For a moderate off grid inverter-charger system from Magnum (some are 120/240 VAC split phase models):

http://www.solar-electric.com/maensiwainac.html

You can also look at integration controller/connecting an AC backup genset. The Schneider will either require a 120/240 VAC split phase genset--The Magnum can work with either 120 or 120/240 (as I recall--with limitations).

The Schneider and Magnum come with 120/240 VAC split phase capability out of the box--The Outback requires further equipment to get more than 120 VAC support.

The standard way of using hybrid and off grid inverter+battery bank is to use a separate Solar Charge Controller (for larger systems, MPPT type charge controllers are usually recommended).

MidNite Solar Classic Charge Controllers and Clippers
OutBack Power FLEXmax MPPT Charge Controllers
Schneider Electric Solar Charge Controllers
Morningstar Advanced MPPT Charge Controllers

Yes, a Hybrid GT capable system is nice and can use the grid to behave (for all the world) like an unlimited AC Battery bank that you can charge in the summer and discharge in the winter.

However, you have to check with your utility to see if they allow solar GT systems--And some scattered utilities are now refusing to connect Battery Based Hybrid/GT inverter systems (worried about buying power at night and selling it back during the day)...

Also, look at how much money you will save... A 3.3 kWH per day hybrid system will "save" you perhaps $0.66 in power per day (if $0.20 per kWH utility rates). Not a lot of savings to go through the effort (a typical GT system these days is probably a 5-10kWatt solar array, not a 1-2 kW array system).

Pure GT systems (solar panels+GT Inverter) can be pretty cost effective... If you live in a place with higher utility rates (east/west coast--In California our summer afternoon rates can be from $0.30 to $0.50 per kWH). But hardly anyone will save money with a hybrid GT inverter system--especially a smaller system.

So--You have some choices to make... A smaller system for backup (and you can run your "backed up" loads too to save a few 10's of dollars per month)--Or you can make a dramatically larger (and more expensive system) that can "save" you more money and, perhaps, come closer to "break even".

An off grid system may cost you around $1-$2+ per kWH... A Hybrid system in GT mode can get down towards $0.45 per kWH (just rough numbers to set your expectations). You have to include capital+installation costs, replacement battery bank every 6-8 years or so, and new inverter+charge controller+other electronics every 10+ years (again--just rough estimates--You don't want to be surprised a 1/2 dozen years down the road when the new battery bank is needed or an inverter needs repairs).

There are lots of options out there--None of them are cheap. Conservation (insulation, modern energy star appliances/heat pumps/etc.) is usually the best bang for the buck to "save money"... Plus if you reduce your emergency power usage through "extreme conservation", you save money with a smaller emergency power system/backup genset/fuel supply/costs.

Getting your needs nailed down--Then figure out what components/system design may meet those goals.

Energy usage is a highly personal set of choices.

-Bill

PS: A 3.3 kWH per day system--A 48 volt battery bank would not be a bad solution either. In general, the 48 volt inverters have larger kWatt output ratings vs 24 volts--So they can cost a bit more. MPPT charge contorllers--You can save money with 48 volt system bus (controller is 80 amps at 12/24/48 volts -- A 48 volt bus will allow the controller to manage a 2x larger array vs a 24 volt battery bank. Power=Volts*Current -- double the voltage, 2x the wattage with the same current).

westbranch

Re: Battery Backup with a twist

Hold on pilgrim,

Bill gave you the basics, now we need to 'truth' the actual choices (of equipment) you want to make against you best estimate of Peak loads, daily usage, etc...
First is the type of battery, our usual recommendation is to use golf cart (GC2) batteries. they are 6 volt and are designed for heavy use and are a safe, inexpensive way to ease into solar power. A lot of first timers murder their first battery due to inexperience.

here is a link to one manufacturer, Trojan: http://www.trojanbattery.com/Products/GolfCart.aspx

look up the 145 model

hth

RichardU

Re: Battery Backup with a twist

You have confirmed my many suspicions (and my power rate is a lot lower). So let's specify an off-grid system (which I might use constantly on my emergency circuits).

Starting with inverters in 48v, some choices include:
Magnum Energy MS4448PAE 4400 Watt Sine Wave inverter 120/240 Volt $2,159
Outback 3600 Watt, 48 VDC Pure Sine Wave Inverter with a 45 Amp Charger $1,770.00

The Magnum is larger and includes 240 VAC, (although I don't really need that). So the bigger question to me is which "family" I'm buying into. I'd love to hear any thoughts about which company is preferable for accessories etc. Or any other recommendations.

I'm sold on the GC2 batteries. One question about them: I was told not to locate non-AGM batteries in a living space. I could put mine in an unconditioned garage (which never gets below freezing or above about 90 degrees) but is about 60 feet from my electrical panel. Would that work?

BB.

Re: Battery Backup with a twist

Flooded cell batteries do generate hydrogen (and oxygen) gases when charging. And vent a little bit of acid mist. So, you don't want them in a living space because of that.

A garage can be better.

Also depends on temperature for your area. Batteries work best at about 50 to 75F--And will age about 2x faster if kept at 93F.

There is a minor issue in that you need an "even" amount of batteries. A 48 volt battery bank vs a 24 volt @ 660 AH battery bank of Three Strings (12 batteries--3 strings of 4 batteries). With 48 volts, you will need either one or two strings of 8 batteries in series (8 or 16 batteries). That will either be 1/3rd less or 1/3rd more stored energy. Or, you can choose another battery besides Golf Cart style (but still choose the less expensive battery style for your first bank).

These batteries are about the same price per kWH of stored energy, but are a larger AH capacity. Trojan L16P 6 Volt, 420 AH Deep Cycle Battery (~2x the capacity at ~2x the price).

• 12 GC batteries * 6 volts * 225 AH = 16,200 WH of storage (24 volt bank)
• 8 batts * 6 volts * 225 AH = 10,800 WH of storage (24 or 48 volt bank)
• 16 batts * 6 volts * 225 AH = 21,600 WH of storage (24 or 48 volt)
• 8 batts * 6 volts * 420 AH = 20,600 WH of storage (24 or 48 volt)

Note that the 420 AH @ 6 volt battery will give you 1/2 the number of cells to check water in (do this about once a month, and check/log specific gravity), but also weight about 2x more than GC batteries (67 lbs vs 121 lbs per battery).

Also, personally, I recommend 1-3 parallel strings of batteries--I would not recommend more than 3 (although many people are happy with more in parallel--It does take more maintenance/monitoring to make sure everything is OK--That is simply a lot of cells to monitor water levels and cable connections to make/keep clean).

Another useful product you may consider is the Battery Monitor such as the Trimetric (Victron is another nice brand). Keeps track of the AH in/out of the battery bank. If you have spouse/kids/guests that use the system--Much easier then telling them to monitor with a hydrometer and usually more accurate than using a volt meter.

What hardware to pick--I am probably not the best person to tell you... My system is a pure Grid Tied unit and I use a small genset (Honda eu2000i) as my backup power.

Each of the units/families I suggested above are good units with lots of happy customers. You can add other models/families if you don't need Grid Tie/Hybrid inverters. Some of them have "generator support"--Basically you can plug in a smaller genset and the inverter-charger will draw current to charge the battery bank or even supply energy from the battery bank if the AC power is greater than the generator can support.

Midnite makes a very nice/capable MPPT battery charger with a new integrated battery monitor (due out "soon") and native Ethernet connection/server. Outback and Schneider have integrated data networks that can use a common controller.

It is like GM/Ford/BMW/Hyundai. You will have to look through them and see which meets your cost/performance requirements.

And, it is an issue in that this is your first system where you will be learning "a lot" over the next few years. And whatever you pick today will probably evolve over time.

Asking questions on the forum about which unit may meet your needs (low cost system, very reliable, data logging, surge power for well pump, ability to integrate with genset inverter-charger vs standalone inverter+charger, etc.). Propane vs Diesel vs Gasoline for backup genset, etc....

Lots of shades of gray here. That is why "westbranch" is asking about the details of your loads. Lots of folks here with a wide variety of experiences. We don't want to see you "waste" your money or make the same mistakes that others of us have already made before.

Me personally--I believe in conservation and a small(er) system. But that needs an accurate estimate of your loads. If you try to draw too much power (or Watt*Hours per day) from a solar power system than it can deliver--It will kill the battery bank over time. An "over-sized" system will run your loads--But cost more to install and maintain.

-Bill

BB.

Re: Battery Backup with a twist

Regarding how far from the electrical panel... If you are sending around 120 or 240 VAC of power--It is not a big deal.

At 48 volts, you want to keep the cables shorter and heavy. 12 volt systems are even more difficult (4x the current of a 48 volt system and much less voltage drop allowed in wiring).

Basically, the battery bank+inverter+battery charger DC cables should all be kept in the same area. Solar array cable (array to charge controller) can be much longer (especially with higher voltages--Upwards of 100 VDC Vmp--voltage maximum power for solar array--when using an MPPT type controller). AC backup power (main panel), genset, etc. can be located farther away as you are sending 120 VAC around.

And for installs with special needs--There are options. For example, if the solar array is a 100' away from the battery bank+charge controller, you can get MPPT charge controllers that can operate >400 VDC Vmp for the solar array--Remember Power=Volts*Current. 4x the voltage is 1/4 the amount of current and about 1/8th the amount of copper needed to move the energy.

Once you start looking a current, wire gauge, voltage drops, you will see what I mean.

-Bill

RichardU

Re: Battery Backup with a twist

Bill, thanks again for your thorough responses. At this point there is only so much I can do for conservation, and I already have an 18KW natural gas generator in place. But that should allow me to test a variety of configurations without harming my batteries. In fact, I'm thinking I can get started with the batteries / inverter and add the solar component later.

I'm studying the Schneider Electric manual, and realize that having my generator larger than my inverter may be an unusual circumstance, in that I want the inverter to run a subset of what the generator runs. So I would have these modes:

Normal Mode. Grid up: Generator and inverter do nothing.
Generator Mode. Grid down, generator on, generator runs both "convenience" and "critical" loads (I can do this right now, and it is automated upon grid down)
Battery Mode. Grid down, generator off, inverter runs critical loads only. When battery level drops to certain point, turn on the generator. When batteries are recharged, turn the generator off.

I don't mind manually switching between Generator mode and Battery mode, it does not need to be automated. There are two reasons for Battery Mode. First if the grid goes down for a week, I don't need my convenience loads 24/7 and I'd rather not run the generator 24/7. Second, if the generator goes down and I have solar power to the batteries, I can still run my critical loads. And if I have solar power, that creates the possibility for one final mode:

Solar Mode. Inverter runs the critical loads, Grid runs convenience loads. (I'm not sure if the wear and tear on batteries would be worth doing this, but it might be something I could switch on and off.)

I believe that's the best expression yet of what I'm trying to achieve. So, my first question is, is this doable, or even advisable? Maybe I should contact Schneider Electric. I really like what I've read from them so far.

Blackcherry04

Re: Battery Backup with a twist

To spend $15,000 on solar standby to backup the backup didn't make a lot of sense ( unless you need the 30 % tax deduction ) and want it. 8 GC-2 batteries and a GTFX 3524 Outback your in business. 24 V is a good choice. It'll be able to be set up as a automatic operation. It still gives you expand ability later for a grid tie solar. You can probably do it for $4,000 or less. Another option would be to have a Honda EU 2000 with a tri fuel kit to keep the batteries charged during any long term outage and some loads. Any plan depends on what solar, propane, gasoline, nat. gas is available.

There are ways to do it cheaper The Outback GFX 1424 is only a $1,000 inverter. It'll give you 1,800 watts for 30 minutes.

Remember, anything " ON " the Inverter is always on the Inverter, it's just being powered by the batteries or the AC input. You need a couple of different bypass schemes to get around things in case something fails.

I have always considered my Solar ( 8.2 kw ) will be compromised in a weather disaster, so I made my plans without it. Of course in 30 years I'v only used it twice. The one time I could have for a couple days the city had to cut off the water because they didn't have electricity to run the sewer lift pumps and flooded the town with sewerage. Hard to stay in a house with 2 ft of sewerage backed into the basement. I now have a cutoff valve for that.

Don't count on Natural Gas either, they will cut that off because of flooded hot water heaters.
.
.

RichardU

Re: Battery Backup with a twist

Blackcherry04 wrote: »

they didn't have electricity to run the sewer lift pumps and flooded the town with sewerage. Hard to stay in a house with 2 ft of sewerage backed into the basement.

I'm sorry you had to learn that lesson the hard way. I've known that as a possibility, but never heard of that actually happening to someone. (Fortunately I am at the high point of our sewer system.)

And thanks for your thoughts on my system.

Blackcherry04

Re: Battery Backup with a twist

RichardU wrote: »

I'm sorry you had to learn that lesson the hard way. I've known that as a possibility, but never heard of that actually happening to someone. (Fortunately I am at the high point of our sewer system.)

And thanks for your thoughts on my system.

It was odd, because I am up pretty high myself. We have two electric utilities in town, apparently one failed first and the other one kept on pumping for awhile, the only problem, it was pumping back into the lines on all the branches instead of up the main trunk line, that way it was able to push it everywhere. Supposedly they have corrected it, but I put in that cutoff anyway.

RandomJoe

Re: Battery Backup with a twist

I run my system pretty much as you want. It's the full Outback setup, VFX inverter, FM80 CC, Mate interface, FNDC monitor. I do have the Mate interfaced through a Linux box to my home automation system so I can do more with it than just the Mate alone can do. I have a six-breaker sub panel with the "critical" circuits of my house moved over, and that tied to the Outback inverter. I also have an A/B switch on the input to the inverter so I can switch it between grid and generator inputs.

Normal operation sees the system go off-grid each morning at 5AM, returning to grid when the CC goes to sleep at dark. If the grid goes down I never even know it half the time, at least until I notice the microwave (not on backup) clock has reset. It's gone down a surprising number of times since I put the system in - we used to have such reliable power here. Haven't had any multi-day outages since I put it in though. (Sat in the cold and dark after an ice storm which motivated me.)

If the grid drops for an extended period, or when I test the system, I flip the Big Switch over to the generator input. Plug it (just a Honda EU2000i) into the socket I installed on the back porch, and the inverter treats it just as it would the grid. I manually select the charger function, normally it's completely disabled.

I like having the backup, and with as many computers as I have it's really nice not having to keep up with all the little UPS batteries. I do offset the utility bills by using the system daily (and I know it'll WORK - one major issue with my backup plans in the past!) but I'll never come close to paying for the system that way. Just getting all the use I can out of it!

When I chose the Outback system I was also considering the Xantrex system. It does have one very cool feature my Outback doesn't - it can "share" loads between inverter and grid. The Outback is one or the other. However, at least at the time it was MUCH easier to interface the Outback to my computer since they had docs on their website publishing their comms protocol. (I don't run Windows and have non-standard desires for my system so the vendor software was useless to me.)


A note on whether to cycle the batteries or not: I choose to cycle them. First, as mentioned above, at least I *know* they'll work! How many times I've had UPSes sit for a year then when the power did go out - instant death because the batteries were shot! Second, I don't think you'll greatly extend the life of the batteries by leaving them fully charged 24x7 - indeed, you may well cause harm from grid corrosion if they're constantly on charge. There's a balance - you don't want to pull them down to 50% daily, but using 20-30% just gives them something to do, lets you see that the system is in fact operating correctly, and offsets the overall system cost by a *little* bit anyway.

Ralph Day

Re: Battery Backup with a twist

"When I chose the Outback system I was also considering the Xantrex system. It does have one very cool feature my Outback doesn't - it can "share" loads between inverter and grid."

What Xantrex product does that? My SW4048 will pass through generator/utility power and charge while an external (ac) power source is present. The inverter/charger can invert or charge, not both at the same time. Have things changed? Are you describing where the loads are supported by the inverter when the bypassed power is insufficient to power the loads? Unit stops charging and inverts putting power to the loads in addition to the bypassed amount? I thought all the big companies did this.


Ralph

RandomJoe

Re: Battery Backup with a twist

If running on generator (or just limiting grid input for some reason) and it isn't large enough for the loads the inverter will supply the rest of the power. The XW inverter would actually use BOTH sources to supply loads - keeping the generator loaded while drawing any more needed from the inverter - whereas most systems simply switch between one or the other.

In practice, I've not missed that feature. I almost never have more than 400-500W load and when I've test-run the system on the EU2000i, the only time the generator can't support everything is when the fridge starts while other loads are going. The inverter switches on while the fridge starts, then a few seconds later returns to the generator.

I think the only system that could very quickly "switch" between charging / inverting is the Sunny Island system, though I don't know how it works - if it could actually "ebb and flow" with the loads or if there is actual circuitry that has to switch in/out to go from charging to inverting. (I'm not well versed on the finer points of inverter circuitry...)

RichardU

Re: Battery Backup with a twist

Joe, Thanks for your comments. I also looked over your web site. I think you're doing some things I've wanted to do, particularly with HVAC. So what's happening with that blower and the hot and cold exchangers?

bill von novak

Re: Battery Backup with a twist

Ralph Day wrote: »

What Xantrex product does that? My SW4048 will pass through generator/utility power and charge while an external (ac) power source is present. The inverter/charger can invert or charge, not both at the same time. Have things changed?

The "older" (Trace) SW4048's would do that. They were effectively bidirectional converters. If battery voltage exceeded AC voltage (multiplied by the stepwise-connected transformer) then power would flow from the battery to the AC side. If it were below AC voltage power would flow in.