Size of Battery Bank?

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I am new to solar -- appears I am in the right place as there is more knowledge here than I could read in several weeks!

I want to create a solar system for my RV using 600 watts of panels. I need guidance in sizing the battery bank. I have heard one rule of thumb is to match the watts of the panels to the amps of the battery bank -- this would beam a 600 amp-hr battery bank.

Looking at my options for batteries and budget, I can create a 500 amp-hr battery bank. The next step up would be a 750 amp-hr battery bank, which costs more than I want to pay.

Thoughts on how to size the battery bank and if a 500 amp-hr bank is sufficient for 600 watts of solar panels?

Thanks!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Size of Battery Bank?

    What voltage battery bank? 12/24/48 VDC?

    And, ideally, you should have some idea of your loads (big solar RE systems are not cheap) and alternative power sources (genset?). I try to preach matching loads, panels, generators to expected loads for cost efficient operation.

    Also, you want to start with conservation regarding your loads (CFL / LED lighting, conversion to efficient devices/electronics, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: Size of Battery Bank?

    Hi Bill -- thanks for the quick response! I am using a 12 volt battery bank. I expect my miniumum daily current needs to be 200 amps and maximium daily current needs to be 400 amps (depending on temperature and abilikty to be outside).

    I am also looking at replacing the lighting with LEDs.

    I have a 3000 watt generator for charging the batteries on cloudy days or when we are parked under trees, and as a backup source of power. It was sized so it could run the AC if needed.
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  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    #4
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    Re: Size of Battery Bank?

    Generally you will want sufficient battery capacity to supply a 3 day weekend. In an RV, you have limits on what you can carry. That usually means some lifestyle changes will be needed. The only way to adequately determine this particular goal is via experience between how you can adapt and what you really actually need.

    Figure on ten usable watt hours per pound of battery. For a 3 ton TT that means you can carry only 1 or 2 kwH of battery energy. For a 24 ton Class A, 4 KwH is nominal and twice that is sometimes possible.

    When you compare that energy capacity to typical use requirements, you can see where very little variance in use can have a big impact on capabilities.

    Solar is as you indicated but I prefer thinking 1 watt minimum per pound of battery to avoid voltage problems in talking about AH. Keep in mind that solar is not sufficient in and of itself in typical RV use. You generally can't get enough solar on an RV to run an Air Conditioner and the tenure of sunlight usually isn't sufficient to assure a proper and full battery charge often enough to keep your batteries in a Vulcan (live long and prosper) state.

    Generally, with RV's it's a polar situation. Either you spend a lot of time off grid where 'the more the merrier' is the rule or you spend very little time off grid which makes any expense much more difficult to rationalize. The key to keep in mind is that the problem is energy storage and there are no magic bullets to solve that one.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
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    Re: Size of Battery Bank?

    with a 500ah battery bank you have just limited your daily ah loads to 250ah so that you will not deplete the battery bank beyond 50% dod. you could charge it with the generator if you must limit the battery bank. btw, 600w of pv doesn't tell us enough as we'd need the specs of the pvs and the controller to be used. i'd recommend mppt. now if i were to guess i could take 600w and divide by a 17.6vmp to give 36.09a. if the mppt recovers 10% then add another 3.6a or so to it for 39.69a. dividing this into a 500ah battery bank gives about a 12.6% charge rate. if the batteries are fla types this will give more maintenance needs, but would be great for agms.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #6
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    Re: Size of Battery Bank?
    RMChad wrote: »
    Hi Bill -- thanks for the quick response! I am using a 12 volt battery bank. I expect my minimum daily current needs to be 200 amps and maximum daily current needs to be 400 amps (depending on temperature and ability to be outside).
    I assume you are talking about 200 to 400 Amp*Hours per day...

    Unless you are living off-grid in the RV (i.e., >9 months of the year)--you can probably plan on taking the battery to 50% level every night and recharging the next day. So, that would indicate a 400-800 AH battery bank.

    That will be heavy use, but even if you assume 500 cycles maximum life--that is 500 days of cycling. Assume you go out 1/3 of the year, that is ~14 weekends or 28 days of cycling per year:
    • 500 cycle life / 28 cycles per year = 18 year cycling life
    And the reality, that your battery may have a 3-8 year life anyway--so 50% cycling (assuming you charge above 75% the next day) is not the worst option.
    I have a 3000 watt generator for charging the batteries on cloudy days or when we are parked under trees, and as a backup source of power. It was sized so it could run the AC if needed.

    Assuming running the genset at 50% of rated output for efficient fuel use--the recommended charging for a 600 AH battery bank would be (using 5-13% battery charge rate):
    • 600 AH * 0.05 = 30 Amp minimum charge
    • 600 AH * 0.13 = 78 Amps maximum charge
    • 3,000 watts * 50% * 0.80 charger eff * 1/14.4 volts = 83 amp charger (generator optimum)
    So, if you had a ~600+ AH battery bank and an ~80 amp battery charger on a 3kW genset, you would be doing OK.

    Solar panel wise, 600 watts of solar panel and 4 hours of sun (9 month months of year) will give you, approximately:
    • 600 watts * 4 hours * 0.77 panel+charge controller derating * 0.80 battery eff * 1/12 volts = 123 Amp*Hours per reasonably sunny days
    So, at this point, your solar power (on a yearly average) about 1/2 to 1/3 of your daily use. The rest will need to be made up with genset/utility power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #7
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    Re: Size of Battery Bank?

    Wow -- there is clearly great experience represented in this forum. I appreciate the information and look forward to learning more.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
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    Re: Size of Battery Bank?

    You are very welcome...

    By the way, a good way to run your genset is to "bulk" charge your battery bank (up to 80-90% state of charge) first thing in the morning--then let the solar panels finish of the charge the rest of the day.

    One of the more fuel efficient ways of charging.

    And just to be clear--the above numbers represent simplifications of real life... Verifying fuel flow vs Watt*Hours / kWHours out of the genset. How much your loads are, how much your system outputs, season, weather, all will have an impact.

    Always question anything you read on the Internet. :roll:

    If it does not make sense, ask questions. Rules of thumb are good starting points, but none of us know the details of your installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #9
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    Re: Size of Battery Bank?

    You should check out the off grid forum on this board. It has a lot of RV stuff.

    I have just (last summer) designed/built/installed (08/01/09) a PV system for my RV (which, without this forum, would be failing to perform miserably.)

    What kind/class RV do you have? Mine is meant to replace a tent. Wife insisted on toilet. So I have more than I, alone, would need. But now she is with me at camp. A good thing, mostly. :roll:

    As a vehicle/shelter descriptor, "RV" is more indefinite than "deep cycle" is for a battery, (apologies to bryanl!:blush:) To me, an RV can weigh anywhere from 400 pounds to some tons. It might be a trailer pulled by an off road 4-wheeler (or even motorcycle) or a self contained unit requiring that someone have a commercial drivers license to move it.

    Is your "RV" large/heavy? Do you wash clothes in it?

    Do you stay at "RV Parks" or do you "boondock" (dry camp?)

    In the same way that defining intended/expected usage is essential to sizing a off grid domicile or GT home, the same is true for "RV" installations.

    BTW, welcome!

    Kamala
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