Mixing battery sizes

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SageOldman
SageOldman Registered Users Posts: 5 ✭✭
in Solar Beginners Corner #1
Is it OK to mix different amp hour 12 volt batteries wired in parallel? Like two 100 amp hour and one 50 amp hour batteries? Does it hurt anything or reduce efficiencies? 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    If the batteries are the same brand and type (i.e., Yuasa Sealed Lead Acid. Interstate flooded cell batteries, etc.), then it will work.

    If you mix types of batteries (AGM with FLA, etc.), then it is less than ideal. You will end up under charging the FLA or over charging the AGM.

    This is usually a "short term" solution (you have the batteries, you do not have the $$$ for a new bank at the time). Longer term, you should buy matched batteries so that months or a year or two down the road, you don't have short(er) battery life, one battery at a time failing and your bank slowly losing capacity, and/or trying to decide when to scrap the balance of the bank and start over.

    Make sure you wire the bank properly. This website does a nice job of showing how to properly wire parallel batteries so that they sure current equally:

    http://www.smartgauge.co.uk/batt_con.html

    Also--Next time, look at "golf cart" batteries that are 6 volts @ ~200 AH... Two of those batteries in series will give you a 200 AH @ 12 volt battery bank. Easier to put a volt meter across each battery for a quick voltage check (batteries are balanced or if they need Equalizing charge), fewer parallel strings of batteries, fewer cells to check electrolyte levels (2x 6 volt batteries = 6 cells to check, 2x 12 volt batteries = 12 cells to check).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • SageOldman
    SageOldman Registered Users Posts: 5 ✭✭
    #3
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    BB. said:
    If the batteries are the same brand and type (i.e., Yuasa Sealed Lead Acid. Interstate flooded cell batteries, etc.), then it will work.
    If you mix types of batteries (AGM with FLA, etc.), then it is less than ideal. You will end up under charging the FLA or over charging the AGM.
    This is usually a "short term" solution (you have the batteries, you do not have the $$$ for a new bank at the time). 
    It is a short term situation Bill. Batteries are AGM. I could only afford one 100 and one 50 amp hour battery right now to get me started towards a 250 amp hour bank. Too late to consider 6 volt this time. I'm curious. Why do they have to be the same brand? I'll be very careful to connect them given the weakness I hear about parallel batteries.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #4
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    Different battery brands/models will have variations in Acid Fill (higher sg electrolyte, higher battery terminal voltage), and different additives to the plates (make plates stronger, generate less gasses, or pure lead for other batteries).

    You put different brands/types of lead acid batteries in parallel, they will behave differently (lighty different voltage and current profiles).

    The batteries, on average, will still work... They will just not work to their maximum efficiency and life.

    If you put "very different" batteries like FLA vs Li Ion--You could have more issues. There voltage/current curves are quite different. And charging a FLA at 14.8 volts (or even higher) is "normal". For Li Ion it can "kill" the battery, or in some cases, even start a fire.

    FLA batteries tend to be more forgiving (NiCads, Nickle Iron, and some others are very hard to "kill"). AGM tends to be more picky. And Li Ion need BMS (battery monitor systems) to prevent damage/fire and balance cells during operation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,144 ✭✭✭✭
    edited August 2020 #5
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    Bill, I agree with your very good, clear explanation. I would like to add the following for the purpose of the discussion!

    Using simplistic terms from my world: At the Absorb voltage, a fully charged AGM battery draws less than .5 amps per 100 amp hours of battery ah capacity.  At lower voltages during Bulk, the current is obviously lower. If the size disparity between batteries is small, the impact of a little extra absorb time on the smaller battery is negligible at such a low current. While the smaller battery reaches 100% SOC sooner, the current draw is very low while the larger battery is catching up. That time differential is dependent upon the charging capacity. If the charging capacity is small compared to the large battery, the smaller battery will spend much more time charging – but at a lower voltage because it has not reached Absorb yet!

    Remember that the smaller battery is not seeing the full Absorb voltage until the larger battery reaches Absorb. In general, it will boil down to about 2-3 hours of extra Absorb time for the smaller battery when the disparity of size is very, very large.

    Note that I have not addressed the differential of DOD between large and small batteries!




    I always have more questions than answers. That's the nature of life.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #6
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    Don't count on any particular wiring method actually resulting in equal currents.   Measure it.

    I am available for custom hardware/firmware development

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