Battery mixing question...again

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lazza
lazza Solar Expert Posts: 336 ✭✭✭
in Off Grid Solar & Battery Systems #1
HI Forum

If adding a new 250Ah 12V battery to an existing 2-year-old 250Ah 12V battery, which of the following would be worse:

a. putting them in series at 24V?
b. putting them in parallel and maintaining the 12V system?

I know that mixing new batteries with old ones is bad practice :blush:, but in this case there may be no choice....

Many thanks
Larry
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Battery mixing question...again

    I don't know which is worse, or better... In series, the cells should be matched so that you can avoid excessive equalization--plus your string AH capacity is limited by the weakest cell (first cell to go under 0% state of charge will die).

    In parallel, the batteries may not equally share current (an inexpensiive DC current clamp meter on-site for monitoring parallel connected strings highly recommended).

    In the end--I would look at the size of your loads and/or available charging energy... If they are >~1,200 watts max continuous, I would look at going to 24 volt inverter/chargers/rewiring panels/etc. And, the needs of your loads (12 vs 24 volt loads, length of wire runs, etc.).

    I probably would not make the 12/24 volt decision on the mixing of old/new batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    #3
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    Re: Battery mixing question...again

    Ok, thanks Bill... will do as you suggest
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