Adding capacity to battery bank

highrail
highrail Registered Users Posts: 3 ✭
in Off Grid Solar & Battery Systems #1
A friend of mine was picking up his Trojans at the factory in California and was given a tour. He was buying four batteries to be wired in series for a 24 V system. After a discussion about adding capacity (if needed), the tour guide suggested adding 2 more batteries by wiring one each in parallel to the first and last in the string.

I replied that this did not sound right, mumbled something about geometry, but couldn't come up with a reason good enough to overcome the "this guy should know!" objection. Can someone help out?
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: Adding capacity to battery bank

    Need to add in increments of four batteries.

    I prefer not to cross strap batteries in parallel. Keep each series string independent so you can check each battery voltage individually.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Adding capacity to battery bank

    Four 6 Volt 225 Amp hour batteries in a series string acts like one 24 Volt 225 Amp hour battery. To add Amp hour capacity you must parallel another such string.

    Tacking two more 6 Volt 225 Amp hour batteries on to the first and last in the original string will only make current balancing impossible.
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  • highrail
    highrail Registered Users Posts: 3 ✭
    #4
    Re: Adding capacity to battery bank

    I see. Current balancing problem is the kicker - the two parallel batteries would be undercharged. Or maybe the two middle batteries would fry in an effort to get the parallel batteries to float.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Adding capacity to battery bank
    highrail wrote: »
    I see. Current balancing problem is the kicker - the two parallel batteries would be undercharged. Or maybe the two middle batteries would fry in an effort to get the parallel batteries to float.

    Yes; those in parallel will act as 450 Amp hour @ 6 Volt whereas the other two would be 225 Amp hour @ 6 Volt. How do you set the charge parameters for that? At the 5% minimum for 450 you get 22.5 Amps - 10% of the other two. So you get to pick undercharging the higher Amp hour pair or overcharging the lower Amp hour pair.

    The same reason why you don't mix single batteries of different capacities.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #6
    Re: Adding capacity to battery bank
    highrail wrote: »
    A friend of mine was picking up his Trojans at the factory in California and was given a tour. He was buying four batteries to be wired in series for a 24 V system. After a discussion about adding capacity (if needed), the tour guide suggested adding 2 more batteries by wiring one each in parallel to the first and last in the string.

    I replied that this did not sound right, mumbled something about geometry, but couldn't come up with a reason good enough to overcome the "this guy should know!" objection. Can someone help out?
    Sounds like the "tour guide" just wanted to sell more batteries and figured your friend would not spring for a full set of four more. Either that or he was clueless. Wired that way the two middle batteries would still define the capacity of the bank.
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