Can you mix batteries of different capacity?

heynow999
heynow999 Solar Expert Posts: 80 ✭✭✭✭
in Advanced Solar Electric Technical Forum #1
A trailer I am working on has a 24v system, 6v x 4 AGM's. They needed more capacity so they bought another 4 x 6v batteris, but the new batteries are a higher capacity. They just paralelled them together, 4 old one in series, 4 new ones in series. Now they are having problems with charging. They do not seem to have as much capacity as they had when they only had 4 batteries. I checked the voltage and they are close, but it seems that the end batteries have a higher voltage, the middle two a bit lower, and the higher capacity string had a higher voltage overall.

Any help? Ideas?

Thanks

Peter
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Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #2
    Re: Can you mix batteries of different capacity?

    Generally not a good idea. The larger ones will take on the limit of the smaller ones. Also, strings of different ages are also frowned upon.

    On the other hand, it sounds like you have strung them together in a very inefficient way, such that the ends of the string(s) take the bulk of the load. Do a search on this site for diagram(s) of the best way to wire parallel/series strings.

    I also suggest that you read some of the following:http://www.rpc.com.au/products/batteries/car-deepcycle/carfaq4.htm#charge http://www.batteryfaq.org/http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    Tony

    PS. I am searching for the diagram but I can't seem to find it. Here is a suggestion from Crewzer (Jim) :

    "In your case, four 8D batteries wired in series/parallel to make 24 V x ~500 Ah will work fine. Just use good interconnection cables between the batteries, and make sure you connect your main battery cables at electrically opposite corners of the bank (i.e., the (+) cable to the positive terminal on series pair #1, and the (-) cable to the negative terminal on series pair #2). This technique provides for essentially equal current paths for each series pair."

    Here is a link to a pretty good diagram with a good explanation: http://www.solarseller.com/readypower_emergency_and_standby_power__trace_drop_shipped_prices.htm

    There is a much better one here on this site,, I just can't find it right now!
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #3
    Re: Can you mix batteries of different capacity?

    You did not say if the new batteries are AGM like original ones.

    You can mix batteries of different capacity with some restrictions.

    1) series string should be of same capacity and age/usage history (you did this correct).

    2) don't mix batteries of different alloy type (antimony & calcium), Gel cells, AGM's.

    Other issue is PH level in the electrolyte. For AGM's and maintance free batteries it is hard to match the specific gravity of batteries of different age. Over time some water will be lost and specific gravity will go up on maintance free and AGM batteries over time.

    Higher specific gravity (higher acid concentration) will make a more energtic discharge and sluggest recharge. This means the older battery will initially give up more of its charge (higher discharge current) then the parallel newer battery and newer battery will initially take more of the recharge current. This is not a catistophic issue as long as you don't have too high a discharge or recharge current on any one of the batteries. Once they make full recharge at float level they should all be recharged to their available capacity.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
    Re: Can you mix batteries of different capacity?

    Here's a great writeup on how to properly wire your battery bank (diagonally)
    http://www.smartgauge.co.uk/batt_con.html
    it explains why, and has several diagrams too.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Can you mix batteries of different capacity?
    mike90045 wrote: »
    Here's a great writeup on how to properly wire your battery bank (diagonally)
    http://www.smartgauge.co.uk/batt_con.html
    it explains why, and has several diagrams too.

    Excellent explanation!

    Addendum: if you have a charge controller and a load (as in most applications) you wire them diagonally opposite each other. Id est:
    Inverter (+) to Bank 'A' (+), Inverter (-) to Bank 'B' (-)
    Controller (+) to Bank 'B' (+), Controller (-) to Bank 'A' (-)

    This may sound crazy, but I've found it advantageous.
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