Adding to battery bank

LBergman Solar Expert Posts: 42
Hi all,

I'm thinking of adding to my battery set, but wanted to check here first in case anyone could see a problem I'm not (not impossible ;) ).

I currently have two 8A31DT Dekas (AGM) wired in parallel (12v system) that are less than a month old. I want to add two 8AGC2 Dekas (6v) wired in series for 200AH @ 12v, then wiring that set in parallel with the current set. My first choice would be to add two more 8A31DTs, but my battery cabinet is a few inches too narrow to allow that. (The other option is to add only one 8A31DT, but three seems a rather odd number for a battery bank...and I'd prefer sets of two. :) )

The Deka 8A31DTs are 104AH rated. The 8AGC2s are 200AH rated. Since age shouldn't be an issue, that would leave the different capacities. I know wiring different capacity batteries in series is a no-no, but wiring them in parallel is generally OK, though not necessarily optimal...?

Any thoughts? What are the downsides to different capacity batteries in parallel? Is anyone out there running such a setup? If so, how's it working for you?



  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Adding to battery bank

    What's going to keep this battery bank fully charged , and equalized ??
    (assuming the Deka is flooded cell battery, it will need equalization)
    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 ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
    Re: Adding to battery bank

    I would strongly recommend keeping the same make of battery in the bank. The different model may be made just slightly different; even if you get the same battery it may not be made in the same run and may not match up as well.

    If it were me I would get a third of the same battery and do it sooner rather then later.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • LBergman
    LBergman Solar Expert Posts: 42
    Re: Adding to battery bank

    Thanks for the replies. The batteries are AGM, so no equalization needed. I'm a little light on the solar panels (~200w), but my typical use is to use the system for an evening (down to 80% charge remaining), then not use it again until the batteries are fully charged. On a full-sun day, the panels will completely charge the current battery set from 20% DOD, hitting float around 4-5pm (the panels get no morning sun). I'm sure by doubling the battery capacity, it'll take somewhere around 1.5-2 days, which is fine with me.

    I'm just looking to gain more run time during the evening, without having to watch the system monitor to know when to switch from the off-grid system back to utility power. Adding one more 8A31DT (+100AH) will bring me close to that I think, but an extra 200AH would give me some margin. But if it means shortened battery life or non-optimal system performance, then I'll go with just the one additional battery.