Batteries - can I add different sizes

I have a remote cabin powered by six sp75s, C40 controller and five concord 2120Ls. I want to expand the battery bank and was wondering if using the 2580Ls was an option or not. Any advice would be appreciated.

Regards, John


  • mike95490mike95490 Solar Expert Posts: 8,207 ✭✭✭✭✭
    Re: Batteries - can I add different sizes

    You don't want the expanded bank to exceed your capacity to recharge it. The rule of thumb I've heard is 1 watt of solar for each amp of battery capacity.
    - your mileage may vary -

    In other words, you an make a huge battery bank that will run a microwave, and toaster oven, but it will take 2 weeks of no usage to recharge it. In that 2 weeks, the batteries sit low, and begin to sulphate. Then you have a large array of dead batteries after a year of this treatment. If you get more batteries, you need more charger (solar or other)
    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: ,

  • System2System2 Posts: 6,290 admin
    Re: Batteries - can I add different sizes

    That's interesting an interesting rule of thumb. Basically, I go there during somewhat infrequently. Usually I have two weeks to re-charge. Sometimes three or four. So I'm not trying to size this for full time use. Between visits I'm finding the bank charged but have on a few ocassions (like a week of solid snow) drawn down on the bank to 30%. I guess I could use a small generator to give it a boost but I'd rather have a bit of spare capacity. My question though was centered on batteries of different AMP capacities and whether one can mix different sizes without risk of damage. I suspect I could damage the smaller capacity batteries but hoped someone could validate my belief.
  • icarusicarus Solar Expert Posts: 5,117 ✭✭✭✭
    Re: Batteries - can I add different sizes

    You won't damage the batteries per se, but you won't be gaining much advantage either. The larger capacity battery will draw down to the capacity of the smaller one.

    You might look into reducing the loads, increasing the solar charge capacity. The consensus is that drawing down batteries more than 50% significantly reduces their life. (20% draw down is even better)

    I know the 1% recharge rule of thumb. In my mind the more useful equation is how much you draw down during a cycle, and how much you can recharge during that cycle. If you can always keep a full charge 1% doesn't matter that much.

    I have lived with 110 watts of panel(7 amps) with ~1600 amp/hours of battery for ten years. (Way under the 1% rule) My batteries have performed well and held up well. I have just added another 110 watts of panel, just to keep ahead of my demand.

    Good luck,

  • westbranchwestbranch Solar Expert Posts: 5,135 ✭✭✭✭
    Re: Batteries - can I add different sizes

    To add to Tony's comment, if your batterries are fully charged when you start to use them and have small draw downs that the PVs can replace quickly, you have it beat.. the problem is that PV power/battery storage is best suited to constant use, where in / out is balanced fairly closely. Slow recharge is your "devil in the details' as it is the 'breeding ground' for sulphation in your batteries, a slow and ultimate death...

    I am in a similar situation in having a periodically used cabin, lots of PV vs use but too low wrt the battery capacity.

    KID #51B  4s 140W to 24V 900Ah C&D AGM
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  • System2System2 Posts: 6,290 admin
    Re: Batteries - can I add different sizes

    Thanks for the advice. I guess I am indeed using a bit too much juice. I recently added a Norcold 460 which is making a difference with the usage. I'll follow your advice and add more panel capacity.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Batteries - can I add different sizes
    The rule of thumb I've heard is 1 watt of solar for each (Ah) of battery capacity.
    Just to clarify: That rule of thumb (which, BTW, I learned from NAWS ;) ), applies to a 12 V battery bank. It somewhat correlates to the "5% rule" we sometimes discuss. For example, the STC Imp charge current spec from a 500 W STC array might be ~29 A, or 5.8% of a 12 V x 500 Ah battery bank's capacity.

    For other system voltages (i.e., 24 V, 48 V), the "rule" needs to be adjusted accordingly. And, as always, unusual load and/or environmental conditions need to be considered.

    HTH, and Happy New Year, all!
    Jim / crewzer
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