Extending Battery Bank

ktersiusktersius Registered Users Posts: 4
I have an existing battery bank of 800Ah and would like to extend this to 1600ah. The reason I want to extend this is up until now I've been using the system as a big UPS due to power failures/brownouts. Now I want to install a solar array to save some money and get partly off the grid. Up until now the load was always on the grid but if I start using solar the load will move over to the batteries and so I need to increase the size of my bank to carry us through the day.

Unfortunately due to my lack of knowledge, and size constraints, I didn't opt for larger batteries and it's too late to change that now. My current bank consists of 8x100Ah batteries in a series parallel setup which connects to a 24V inverter/charger. My batteries are about 9-10 months old and have been kept 100% float charged and hardly used during this time. I think if I have done 5 x 30% DOD cycles it might be too many.

I was thinking of getting 8 more batteries and inserting them in between the existing batteries, is this a viable solution? I'm going to try and get the exact model batteries again.

Also I'm not sure if my batteries are wired effectively?
I've had a look at smartguage and it seems my wiring matches option 2 which is OK but not optimal.

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  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Extending Battery Bank

    I personally do not like placing batteries (and cells) in parallel. It makes it very difficult to measure per cell/battery voltage to find bad cells.

    I would suggest:

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    I do not like large numbers of parallel strings (I suggest that 1-3 parallel strings is a better solution)--But you have what you have for now.

    Also, any battery bank with 1,600 AH is a lot of stored current. How much power are you planning on drawing (peak watts, average watts) and how much charging current are you planning on supplying (solar panels, AC battery charger, etc.)?

    We suggest 5 to 13% as a good range of charging current for solar panels... And 10%+ is recommended for an off grid/daily use system.

    10% current for a 1,600 AH battery bank is 160 Amps of charging current or around 6,000 Watts of solar panels and two 80 Amp MPPT charge controllers.

    And mixing old and new batteries--Not a great plan if the batteries are more than a year or more apart in age. Old and new batteries have somewhat different operating parameters (voltage, internal resistance, etc.). Maintenance/monitoring of a large number of parallel strings of batteries is a pain too... Especially if these are flooded cell and you have to check water levels/specific gravity once a month or so.

    You are heading down a dark road here--At some point I would recommend cutting your losses and redesigning the system (large AH 2 volt cells, going to a 48 volt battery bank, etc.). But it will not be cheap and can be a lot of work.

    My last suggestion is always my first... Measuring/understanding your loads, power requirements, and have you done as much conservation as you possibly can. In general, conservation (new energy efficient appliances, reduction in power use, turning stuff off when not needed, etc.) is almost always cheaper and easier (in the long run) vs just adding more batteries and panels to a solar power system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,362 ✭✭✭✭✭
    Re: Extending Battery Bank

    IF you are adding more solar, you are going to need more PV controller to keep the batteries up. So maybe plan on a 48V system, run what you have till it's tired, and then replace the inverter with a 48V model, and install the new battery bank.

    And you need to learn how series/parallel works
    does not give you a 24V battery @ 800ah, unless you have some special 24V batteries:confused:

    Your sketch shows you have created a 24V 400ah bank from the 8, 12V batteries you have. If you want to stick with 24V, you can double the amp hr by using 6v, 200ah golf cart batteries. Wire 4 in series, and as many parallel strings as you dare !
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Extending Battery Bank

    Start with the expected loads, and design a battery bank to handle those loads and thier duration. Then design a charge regimen for that battery bank.

    One of the biggest errorors people make is over sizing the battery bank and undoing the charge regimen.

    Welcome to the forums ns tell us more about your intentions,

  • ktersiusktersius Registered Users Posts: 4
    Re: Extending Battery Bank

    I've decided to rather upgrade my inverter to a 48V system. For now I'll reconfigure the batteries to have 4x12V and two parallel strings. Might run the batteries at 50% DOD and see it goes. Thanks for the advice.
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