batteries in parallel

I had been working with 24 batteries giving me 48 volts. My control charger was floating them at 54 volts (2.25 v per battery). I decided to add another set of batteries which I hooked up in parallel. The first set is rated at 168 amps for 8 hours, while the second set is rated at 87 amps for 8 hours. Since they have been hooked up, which is about 8 months now, I haven't been able to get any of the batteries (either set) to register more than 2.05 volts at any given time. The charge controller is still set at a float rate of 54.0 volts. The charge controller is an Outback 60. There is nothing in the manual that says anything about parallel battery strings.
Anyone know why I am not getting the batteries to float at a proper level. Could it be the time that is set for the flost to take place, or does the charge controller sense something different when there are battery banks hooked up in parallel???


  • PhotowhitPhotowhit Solar Expert Posts: 5,805 ✭✭✭✭✭
    Re: batteries in parallel


    Never a good idea to mix old and new batteries or different sizes/capacities of batteries.

    I'd remove the new set, sounds like it's drawing down the old set for some reason.

    There is good battery information here;
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: batteries in parallel

    Were the batteries you added new batteries?

    How much solar panel do you have?
  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin
    Re: batteries in parallel

    You may have a shorted cell? Put a healthy charging current on your bank and use a Digital Volt meter and check the voltage of every cell... See if one is much lower than the rest.

    You don't want to run the cells at that low of voltage for long periods... That is just not enough voltage to properly charge them.

    And, double check the accuracy of the meter--it would not be the first time a meter "went south" and started giving strange readings.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: batteries in parallel

    Thanks for the help.

    All my batteries are ones I got from closed down telecommunication installations. The first set is about 15 years old and the second set is about 11 years old.

    There are 8- 200 watt panels charging the system. Being winter, now, and this system being in a remote area, the temperature can get to -20degrees Celcius and we are only getting 6 hours of sun at this time of year. We only get out for the weekends, so it is hard to have the temperature in the battery room consistent as the structure is heated with only a woodstove, and only when we are there.
    Having checked all the batteries, I found that the new set are all equal in charge, but one battery in the old set measures about .05 less than the others. Eg when they are at 2.05 this one is at 2.01
    The first set, has worked pretty good even with the weak cell, and measured 2.25 when the float finished. This has been consistent for the four years I have had them. It has only been since I added the parallel string that I haven't been able to achieve these numbers.
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: batteries in parallel

    You might think about putting the batteries underground (root cellar) that, my help with the battery chemistry. Make sure you have a vent for the out gassing. -20C can be hard on batteries.

    That weak cell may have gone south. I think you have a bad cell or two in the new bank.
  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin
    Re: batteries in parallel


    Did you measure the battery voltages under charging conditions?

    If so--it is acting more like the bank is simply undercharged (and/or too much load for too little charging).

    I am not sure you have one bad cell (I would expect one cell to be dramatically lower or higher than the rest if it was shorted or open and you were pumping a high charge current into the bank).

    What is the current flow into the bank (and are you sure it is a real value)?

    What are the daily loads (Amp*Hours or Watt*Hours)?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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