Battery Bank Problems

ChiefaChiefa Registered Users Posts: 1
Installed a 1800 watt off the grid system about 8 months ago.   I have 3 X 4 - 6 volt golf cart batteries.  When I first installed them they would charge up to 28.6 volts which would trigger the absorb charge and then the float.   I did let the system sit unattended for about 60 day with everything off.  It seem now that the battery bank will not charge above 26.4 volts.   Is this a problem?   I am running midnight charge controller.    Do I have a bad battery in the bank?  


  • VicVic Solar Expert Posts: 2,933 ✭✭✭✭
    edited September 2016 #2

    Hello Chiefa,   Welcome to the Forum.

    Lead Acid batteries will discharge,  when just sitting,   with no loads,   and no charge sources  --  Self Discharge,  this is called.   Elevated temperatures,  generally,  increase this rate of self-discharge.

    In general  batteries that are in good condition will discharge a few percent (of remaining Capacity)  per week,  at a nominal average temperature of about 25 C (77 F)   Assuming that your Golf Cart batteries are Flooded (with  easily removable caps on each battery's cell),  measuring the SG of each battery cell with your good quality Hydrometer,  will tell you weather there is an imbalance in the State Of Charge (SOC) on any cell,  compared to the others in the battery bank.

    Parallel battery strings,  can cause problems,  as small differences in the average SOC of each string can cause large differences in the amount of charge that individual battery strings receive.   Strings that receive less charge,  will tend to discharge strings that have better levels of charge.

    With three strings of batteries,  you will WANT to have a DC Clamp Ammeter,  that will allow you to monitor the charge (and discharge) currents of each string.

    It is quite possible,  that you will just need to allow some additional days of good sun,  to fully charge your batteries.

    Using a Remote Temperature Sensor (RTS) for each charge source will help you assure that the battery bank is receiving as much charge as is possible.

    You could also need to try to do an EQ,  with the battery as fully-charged as you can achieve,  especially,  if there is a fairly variation in the SG readings between all cells in the battery bank.

    The Classic CC includes the BTS.  For the KID,  this is an option.  It IS important to use the BTS on almost all systems,   which are not in a carefully Temperature-Controlled environment.

    Also,  if you ARE using the BTS on the Charge Controller,  be aware,  that warm/hot batteries will need a lower Absorb,  Float,  and EQ voltages.   And,  the BTS will indicate to the CC just what IS the correct voltage,  for the measured battery temperature.  Perhaps this could be part of what you are seeing in the Absorb voltage.  Attach the BTS to a battery in the center of the battery bank.  Place it about half-way down the side of the battery,   and cover it with some form of rigid foam insulation  --  foamed Polystyrene (called Styrofoam (r) in the USA) is a good material for this.  Use a rectangle of foam,  about 25,  to 50 MM thick,  and tape or strap this foam to the battery.  Place all BTSes on this one battery,  under the foam.

    The foam insulation is not absolutely necessary,  but will help reduce the effects of variation in the ambient temperatures on the reading of the BTS.

    FWIW,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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