This may be caused by multiple parallel strings of batteries in a bank, which often result in charge imbalance. U.S. Battery does not recommended that a system exceed 3 strings of batteries connected in parallel. Charge voltage may be too low. Verify they meet U.S. Battery recommended charging parameters for Flooded batteries. An increase in Absorption charge time may be necessary. Increase in 15 to 30 minute increments as necessary. If specific gravity readings vary by >0.050 points then it is safe to assume the battery has a bad cell. Charge Current = C/10 Amperes Charge Voltage = 2.583 Volts per Cell Charge Time = 10 Hours Battery temperature adjustment: reduce the voltage by 0.028 Volts per Cell for every 10 degrees (F) above 80 degrees (F), increase by the same amount for temperatures below 80 degrees (F). Download the full detailed charging instructions