We just installed six 12 volt lithium batteries 200(ah) in series (24v) and then in parallel. There seems to be a problem during the charging phase as only one battery in each of the series is charging to 14.8 volts or higher while the other battery in the series is slowly charging at 13.5 with none getting to the 14 volt stage. Do we need a battery balancer? Also two of the batteries in the series that is getting the higher voltage is connected to the positive while the third one is connected to the negative.