Last year I replaced my two trolling batteries with the biggest Interstate makes. Shortly after that one of the batteries had reversed polarity. The guys at the store said it couldn't happen but it was showing a negative voltage. They replaced the battery with a new one and now I am still having problems. If I use them a little to troll, one may read about 11 volts and the other about 3 or 4 volts. When I plug in the charger and charge them, the one that holds the better charge is showing close to 13 volts and the other like 11.5 volts. Do I have another bad battery or could it be something with the charger? Both went dead last year within a month of each other and then all these issues.