I have two 12 volt Interstate deep cycle batteries connected in parallel in my cruiser 5er. The are origional equipment and about 18 months old. I winterized the 5er and brought the batteries into the garage, checked the water level, all good, and put each on a automotive 12 volt charger which shows the charging amps. The first battery showed 15 amps at the start and over a two day period went down below zero. While charging the voltage read 15 plus volts. Ater 2 days off charge the voltage read 12.75 volts.
When charging the second battery the amps only started off around 10 and went down below zero within hours. The voltage during charging also read 15 plus volts. Even though the amps dropped so quickly I left the charger on for 2 days, as the first. Now 2 days later, off the charger, the voltage is only 10.5 volts.
Any thoughts about why one battery does not hold a charge? Also when you have 2 batteries in the 5er does anyone "rotate" or switch out which one is first in line.