Originally Posted by drynoc
They are identical batteries bought from West Marine three years ago. And right, I meant to say that they are in parallel. But I tried to charge them both at the same time with the switch set to both, and it didn't work. The charger said "full" after a while, but I was able to hook it up to the other battery and it charged that one separately.
The easiest way to make sure you are hooked up correctly is to measure the voltage on both batteries as you are charging and switched to "both". You should measure 13.3 to 14.5 volts, depending on the battery charger and battery type. If you measure less then 13 volts you have something hooked up wrong. If you are using a "smart" charger that calculates amp-hours out vs. amp-hours in you will get an incorrect "full" indication.
Lead acid batteries take 12 to 18 hours to charge to their full capacity after they reach their float voltage level. As long as your charger is maintaining the voltage at the correct level you will charge your batteries. If the voltage drops after the charger indicates full, you have a problem. There are so many different types of chargers out their it is difficult to know what it does without a full specification or operating characteristic.