Just to expand on what Allan posted, lead Acid batteries, ie, car batteries, have a peak voltage at full charge of 13.6 VDc. I believe Gel Cells are pretty similar. The require a 'battery eliminator' type source that is capable of putting out a 13.6 volt charge voltage with enough current to charge the battery in a reasonable time. If you have, for instance, a 20 amp hour battery, well, you need to charge that at one amp for 20 hours... well, given losses, that will actually be around 24 hours at one amp. Do the math.. charge current at voltage for the total length of time from a fully discharged battery to fully charge it. Car type battery chargers work just fine, as do, as I said, battery eliminators available at Radio Shack for instance.

Dick.