Pulling this thread back to the top.
If you gang charge a few batteries, don't leave them connected after you have finished charging them. No two batteries ever have the same voltage or potential. If one battery shows 12.4 Volts and another battery shows 12.2 Volts, the 12.4 Volt battery will try to balance the voltage between the two batteries. The 12.2 Volt battery will leech from the other battery. If left this way for a while, both batteries will eventually decrease in voltage, till they aren't able to start anything.
Same thing with storing a battery on a cold, concrete floor or in the basement. The bottom of the battery is cold and the top of the battery will be warmer. The acid inside the battery will be cool at the bottom of the plates and warmer at the top. Now the plate will try to balance the voltage difference between the top and the bottom of the plate. This difference is measured in millivolts but it will deplete the battery over time.
Always store a battery on a bench or a shelf where temperatures are more consistant. Use a piece of wood to set the battery on, to isolate it from a metal shelf. The shelf acts like a heatsink.