[masraum]

My contribution is that a car battery is 6 cells at 2.1V per cell, so a fully charged battery should be 12.6V. If you use a fast charge, then yes, you can over charge, but it'll settle down to normal. When you charge slow and low is better than high and fast. It's not unusual for a high/fast charge to be considered "surface" and then drop back down to where it really is in a fairly short period.

Still, 12.4 isn't technically fully charged. Other than that, I'm not sure what you're dealing with. I wouldn't expect a battery sitting to discharge quickly. Batteries sit on a rack at an auto parts store while fully charged, sometimes for quite a while and are usually still fully charged when you buy them.