I have the BMW charging system. I charged the battery to 'full' over the weekend, and decided to hook it up last night (wednesday). The green light wasn't flashing immediately, which supposedly means the battery is under 80% charged. Left it charging overnight and it was 'fully charged' the next morning. I'm not sure how accurate this system is but it seems that it lost significant charge in the space of three days.
The car did seem to start a little better.
The battery is 2 years old (original battery) - to test it should I just be looking at it's resting voltage?
thanks for the help.