My 735i was running out of power. Maybe a constant drain on the battery, maybe a bad generator, or maybe something else... After a long drive the battery seemed to come to life, but within a few days it would die completely. Using a charger produced the same result: it went dead again in a few days.
The easiest way out was to buy a new batteri and hope for the best, so I did that. Unfortunately, that was not it.
I removed the ground wire from the batteri negative terminal and put an amp-meter in between the two, in a position were the display was visible from outside the car. That way I should be able to remove the fuses one by one and keep an eye on the power drain.
The moment I turned on the amp-meter, I knew something was wrong. A drain of 0.6 amps does not sound healthy to me, especially since I checked the drain a few months ago and it was almost at zero back then.
I opened the hood and the drain increased. Don't know exactly why, but I do know that there's a switch or two that reacts when the hood is opened, so I wanted to tape them down while checking the fuses. I pushed down the one close to the passenger side firewall without much change in powerdrain, but when I pushed the one located on the hood (don't know if that one is original, or part of an old alarm system), the hood light was turned on! When I put my hand on the glas, it felt very warm, but it looks like it has been lit for a long time now, so no wonder!
A few weeks earlier I removed the glas to check why there was no light in the engine compartment. Well, no bulb in there! I put one in right away, but it didn't work. It is pretty cold in Denmark this time of year, so I didn't bother to do anything about it and let the bulb stay in. This means that I have been driving around with the hood light on for some weeks, 24 hours a day, due to a bad wiring! No wonder the battery had problems keeping up! :-)