This is my first post, so I'm hoping there's someone out there who can help me here. I have a 2001 BMW 325ti Compact that is losing battery power. The other day I noticed that while I was driving I couldn't run anything electrical from the cigarette lighter. I thought this was strange but didn't think any more of it until I tried to start the car after a shopping trip and found myself stuck in the car park with a completely flat battery that just made a clicking noise when I turned the ignition key. Fortunately I was right by a car accessories shop and was able to buy a brand new battery. The following day, to my horror I noticed the exact same problem after only having driven the car for a few hours, then the sme again that evening, a completely flat battery clicking when I turned the ignition key. The manual says if the alternator was not delivering enough charge, the red battery warning light would come on, but in both cases this did not happen. I therefore don't want to go running out buying and fitting another alternator if there's a chance it could be something else (a simple fuse or something), since it would seem there was obviously nothing wrong with the battery I've already replaced. Any suggestions guys ? Please help me. Thanks.

Have the alternator load tested. Many auto parts houses will do that for free.