I have just started having some difficulty topping off my battery. I use a battery tender (BMW branded but similar to the good ones we all know and love). It is connected directly to the battery posts. In months past it would top off the battery (indicator light on the tender green) in about 12 hours.
This Spring I haven't used the tender much because I drive the car about once a week. There is no short drive where I live - 20 min is the shortest, 30 to work, and speeds/revs are pretty good
Last week I went to move the car (88 911 with 87k miles) and it wouldn't turn over (solenoid clicks, lights, etc all worked). This after just over 2 weeks of not being driven and not on the tender. So I connected the tender and it took nearly 3 full days to charge. Ok, it was pretty dead so although it seemed about 24 hrs long, I can roll with it.
Once charged fully, I drove it to work. No issues, no warning lights, everything worked as much as before (nothing was magically fixed, like say the windshield squirters). Actually I was careful not to use fans, radio, or anything else electrical and didn't need my headlights. So I thought once home it would top off pretty quickly. Immediately put it on the tender, 36 hours later it is still red - not fully charged.
Ambients are high and the garage is particularly warm (an insulated garage in summer isn't all that).
The battery does eventually top off, so I assume its cells are all good. The key is out and no accessories are on (except those I can't control, I suppose).
So either something is causing the battery to drain fast, or the alternator/tender are not able to bring it back as quickly as they should.
Would appreciate thoughts on where to start troubleshooting this one. (sorry for the long post, but details seem to help narrow the possibilities).
Thanks!