My 1986 BMW 635 CSi seems to be eating its batteries. I’ve lost track of how many I’ve installed from my stash of used batteries. It’s not clear to me whether these are all coincidentally bad or if the car has some sort of drain.

This morning, the battery was flat again.  My contractor and I towed the car (which also has transmission-related problems) to our local BMW guru, and he is testing the battery.  He made a point that there’s more to testing battery quality than just checking the voltage.  I didn’t know that anything else could viably be tested, but he explained that they have a $380 battery tester that provides far greater insights than a mere voltage tester does.

He also mentioned that, unless it has 11 volts, the computer won’t even work.

I don’t know if the same holds true for the 3-series.