I've avoided a lot of problems with my E39 and now with my E92 by regularly charging the battery. I've got this charger from Northern:
http://www.northerntool.com/shop/too...2299_200442299
I start charging at 12A and it ramps the charge current down automatically as the battery voltage rises. Then it goes into "desulfation mode" once the charge current goes below 0.1 A. That's when it really reconditions the battery.
This avoid electrical problems due to low battery voltage (on the E39 the DSC stopped working for no reason - turned out to be low battery voltage). Now I've got my E92 with a nearly 4 year old battery that is not covered by CPO. So by charging and desulphating it I can extend it's life.
Our cars consume so much electrical power than you need to charge the battery with a relatively powerful charger (not a little trickle charger) even if you drive it regularly.
I know this sucks for apartment dwellers. I had thought about the idea of a DC-DC battery charger. But no one makes this AFAIK. It would work if you could power it from an 18-24V battery.