I am having a problem such that I have to jump the car, because apparently the battery is being drained.
I did not have this problem before. But I did not drive the car for about a month, and since then I am having to jump it to start.
I have driven it weekly now, and drove it for about an hour the last time. Still no help.
If I turn it off and restart within a few hours, it is fine. But if I try to restart the next day, same problem.
What could be the issue? Do I need a new battery? Can the battery be professionally recharged?
Please help if you have had this issue, or if you have any suggestions.
I would appreciate any help you can provide.

There are two batteries - one on each side of the trunk. They need to be charged fully and then load tested individually to make sure they are still both holding a charge and not draining each other.

There are two batteries - one on each side of the trunk. They need to be charged fully and then load tested individually to make sure they are still both holding a charge and not draining each other.

...and if you can't charge one or both, you need to REPLACE them BOTH with the same type and size of battery. Again, charge new batteries individually outside of the vehicle before you install them.

As always, thanks for your help. Will do as suggested. If the batteries need to be changed, should I get BMW batteries froma dealer? Or will any battery do?

INTERSTATE MTP-93 or the equivalent BOSCH battery. Do not go to the dealer for the OEM battery - you can do much better (price and performance wise) on your own.

when the car sat did the batterys drain completely?
if they did thats not good at all, i would do a overnight or weekend charge.
if you can pop off the tops on the batteries first to check the electrolite level
if there low fill them to the level sugested close them back up then overnight charge

With the fine and rare car you have '97, make a small investment in a Ctek-7000 charger (look for a distributor out of KY on Ebay for best price and service) I would agree with TerryY about your current batterries, but the Ctek will recondition and charge them properly to see once and for all. At the least you will have a high quality charger/maintainer that you'll probably never replace even if you have to replace the batterries (likely). Steffen has the Interstates, I have the Bosch, both are great. When he said same size and type he meant of whatever you choose to get together, not of OEM. OEMs are smaller in size, more expensive, and only have about 550 CCA compared to 850 CCA of aftermarket, but they have the cool propeller logo on'm and would be necessary for a concours event. Your car will be very happy with either choice above. If you get the larger batteries you will notice there is an additional mounting hole for the rearmost bracket rod. Please report back if that hole is threaded or not....... there has been discussion and evidence that some cars have it threaded and some don't....neither of mine do but no big deal if not, those big 'ol 850 CCAs will sit there just fine. Make sure to reattach the battery vent tubes!

I don't know if this has been mentioned here on BF, but a superior way to keep the batteries conditioned is to add disconnects, so that they can be hooked up to float chargers separately. That way one battery provides the dormant vehicle power while floated, and the other battery is floated disconnected. Harbor Freight carries disconnects that don't look bad in an 8er.

Fwiw, Randy - my ''92 came with a battery tray threaded for the larger battery. Not sure of the '91.

my 91 parts car, as well as Esmerelda (93) have the second set of threaed holes...