What's running down the battery ??

The usual culprits - alarm system when left on for weeks at a time, glove box lights, underhood lights (don't remember if the 328 has any), and the courtesy lights inside the door. Other than that, I'd switch to an Optima battery. Let us know what you find.

 

Alarm/ Immobilisor? Clock?

 

An Optima battery wont solve the "problem", maybe just give you more time before it is dead. The battery will run down after a few months if not used. Mine did it in 8 weeks with a brand new Optima. I blame the clock, the alarm, and cold weather.

Same battery in my truck has gone four months and still started. Same weather, alarm and clock in the truck.

Disconnect the battery, trickle charger (lots of threads) or start it up every now and then (but look at the threads on how long to run the car to charge the battery, dry out the exhaust, circulate the fluids, etc.)

 

If you want to check the amp draw on the system there is a simple test.

After fully charging the battery, disconnect the positive terminal. Connect the negative lead from your volt/amp/ohm meter (set in the 1-3amp mode)to the battery and the positive lead to the disconnected end of your positive battery cable. If your reading is above .05 amps you have an excessive draw. If you have an excessive draw, the best course of action is to begin removing fuses one at a time and see if the draw goes away. If it does not go away after removing all the fuses, disconnect the main alternator lead and check the results. The alternator is a common point for discharge.

All cars have some draw. There is even a draw across the top of your battery. All batteries will eventually discharge completely. Six weeks does not seem excessive. Newer cars with multiple computers will run batteries down much faster due to the computer memory that must be maintained.

Hope this info is helpful

 

The battery on my 1989 328 GTS also runs down when the car is left unrun for a little over a week. One week hasn't been a problem, but 11 days is too long. In contrast, when the battery had been disconnected for 6 weeks, the car started right up after reconnection.

The prior owner was somewhat concerned about this, and either kept the trickle charger on, or disconnected the battery when not in use. Both he and his mechanic attributed this to the analog clock, although I've not heard any confirmation that this has been positively identifed as the source of current drain. I've also heard that a clock rebuild might help, but, as the clock keeps good time, I've chosen not to pursue this.

I've noticed trickle chargers on other 308/328s, so I assume battery drain is a common problem. The trickle charger is an easy solution, with the disconnect being a good alternative when the vehicle won't be run for extended periods, as long as you're willing to reset the clock, radio stations, etc.