BMW Z3 battery drains
Z3 battery drains in winter: is a 30 mA current draw too much?
About 30 mA is usually a normal draw while the car is parked. Disconnecting the negative terminal prevents the battery from draining in winter. Deep discharge damages a lead-acid battery, so you can also take the battery out for the winter and charge it a couple of times.
Can a deeply discharged Z3 battery be brought back into use?
You can try to revive a deeply discharged battery with the recovery function of a smart charger, for example the reconditioning program of a CTEK charger. The battery's condition should be measured with a load test, because a badly damaged battery has to be replaced.
Why does the power stay on in the Z3 and the car does not switch off when the key is out?
A known fault is a defective diode in the instrument cluster, which causes a short circuit and keeps the power on. This drains the battery. The fix is to replace the instrument cluster. The fault has been seen in the Z3 Coupe 2.8.