Unofficialbmw.com

[ErikNet]

I have taken my 1990 BMW 735il to several mechanics and Service Centers and no one can seem to tell me why my battery dies every week. I have replaced the battery and tested the circuits. I have no idea what is draining it.

Does anyone have the same problem or have a solution? I appreciate any help. Thanks!

[edjack]

How did you "test the circuits"? Have you measured the current drain with the key off? It should be in the 100s of milliamps, at most. If more, you'll need to start pulling fuses to find the culprit.

BTW, with the engine running, you should get 13.5 to 14 V at the battery terminals. If lower, the alternator is not putting out enough power to charge the battery.

Do you drive the car every day, at least 20 miles one way? If not, it may just be a case of under-use. There are several systems in the car that draw current when the car just sets. If you don't drive it much, the battery doesn't get a chance to be charged.

Also, check the integrity of the ground wire from the alternator case to the engine block.

[ErikNet]

Thanks for your reply. I conducted a parasitic drain test to no avail. What I know believe to be the culprit is the battery.


BMWs come with lead-acid batteries, but most maintenance-free batteries (which I think was installed in my car, I need to check this weekend) are lead-calcium batteries. Interstate's new Pinnacle battery line uses 'silver calcium" chemistry. These battery types differ in their characteristics and operation. Maintenance-free batteries require a higher charging rate than lead-acid batteries, but BMW voltage regulators are calibrated at a lower charging rate than the maintenance-free batteries require. Another difference between the two battery types is that lead-acid batteries have a different internal resistance than maintenance-free batteries. This internal electrical resistance difference causes the voltage regulator to set the charging rate even lower than it would if a lead-acid battery was installed. In effect, it "fools" the voltage regulator into thinking that the battery is in a higher state of charge than it really is. What occurs is that the maintenance-free's need for a higher voltage is not met, but the battery's different resistance causes the charging voltage rate to be lower resulting in a perpetually undercharged battery condition. Recharging a lead-acid battery requires around 14 volts.

So you are right to check the voltage, I do not use the car everyday, however, every weekend for a good amount of time.

I will definitely look into your suggestions and I thank you for your feedback. You sound extreemely knowledgable.

Thanks again.
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[edjack]

I have used Delco Freedom batteries in a number of different cars in my stable, including my old E12, with no problem. I believe that this Delco model is lead-calcium, and the old Bimmer kept the battery properly charged over the lifetime of the battery.