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Date: Mon, 15 Mar 1999 18:54:20 EST
Subject: [E36M3] Actual Content - Tech tip. Long but YMMV :)

I will try to retype this but the phones are ringing so if it reads funny, to bad. This is a little something to go with the underdrive pulley thread and is reprinted without permission (sue me) from the Allegheny Chapter Jan, 99 newsletter and was written by Bill Ballon.

Ever wonder why BMW's don't come equipped with maintenance free batteries?

BMW's come with lead acid batteries, but most maintenance free batteries 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 2 battery types is that lead acid batteries have a different internal resistance than maintenance free batteries. This internal electricalresistance causes the 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 batteries 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.

Many maintenance free batteries need close to 15 volts to fully recharge (a Delco Dura-Power for example). A typical BMW charging system ranges from 13.8 to 14.5 volts. I have personally seen up to a .5 volt drop caused by a maintenance free battery installed in a BMW. Dropping 1/2 volt of charging rate causes the charging voltage to fall out of the acceptable range if that particular car is marginal (editors note: underdrive pulley?) to begin with.

One case in particular stands out in my mind. A BMW owner was mystified that his battery would discharge after sitting in the garage for only a few days. The combination of maintenance free battery and a typically low charging rate setting for that model, caused the battery to never attain a full charge level. In normal driving the battery would slowly discharge, so that when he garaged his car it already had a partially discharged battery. The normal radio memory, accessory draw and computer memory would finish draining what little charge that was still left in the battery. When he went to start the car, the battery would be stone cold dead. Testing for parasitic drain would always come up with no drain, and the charging rate was a bit low, but changing the voltage regulator or testing the alternator and circuits would yield no conclusive fault. Installing a new lead acid battery cured his problem. It is amazing to me that many of the chain stores' application guides continue to recommend a maintenance free battery for BMW applications, even when it is clear that they aren't correct for use in BMWs. Be sure to specify the correct lead acid battery fitment for your BMW.

Jim Powell typing from here on:

Don't assume ERT or anyone else knows what they're doing. Sears ought to know what battery to sell. So should Interstate. But they don't. Long term exposure to voltage sags stresses electronics and shortens its life. I can't be the only Electrical Engineer on the list. If you install an underdrive pulley on your street car you are taking a risk. Do what you want, its your money.

Jim Powell

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