In Reply to: Re: battery posted by rcbmw on May 30, 2002 at 13:10:32:
You seem to have found the correct part number for the E28 M5 battery. Additionally, there is a considerable amount of debate in regards to the use of maintenance free batteries which are typically lead calcium and or silver calcium types, such as Interstate's Pinnacle series. There is debate around the characteristics of these batteries as they require a high(er) charging rate and it is thought that BMWs charging systems are calibrated at lower levels. Some time ago when this was discussed at great length, we looked at the different types of batteries their internal resistance which is thought to be be different than lead acid batteries. It is also thought that 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. The average charging rate for these batteries is thought to be 15 volts and most BMW models have a charging rate between 13.8 and 14.5 volts. Jim Powell tells the story of a BMW owner who "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." So for what its worth, it may be worthwhile going to your local BMW Center to purchase the OEM Douglas lead acid battery. However when I recently replaced the battery in my M5 I had a very difficult time finding the Douglas type, and ultimately replaced the battery with an Interstate battery. It is thought that Douglas batteries for the E28 M5 can be found at Mercedes Benz dealerships. For now I'm happy with the Interstate battery.