About 2 months ago my wife was sitting with the engine off listening to the radio waiting to pick up our son after school. After about 10 minutes a yellow battery light flashed on. She turned off the radio and the car started fine. Didnt happen again until about a week and a half ago. That evening I put my volt meter to the battery and got a reading of 12.2 volts. I then stated the engine and at idle I was getting a reading of 14.5 volts which I believe tells me that the alternator is working fine. The car has started fine since and my wife hasnt been sitting with the engine off and listening to the radio. This past weekend we didnt drive the car at all, when I checked the voltage on friday night it read 12.4 about 2 hours after it was driven. I checked the voltage again saturday afternoon and got a reading off 12.2. Sunday afternoon I checked it again and got a reading of 12.1. I called my local independant service guy and told him my findings to see what the cost would be to replace the battery but when I told him the readings, he said that he thought it might be the charging system that had a problem not the battery. But since I'm seeing the battery losing voltage slowly I believe the battery is getting weak. I purchased the car in Sept of 2008 used but I would believe the battery is the original. I thought I would get some expert opinions from the club members before I commit to the new battery. So far the car has been starting just fine but my wife drives the car and dont want her to be stranded one of these days. Thanks for any input Robert Weston
You could try purchasing a battery tender. My local dealer states if the vehicle sits for longer than a day it would benefit from a battery tender. Short trips can also effect the battery as well.
Thanks for the reply, since I posted I have e-mailed Mike Miller and he said because my wife only runs very short daily trips and the e90 has lots of draining computer stuff the battery will look weak and he also suggested a battery tender. I dont have a "battery tender " brand but I do have a Schumaker brand battery maintainer which is the same, it fully charges and then adds as needed. I also just need to swap cars with my wife every so often since I drive 24 miles one way to work and give it a good run for a week or two every now and then and I havent done that for quite awhile.
Interstate make a good heavy duty battery tender. Prior to the Interstate I had on made by BMW however it finally failed. I only plug mine in over the weekends.
A car battery should be fully charged about every 30 days to prevent sulfation. If you only drive short distances, your battery will not get fully charged and an occasional top off will extend battery life. The average car battery life is about 4 years, if this is your original battery you are way ahead of the game. You can't accurately test the battery capability by checking the no-load battery voltage. Your service station can easily do a load test.
I had similar symptoms on my 06 and it was a bad cell in the battery. When it dies it dies, I had to have it towed to the dealer due to it not holding a charge at all. It is 6 years old so, so it might be time for a new one. I had similar problems to what you had and it was the battery.
