Battery Question - The Unofficial BMW M5 Messageboard (m5board.com)

bdeutsch · 6th April 2008, 22:56

I have a 2003 that has approximately 14,400 miles on it.

Obviously I do not drive it much, but of late due to work travel, sometimes weeks (up to 3) elapse before I get a chance to start it. In the last month, after prolonged period of no use, again 2 to 3 weeks, the car's battery does not have enough juice to turn the engine over. The battery in question is one that came with the car when new.

When this happens, I then connect the battery, via the connections under the hood, to my trusted Sears Battery Charger. In about 5-hours, the charge is brought up to about 12.75 volts and life is good again.

Is it normal for a healthy battery to drain without any noticeable load while car is dormant? Based on the voltage reading, there does not seem to be a problem with the battery itself.

As always, appreciate any insight fellow owners can pass this way.

Thanks,

Bob

jclyman · 7th April 2008, 00:18

Yes, there is a drain ... components that need power all the time are still on ... When the battery is new, it has enough charge for months without starting. Your battery is getting near worn out.

I wouldn't use a regular charger, although many here do. I would hook up a trickle charger to keep the battery up ... a good $ 40 spent. Mine is on one now. If I know I am not going anywhere for a week or longer, on it goes.

I have the original battery and it is fine.

Also, BMWs have a threshold below which everything that uses power when the car is OFF is shut off to protect the sensitive electronics. If your battery won't turn the engine over, SLOW CHARGE it for an hour or so, and then try. The threshold voltage is set to avoid "shocking" delicate electronic when starting the car.

El Diablo · 7th April 2008, 01:16

Despite the low mileage, the age of the battery along with many factors such as temperature and suphatation build-up in the plates will cause premature failure. I just recently replaced mine after 75 months. I will be buying a battery tender or trickle charger very soon because sometimes I do not drive her for up to 2 weeks due to travel. Not re-charging a lead sulfide battery will accelerate the formation of lead sulfide. Hope this helps.
-Roberto