We have had the 2008 BMW 528i 2 1/2 years. After two years we started getting a dead battery with the car sitting

Hello,

This is something that is more common than not. Energy problems on BMW's can be a difficult thing to diagnose.

The first thing that should be checked is to see if the vehicle has a waking problem or a draw. To do this the dealer should use an oscilloscope in writer mode and monitor the vehicle over an extended period. I have found that even when you do not have a big drain that can be identified you can still have a "waking issue". A waking issue is when a module "wakes" up and causes the vehicle to have a quick amp spike and then it shuts down again. With progressive wake ups this can cause the battery to drain and it will not show as a draw in the energy diagnosis. This can only be checked using writer mode over an extended period of time. I like to hook this up overnight and then check the wake ups in the morning. This way it runs over a 12 hour period.

It might be worth using another dealer as it might be that the individual working on your problem is not used to this test. You can also ask to get the foreman involved as this test takes some know how of how to look for the amperage spikes.

The most common cause of the spikes has been the telephone communication module but it could be caused by other modules as well. Once a wake up is identified then it is trial and error by disconnecting modules and moving the amp clamp around to try and narrow down the culprit.

It also does sound like your driving could also be playing a role. Your BMW uses a lot of energy when you start the vehicle, then after a short drive it takes an hour for it to assume sleep mode, and then when you drive home it does the same. This can lead to a battery being depleted over time unless a long drive is performed. The best course of action is to use a battery charger that you can attach to the jump terminals under the hood. I would recommend and automatic charger that will maintain the battery and shut off once fully charged. If you do a full charge at 10 amps once or twice a week this should prevent the reset of the 30g relay which occurs once the battery voltage drops to the 50% threshold. The problem you have actually occurs when the battery is at a state of charge of around 50%.

I would be glad to help you further if you have more questions. Please let me know and please accept. Thank you and I hope this information is helpful.

-Roger