This has been going on for a while. The battery seems to be draining when electrical components are off overnight. Then it became worse during driving. All of a sudden all power would be lost, including transmission brakes etc. I would turn off the car and it wont start. I wait an hour, then it starts and everything is fine again.
The first thing to do with a draw is to check and make sure nothing is on that can be turned on by a human. Some common sources are interior lights, exterior lights, after market stereo systems or other after market systems. Followed by those items are glove box and trunk lights. These lights are turned off by switches that can fail. If none of these items reveal problems, the next thing to do is to test for a draw. This is done with an amp meter. In most cases a multimeter will be used. When doing this, the amp meter will need to be placed inline between the negative battery post and the negative battery cable. Do not turn the key on while the amp meter is connected inline. Few amp meters can handle more than ten amps. If you turn the key on, you will damage the multimeter or hopefully just blow the internal fuse in the meter. Either way, you won't be able to complete this test. Once you have an amp meter inline on the negative side, you will want to wait for up to an hour to be sure any systems that normally stay on have time to time out and turn off. Some cars require a wait of as much as several hours. Once you are certain there are not any modules on that should not be on, whatever the amp meter shows is the draw amount. It is normal for your car to have no more than a 50mA draw. If you have something over this by as much as 100mA, I would not see this as a problem. If you find you have a draw, you will need to remove fuses one at a time until you see the draw on the meter drop. At this point you will need a wiring diagram to identify what on that fused circuit is the problem.
The above is how we track down a draw, but I don't believe that is your problem, or you have more than one problem. If you are losing power while driving, this is not a drain, it is more likely there is a main power problem. The first place to check for such a problem is at the battery. Check all fusible links and battery connections. With the complexity of your 750li, it can be very challenging to track down other potential sources of such a problem. There are many relays that power the many modules. I would begin by identifying any modules that are still powered. If there are any lights of the dash, these will be clues. This will aid in the process of elimination to track down the source of the complete power loss.
The fact that the car won't start immediately and eventually starts is of no surprise. Intermittent conditions such as this are very common with electricity. I definitely recommend you have some one with BMW experience diagnose your car due its very complicated nature. I recommend the following inspection to help you out. Electrical components Inspection Send the technician a note about the exact symptoms so the technician can come as prepared as possible to your appointment.
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