'10 35D no start then no crank

[torqueisking]

Posting this in the hope that it helps someone as I have yet to find any a thread about it on the interwebs.

Put my X5 on a trickle charger during the summer and didn't drive it (use my 335D in the summer).

Went to start it, it turned over very slow then nothing. Tried again and no crank. Connected my 400A booster battery and it fired up. Drove around for a bit to charge it; total run time about 15 minutes. Try to start it 30 minutes later, slow crank no start then no crank again.

Dead battery was the first theory. Pulled the battery, had it tested and there is nothing wrong with it.

Read about how the starter motor windings can short causing the starter to draw excess current from the battery and crank slow if at all. Measured the voltage at the starter motor while trying to start the car and witnessed the voltage drop to zero. Out comes the starter motor. DC resistance of the motor was 150 milliohms. At 12V, that's an 80A current draw which is apparently typical. Spun the starter with my booster battery and found no problems. Back in it went.

Some of you are probably thinking IBS and so did I. Pulled the communications connector out of the IBS. Connected the booster battery with the vehicle battery and tried again. Slow crank no start then no crank again.

While the battery charger connected to the battery terminals, I measured 12.2V at the battery terminals. Went to the front of the vehicle and measured the voltage at the jumper terminal and the alternator ground. It was 12.0V. That difference seemed odd so I moved the ground to the chassis. 12.2V
Bingo.

Measured the resistance between the engine and chassis grounds: 50 ohms! Used a pair of jumper cables between the two grounds and the X5 started up.

The ground strap was extremely corroded. It's located behind the driver side front wheel under the frame behind a plastic panel. I could not easily get to the engine side of it so I installed my own ground strap.

You may wonder why my booster battery wouldn't start the X5. After finding the root cause, went back to the booster and measured the voltage. It was zero. Trying to start the motor without the battery contributing much eventually burnt out the the battery booster.