Greetings folks- chasing a serious electrical gremlin on my 535. This has been going on for a couple of weeks lately. Occurred in a similar fashion about 6 moths ago- went to dealer and of course they found nothing.

Symptoms- battery low, intermittent no start. If you walk away, or keep hitting the start button, it will eventually fire the starter,

Replaced alternator- Bosch reman- was not charging the battery.

Still had no start issue. Unplugged the IBS- no change. Also did not change the alternator output.

Fully charged and load tested battery- battery is good. Battery is 6 months old and properly registered.

Wired in an analog VDO voltmeter into the cigarette lighter plug to watch system operation.

No codes in system other than water pump not reaching commanded speed 2E81.

Drove 250 mile round trip today while watching the voltmeter. Immediately after engine start, system voltage 13.5-13.8. Remained here for 75 miles or so. After that, system voltage dropped slowly to just above 12v at the 125th mile. Parked car, was at location for 2 hours or so. Engine started fine. System voltage back to 13.5-13.8. Remained there for 100 miles, then began the same drop. Lowest this time was 12.5 or so. Stopped car in parking lot to take phone call, engine idling. Voltage climbed back to 13.5-13.8 while sitting still. AC was active and radio on. No lights or other accessories were operational throughout the trip. THIS SEEMS LIKE ABNORMAL OPERATION.

Having done some research, it appears as though the CAN node for the IBS also contains the water pump and alternator. Wondering if the water pump issue is related electrically.

In addition, I would like input from the experts as to where to proceed next. I have access to DIS/TIS and INPA.

Thanks in advance!