I don't even know how to describe this, but it's very similar to the last time this happened to me here. I'm starting a new thread as I never resolved the last one as the issue just "went away".
E99 essentially stock, ZF6, 4wd.
Symptoms:
I had driven it this morning for a short distance with one restart in town and then it sat in the driveway for about two hours. Bright sunny day, >60°F.
BT ELM27 adapter cannot talk to the ECU. The first real clue I have, and the only thing that's different from the last time.
All fuses are good.
Batteries fully charged, but several years old
Cranks very well, Denso starter also several years old.
No smoke from tailpipe
Eventually batteries drain and I get the relay clicking under the dash. And if I let them discharge enough the cranking stops and the wipers go on.

Charge batteries again, still no start. Have RPM indication on tach. The last time this happened it was after driving in heavy rain. I don't have any known windshield leaks, but there is the typical east coast under dash surface corrosion on unpainted stuff, but nothing severe. All I ever did before was check every electrical connection I could find, cleaned things, recharged batteries, no avail. Next morning it started.The OBD adapter not talking the ECU is the new thing, before I was able to grab codes. Thoughts?
Thanks again,
Tom

 

Check for power at the 12v power port in the dash. If that has power, check for +12v on pin 16 of the OBD-II connector and ground on pins 4 & 5. Be careful not to spread the contacts with the meter probes. If that checks out, check the fuses under the dash with a multimeter.

 

I'll check, but if pin 16 and pins 4 & 5 power the adapter, then they are good as the adapter is on and connects via BT to my phone. All the fusses are under dash and under hood are good.
Tom.