Listened to the video. Doesn't sound like that is the problem. I can either start it up and all is fine or I have absolutely no power getting anywhere even though the battery is fully charged.

I have been busy, planning to do more troubleshooting tomorrow. Started it a few times and it has been fine. Haven't driven anywhere, don't want to get stranded. If I could make it happen in my garage I could find the problem. There is one connection in the quick disconnect negative cable that I want to check again.

I imagine there is a possibility of the connection in the BST being poor/corroded even though the BST hasn't blown ?

Did you scan the vehicle to see what the pending codes are? I had a very similar situation with failing EWS module.

I also had a very similar problem that sent me crazy trying to work out because it was intermittent.
It was random no start- no cranking at all- then all would seem fine, checked out ignition switch, battery, etc, I changed my starter motor twice, because it tricked me into thinking it had something to do with it, because there was no crank , no power getting through, then sometimes there was power;
but in the end it was the EWS out of alignment-
I didn't replace it, just realigned- took a few goes for some reason,
I wish there was a way of disabling the security, it would have made it much easier to diagnose, after a year now ,
All is good- touch wood.!

Haven't checked for codes, will tomorrow.

You mention your problem was "similar". When it would not turn the engine over at all did you have interior lights, dash lights anything that was getting power?

I thought when there is a problem with the EWS the engine would turn over but would not start. I was not aware an EWS failure would result in not having any indication there is a battery in the vehicle at all.

Yes, I had interior lights, everything was working correctly- except- no crank- but just sometimes, no rhyme or reason, no crank.
I thought it was the starter because it wouldn't crank ;
But changing it made no difference;

Only realigning fixed it.

The only code I had was 4A62 or 4A63 or something like that related to DDE EWS alignment;
but I didn't want to believe it had anything to do with it,
because it refused to even crank,
and then suddenly it would crank and start like everything was Aok;
as time went on I didn't trust it to start;
I almost gave up and was going to take it to BMW and leave my wallet with them while they would have probably replaced DDE EWS, keys , who knows leaving me broke;
I even had problems resyncing or realigning;
In my case the problems started after I took my X5 to local mechanic who read my codes with a small hand held reader-
An error occurred- I didn't pay it no mind at the time- but my problems started after that-
It was confusing as I thought if EWS wasn't in sync,,then it should never start and should still crank-
But that's not what happened-
completely random no crank-
I had to align at least twice-
Then no problems since , and it's been over a year now all good.
Bear in mind mine is a diesel, but I guess BMW use a similar system, considering my e53 was built at South Carolina with every other e53.

Brian I can tell you that when my EWS failed on my Touring, it was a no start/crank. It was intermittent and led me to believe it was the starter. As it got worse the code came up for the EWS. All I am saying is check this part of the vehicle because it can trip you up in the diagnosis. And yes all of the lights came on in the vehicle but no crank to start.

Certainly will check for codes and add that to the list of possibilities. The only thing that lights up when I have this problem is my frustration.

Update--am in the garage. 2 days ago after I shut it off I checked the voltage at the under the hood terminals. All was fine. Today there is no power to anything. My battery charger will not start a charge cycle because it does not sense there is a battery to charge. The hatch is closed and I can't read codes. Before I climb in and remove all the nitrous stuff and subwoofer so I can start the painful process of getting to the battery from the back seat to see what it reads, expecting it will read 12volts as it has previously, any other suggestions? Is it possible that a bad EWS will cause this? I have never experienced not being able to charge or read battery voltage from the front terminals.