Precision 7740 won't boot from battery

Before I talk to Dell, which is sometimes painful, I figure I'll ask in here.  HOPEFULLY it's something simple and stupid.

Precision 7740 - something less than a year old.  Works fine.  Plugged in, it boots fine.  From battery, it churns on, then displays the logo, then progresses across the blue line below with the tiny yellow line of text about the boot menu in the top right corner.

Then it sits.  With the logo on and a black screen.  It'll sit there for an hour, but it WON'T boot...  The battery is FINE - AGAIN, the battery charges and will run the laptop for HOURS.  But the laptop won't BOOT without being plugged in.

If it's plugged in, it does exactly the same thing, but a few seconds after the blue progress bar it starts the circling dots and boots fine. 

I figure it's GOTTA be something stupid I've done, but I stay OUT of the BIOS and try not to play with things.  So I have no idea WHAT has it's undies in a bunch.

IF I hit f12 I get the boot menu and it SAYS it's using - Boot mode set to UEFI:  Secure Boot off

I TRIED to run the diagnostics.  With the charger unplugged.  I got a WARNING the charger was unplugged but it ran them anyway.  Then it "finishes" and says

Critical Error Message

Testing completed.  One or more errors were detected.

Except I don't GET any critical error messages or errors of any kind.  I presume it's mad about the charger, but either way, it didn't show anything useful.

WHAT would cause an ordinary laptop, OTHER than a bad battery, TO NOT boot.  And what do I need to set to have this thing go back to normal so I can boot it when the power is off?