Seems that early boot is getting wrong info about the battery. If You add strange behavior when it's running ("thinking" that it's connected when it's not, and booting up immediately after inserting battery - it's *not* normal), it may mean hardware failure.

Ho ever, have You tried reflashing, just to be on safe side? Also, flashing framebuffer kernel will reveal which part of boot *exactly* fails.

// Edit

Before trying to reflash or flash framebuffer, I would check if it act the same way with R&D mode enabled. There was a guy here, whose device failed to recognize raw BSI value. It can't be true in Your case, cause You can boot with charger, but I'm 99% sure it have something to do with detecting battery.

As R&D mode with watchguard disabled flag turn off watchdogs, it may permit phone to boot normally. Worth checking.