Thank you both for your advice. The battery connection was seated okay and I tried reinstalling the driver to no avail.
I ended up taking the battery out and replacing it again, but it was used (but newer) battery from a 850 G3 model (same battery - thank God for close generation laptops using similar parts)
I have had no issues with this battery. So I surmise it must have been the new battery after all that was the problem somehow. Which is odd considering the voltage showed okay and the repeated battery tests from HP showed okay.
All in all, this chapter is closed. On to the next mystery.