It is most likely due to poor quality batteries. The "cells" in a 9-cell battery are 18650 batteries, which vary in capacity from 3400mAh at the high end, to counterfeit ones that are labelled to have 1200mAh capacity, but in reality have only a fraction of that. Lithium-ion batteries also have a limited shelf life, so even good quality aftermarket batteries will have a fraction of the rated capacity if they have been sitting in a warehouse for years.
Battery had been disconnected, also the CMOS battery. I usually remember to hold down the power button for a few seconds. Dang.
Didn't chase this further, as I am clumsy with board-level repairs, and my friend didn't want to throw more money at the project.
Edit to add: Obviously this is an i5 laptop from 2012, much newer than some of the other machines in this thread. Although it's not my daily driver anymore I do still use it for music production.
I wish the multiple replacement batteries I got for it over the years were as performant ;o)
If anyone here is planning to do the FHD upgrade, be very careful with power, you need to remove the battery/power cable and press the power button a couple of times to discharge all capacitors etc, otherwise you will blow a microscopic fuse (I did!) and it's near impossible to replace it. I just soldered a bridge between the contacts! I just can't do anything wrong again or I'll fry something.