His charger was putting out close
to 18 VDC - when I asked him to check it - via phone. So, I told him,
there is your culprit.

That's what I suspected, unregulated output, without the battery in the loop
to absorb the energy.

This comes from using a "charger" as a regulated power supply.
Note that a regulated power supply makes a much better charger, than even a
"charger".