Bad batteries–vaccum has 13 sub C NiCD cells. Tested voltage on the individual cells and replaced 6 bad cells.
The vaccum still wouldn’t charge–checking the charging circuit board showed a crack in a trace near the edge where the indicator LED is located. A bit of solder fixed it, and now it is charging.
FYI, the main IC on the circuit board is a ABOV MC96P0202, a 8-bit OTP CPU.