The regulator/rectifier which supplies the charge current has likely failed Dutch. Although prone to failure its easily accessed with a Harley unit recommended as replacement.

If you measured 12 volts with engine ruining that equates to a near flat battery with absolutely no charging taking place. When working properly the regulator should hold the battery voltage around 14.4 volts, with engine running of course.

I would stay well clear from any "repair guy" that told you it should be 20 volts!

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone