I had a problem with my battery too (so I tought).
Sometimes the gauge showed me 12V and sometimes 9V or 16V.
Of course when it showed me less than 12V I was unable to start the engine, but when it showed me 12V or more the engine started fine.
I fact it was the regulator (which should be between the generator and the battery) who was out.
Changed this piece and no problem more (keeped the same battery)
Hope this will help.