Two similar issues Iíve experienced.
As others have mentioned check all your connections.
I had nut on the fat wire on the downstream side of the starter contactor come loose. It arced and welded itself. So it still worked but super patchy.
The other one which I have seen a few people do (myself included) is inadvertently wire the voltage sense on an external alternator regulator to the battery bus.
That regulator will draw a tiny current (about 20mA) with everything shut off.
Obv not a problem if you fly regularly but if you leave it a week or two..
Easy to test. See if thereís any parasitic drain on the battery with the master off.
RV7 VH-XRC Sold :-(
RV10 in progress.