If it was me, I’d start by load testing the battery which might give info regarding a bad cell and at least give an idea of overall battery health. Then I’d get out the multimeter and check for the amount of parasitic drain. Next, check all the wiring looking for bad crimps or loose connections…jiggle wiring in the primary circuit with multimeter attached. Maybe just empirically clean and tighten all the connections. Then, I take a hard look at the maintenance charger…there is a wide variety of quality and reliability in those things.
On my RV, I run two PC680s in parallel and keep them 24/7 on an Odyssey OBC-6A charger when in the hangar, and I have some kind of Amazon regulated power supply that I use for avionics updating etc. When I bought the airplane, the owner included the float charger he’d been using. I was getting some occasional low battery issues, threw that charger away and bought the (rather expensive) Odyssey charger. Never an issue since. I use Battery Tenders for maintaining all my seasonal home rolling stock and have never been disappointed but I went all out for these airplane batteries. I believe that the thing that kills lead-acid batteries is sulfation and the thing that causes sulfation is discharge. Hooking my airplane up to a good quality float charger when I put it in the hanger is a very, very simple way to eliminate that variable.
RV-9A, 2011, bought flying
IO-320D1A (factory new), C/S
AFS 5400/3500, G5, IFD440 navigator,
bunch of other stuff