P42 starter battery discharges when off; why?

[BBC16385]

I have a 1997 P42. I close up the boat on a Sunday. Starter is showing 12.8 or higher. Come down Friday afternoon and starter is showing 12.4. If I go two weeks away from the boat, starter battery shows 12.2 or worse. The battery off switch is off and the 12 volt panel switches are off. Bilge pump is not cycling. Battery is in its third season. A Lifeline AGM. I think this has been going on for years and I just didn't notice until a few years ago.

Something is draining the battery. Any ideas? Thanks.

 

[Stu Jackson]

Consider disconnecting that battery completely for a week and keep up the measurements to see what happens. >12.8V is fully charged. You can find % of full via Voltage tables via Google (where 11.8V is empty).
Then you'll know if there's actually anything "on" and can do more detective work, if necessary. Could be a chronically undercharged battery that's dying.
Good luck. Let us know.

 

[JamesG161]

These are items on my H430 1998 Start Battery that cannot be shut off by the Selector Switch.
1) Bilge pump
2) OEM Solar Panel
The only thing that it shuts off is the Motor Starters , Engine and Genset
______

Bilge pump is not cycling.

Are you sure?
It is rare to see mine cycle.
______
When is the last time you started your Engine or Genset?
It is those Alternators that Charge your Start Battery.

On good Start Battery, it takes about 10 minutes to Re-Charge a good battery using my engine alternator.[after a month of no use]
I just replaced my Start Battery 2019 after 3 years of use.
Why?

I saw it becoming weaker [like yours] and it is a very important battery. Cost $80.
Jim...

PS: My Solar Panel charger is not working.

 

[splax]

maybe it is your solar charger controller, and it contributed to your battery loss