Our AGM start battery appeared to be failed. The indications were a) low voltage and b) the Magnum Energy - Smart Battery Combiner (SBC) reset itself every 10 seconds. The latter is a recognized indication from the SBC. I called Magnum Energy's tech support and, cutting to the chase, found a fuse, protecting the house battery lead to the SBC, popped. Replacing the fuse ended the 10 second reset cycle.
The battery in question is a Lifeline Start battery, GPL-1400T - AGM - 43 AH - 850 CCA
NTL, I had doubts about the batteries health. I took the battery to Treasure Coast Battery (Stuart, FL) for a load test. The tester showed about 220 CCA or functional but fading. I replaced the battery.
Now, the question: What kills AGM batteries? This battery was in place for 4 years. Its sole job is to start the motor (32 hp diesel) and run the instrument panel and booster pump while the engine is running. AFAIK, the battery was never fully discharged. When the boat has been in storage, the house and start battery are charged by the boat's solar panel. What killed the battery?
Eat more moose, 17,000 wolves can't be wrong...