Starting Battery Dead?

[HookJet]

Got a late start to my boating season, and yesterday was only our 3rd outing. I replaced both batteries 2-3 years ago, and I haven't had an issue until today. Both are AGM 24M Interstate batteries, one is a cranking the other a deep cycle. I have always had these batteries on a Noco 2 bank charger, and when I get in the boat the light is always green, except for yesterday morning. It had been about 2 weeks since our last outing. Went to check the condition of the connections and the starting battery was very warm. Warm enough to feel it through to cover. Connections were tight and in good condition.

Was out 'floating' for 5-6 hours and both batteries were turned off as I was on a friend's boat. Went to leave, and it would not start or turn over. Checked everything, still nothing. Put the batteries in parallel and it cranked right up. Before putting them in parallel, SYS was reading 12.3V and AUX was 12.6V, while underway they were both 13.9V -14.0V. Got up this morning and put a voltmeter on the batteries and it was 10.8V for the cranking, and 12.9V for the deep cycle.

Is this simply a bad cranking battery? Bad charger? Something worse? I'm going to change the battery tomorrow, and hope that's the end of it. It just seems odd that a staring battery would go before the house one.

Always appreciate the help...

 

[Dixemon]

Sounds like a typical bad cell. It happens to these batteries that pound in the choppy water.