2017 Cutwater 28. Relatively new starting battery (this is the second time this has happened. After the first time I replaced the starting battery). We're on the hook very seldom. We're snowbirds ... until mid April the boat spent the previous six months connected to shore power with the battery charger on. Anchored out last night for the first time recently. Overnite the house batteries were used minimally (fridge, charging cell phones, etc.). We have a Victron Connect monitor for the house bank. This showed the house bank at 85% just before I tried to start the engine this morning, which just barely cranked but not enough to start. It did start (happily) with the crossover switch turned on, but just barely. On the assumption that the battery is OK, what might have caused it to discharge overnite? A defective ACR maybe? Any ideas on how to figure this out? What would be the downside of totally disconnecting the starting battery when the engine is off, like with a switch on its ground cable? Thanks!