First night on the hook in the "new" boat and woke up at 4 AM to a tapping/thumping sound. The frequency of the "taps" increased quickly until it was a steady hum. Traced it to the starboard cabin speaker. Stereo wasn't even on(!). Not quite sure, but I think I opened the port battery solenoid and the sound stopped. Looked like I drained the port battery bank pretty badly (was running a few lights and the fridges). I started the starboard engine. All is fine. Then closed the port solenoid and used the emergency start to start the port engine. The port dc volts (analog cockpit gauge) pegs high and I get a "High Volts" alarm. So I stop the port engine. Alarm does not clear. Port volts reading very low (~10 VDC). Try to start the generator to run the battery charger, but can't close the generator solenoid. Genny starts from port battery bank (of course). I don't see anything in the manuals or google about using the emergency start switch to start the generator, so I don't try. Can't find any info on a High Volts alarm. Keep searching and try the port engine again with the digital meters up on the display. Port engine starts (emerg start again) and the volts go up to 28 VDC. Shut off the engine again. Spent a couple hours trying to figure out WTF was happening, but to no avail. At one point, I did have both engines running with VDC at nominal and all alarms cleared, but eventually got the high volts alarm again. Decide to limp back to marina on one engine and managed to get it back in the slip (thanks bow thruster!). Hooked up to shore power and let the batteries charge all morning. Looked at the battery charger and it was pegged high at 60 amps (later was reading a more reasonable 20 amps). Went out that afternoon and everything seemed to be fine. I started the generator in the slip and kept it running the entire time, just to ensure the batteries stayed charged. The next weekend, I checked all four batteries and all tested good. At this point, I chalk it up to "extreme battery discharge." Any other ideas?