Another CO death on a boat

[trackman]

The reason for charging batteries setting off gas detectors is methane. This gas, related to propane and butane can trigger both CO and LPG alarms.

Batteries being over charged are likely to give off methane as well as hydrogen. It can happen to new ones if the voltage goes too high, or old tired ones at normal voltages.

The OP's 16V is plenty high enough to cause methane emission even from good batteries.

We had this on CN earlier this year when our alternator regulator went mad, pushing the voltage over 16V regularly. Both our CO detectors and the LPG one would go off when this happened. Ear defenders were highly desirable!

The OP needs to get his solar regulator fixed or replaced asap before his batteries are cooked.

Oh! Methane is pretty poisonous too, so it's not something to neglect. Listen to your alarms folks, they will keep you safe!