DO your batteries go dead, when not connected to shore power?

[pascavone]

The carbon monoxide detector could the casue of your dead batteries.

The original one draws 400ma, and a new on draws 60ma.

Plus, it need to be replace every 5 years, and the old was going on 11 years.

[BayBoy]

Not sure if you are asking a question or making a statement. Bilge pumps also draw battery power as well as the Mercathode system (if equipped) even with the battery switches turned off as they are wired to the hot side of the battery switches.

[ss3964spd]

All batteries that I am aware of lose their charge over time if they are not maintained on a charger, and some more than others. Lead acid, for example, looses its charge faster than AGM's.

Depending how your boat is wired anything that remains "on" when the ignition is off and the battery switches are off will slowly deplete the battery(s). Co detectors, MerCathode, clocks, stereo memory, anything that goes into "standby" mode when it is switched off. Bilge pumps - yes and no, because when they are not running they do not consume any power. An exception might be those equipped with electronic "floats" or electronic high water alarms - which may require a constant 12V feed for their sensors.

You should be able to figure out what the total draw of the consumers, then divide that into your batteries amp hours to get an idea how long it should last.