AR230 - not charging - Troubleshooting step

[Blazin14]

My voltmeter readout on my dash (aftermarket) on my AR230 would usually show 13+ volts while under way. But now it is showing 12 volts underway or when engines are off. My question is this, in cars to troubleshoot a bad alternator, just loosen you batter terminal and start the car, and then pull your battery terminator off, if it dies, it was running on your battery only and your alternator is not working. Can I do the same for a boat, or not because it's not an alternator, it's a rectifier / regulator?

 

[anmut]

How old is your battery and have you had it tested?

 

[Blazin14]

I have 3 batteries... 2 are new (selector 2 on switch for stereo when engines are off) and one older - 4 yrs. (selector 1 on switch). When I switch to 2 to just both my new batteries, still the same...12.x, engines off or running.