Mercury 115hp battery should be charged by engine?

[brooks]

Hello! I`m beginner in this field!
Does Mercury (1996) outboard 115 hp should charge battery? Currently it does not.
serial nr. 0G335231

Thanks!

[ss3964spd]

Welcome Brooks. Yes, the engine should charge the battery. A fully charged battery should read 12.5 or so volts when the engine is off. When the engine is running the reading should be around 13.5 volts. It could be that your battery is bad and will no longer take a charge, or that the alt on the engine is not putting out a charge.

[brooks]

I was take measurement on a running engine and it was the same as engine ir off.
Previously had a problem with the tachometer, it doesn`t work. I bought used regulator rectifier, and now tach is working, but on low RPM it is jumping. Could it be a problem with regulator rectifier and therefore not charging battery?
Where should I star looking for a problem and how can I test Alt or voltage rectifier?