Is the engine battery charged if the motor main switch is off?

[Krumelur]

My boat does not have a starter key so I turn off the engine Main switch when leaving. The moment I do this the panel is showing me an alarm and I can see the voltage of the starter battery beginning to drop. I don’t know why that would be but my question is: is the starter battery still going to be charged while I’m away or is the motor main switch disconnecting it from the charger?

[tiger79]

Depends entirely on how it's wired.  As the boat is over 10 years old, all sorts of changes could have been made to the wiring.

[symphony2]

You should have an isolator switch for the starter battery. The only load on the starter battery is when you activate the starter motor so it is not affected by whether the switch on the engine panel is on or off. How do you see the voltage on the start battery dropping? If you have been running the engine it should be fully charged and at rested voltage.

[Krumelur]

There are two main switches on the boat. One is for lights, shower etc - all consumers.
The other one is labeled engine main switch.

If I turn off the latter one the panel shows the starter battery’s voltage drop. I doubt it really is dropping. And before switching off it’s showing the expected 12 volts.

There’s no additional wiring been made on the boat.