boat's charging system/electricals question

[blacksapphirez]

Does the boat's charging system designed to charge the start battery before the house battery? I am not sure how it is wired or how it would know to do that if the battery switch was already in COMMON.

I've had a weird experience and would like to just understand the charging system a bit.

During the Bimini trip, I did not plug in for 6 days. Ran the stereo very little. Bilge was off. so the battery usage was very low. Now, I did start the engines a lot during that time and ran the boat a lot, thinking it would be enough to charge the system. However, about 5 minutes after leaving the Bimini heading to Fort Lauderdale, I've noticed the following.

1. trim tabs would blink when engaging both sides simultaneously(aka didnt work), which i assume it was drawing a lot of power and I didn't have enough. Operating one by one was okay.
2. the chart plotter would start but never fully turn on so I had to follow someone back(no problem)

All of this while the engines running and at 6000~7000rpms.

When I got back, of course I charged the batteries and ran the boat...No issues with the trim tabs or the chart plotter. I'm beginning to think that the house battery was never charged and it was just running low.

Does this make sense? I am thinking it should be charging BOTH while the boat is running if the switch is on common