Electrical issues

We were using the boat yesterday and when the time came to leave my right side batteries were dead. I knew I needed to replace them so I wasn’t too concerned. I jumped it using the switch and they started no problem. As we were leaving I smelled smoke so I stopped the boat and looked in the hatch. There was some smoke down there but I couldn’t tell where it was coming from. My vessel view had an alarm also. It said battery voltage low. They were low on the left side. Those batteries have been replaced already. When we got back to the dock I checked further and it looks like the starter solenoid was a little melted and burnt. Does anyone know if this could be from jumping it? Or any ideas?

Low enough voltage from batteries could have caused extremely high current when you started the motor cause the wire to burn down .This is one possibility.

Dirty connections is another reason. If that fuse link would have blown you would be dead in the water because the alt output and ships power are connected together. I remove that block and install a Maxi-Fuse in its place with a 50 amps fuse on the Orange wire .Should never need more than that to recharge a battery.
I also cut the Red wire and connect it to the starters battery post. That circuit is protected by the 50A circuit breaker . This way you could lose the alt but still be able to run the boat on battery power to get back in instead of being towed in.