Replacing a battery charger is very easy. You don't have to be an engineer, just reasonably skilled with a screwdriver and drill. You just label each wire before you remove them from the old unit and follow the instructions for the new one.

The new charger,
installed and working!

There are only six wires on our unit. On the DC (battery or -) side there is a Ground, which you connect to the Negative terminals on both batteries. Each hot wire (+) goes to a Positive terminal on one battery. This lets the batteries charge independently, even if one battery goes bad, the other will still hold a charge. The AC side had three wires. Just cut the old AC feed and follow the color code. If your boat doesn't have a white, black and green wire, STOP, get a professional to look at it. Wiring it wrong can lead to all kinds of problems.

Finally mount your new charger. Pay attention to the manufacturers instructions. Battery chargers give off heat and must have proper ventilation.

Well, that's done. I guess I can put the automobile charger back in the garage.