If you use the right size wire from your alternator to the forward battery, then most of your problems are solved. Run a charging wire from the alternator to the battery in the bow that serves the thruster. Use #8 wire which can carry 80 amps safely and install an 80 amp fuse to protect the wire, not from the alternator output, but from the starting battery that is undoubtably connected to it. If the fuseshould blow, the alternator will not blow its diodes because other loads like the starting battery are still connected to it.

But #8 wire will have a very big voltage drop, about 2.5 volts for 60' of round trip at 70 amps because that #8 wire acts like a long resistor. That much voltage drop will limit the actual charging current that goes to the forward battery, probably to 25 amps which gives a 1 volt drop.

So after using the thruster if the forward battery is down to 12.4 volts due to extensive running of the thruster, and the alternator is trying to put out say 13.6 volts, with a 1 volt drop, the forward battery only sees 12.6 and that is why not much current will flow. The whole thing is self regulating.

The only time heavy current will flow is when you are actually using the thruster (and I doubt if it will pull 500 amps). Then the forward battery will drop to maybe 11 volts just like a starting battery in a car. But there will be a 2.5 V drop if it trys to pull 70 amps, so that means that it would take 13.5 volts at the alternator. When the alternator is tying putting out 70 amps its voltage is going to be less than that so it is self regulating at that high current level as well.

I realize that this has been a pretty confusing explanation, but give it a go. It should work fine.

David