what is this amplifier/rectifier looking thing?
 

So I got the boat end of last year and only used it a handful of times before it was time to put away. The boat had two batteries with a switch but it and the batteries went bad so I just had one battery in it last year. It's finned aluminum like a car amplifier but small. It has 3 posts, the alternator goes to the center and it's got Two white leads that go to the batteries. I'm thinking they each go to a battery positive.


But what does it do?

It's not letting me post a pic so you can see it, I'll work on that.

Thanks

That sounds like an external voltage regulator.

It's a battery isolator. It keeps the two batteries from discharging one another while allowing the alternator to charge both batteries at the same time.

With one of those you do not need to use the battery switch to select which battery to charge.

They work well and are dependable but they have the disadvantage of requiring slightly longer charge times because of the voltage drop across the isolator.

Yep, battery isolator. They are used all the time for campers and trucks with dual batteries.

Great information guys. Thanks! So, am I right that one lead goes to each battery's positive then ? I guessing has to.

I would say that is correct, but I would pull the model number off it and google search to look at an online manual.

yep. The offset terminal goes to the alternator.