can someone explain to me how these http://www.basspro.com/servlet/catalog.TextId?hvarTextId=74190&hvarTarget=search&cmCat=SearchResults work? do they charge while you are on the water when necessary and if so, then how? or is it something you just install on board and plug in when you get to the dock or home? considering getting one, just don't know much about them. does anyone have one and if so, which one and do you like it?

they are something you use when the boat is at home.  You plug it in and leave it.  It will charge all your batteries (assuming you get one with enough banks) and when completely charged it will go into a float or maintenance mode.

Once one batt is charged it will take the amps from that bank and send it to the bank still charging the other batt(s).

you install onboard so that's where it gets it's name.

I highly rec one.

You just leave them plugged in all the time while you are at home.  I "think" that "some" of the units will charge your batteries when your gas motor is running.  Not sure though.  

Those units are built by ProMariner and only charge by plugging them into an AC outlet.  You can upgrade to the ProMariner ProTourney line and they will charge from the outboard as well.  

Now, with that said, I run the ProMariner, ProTourney 300's, my motor has a 35 amp charging system and have seldom seen any increase in TM battery percent of charge when running the big motor.  They will not charge TM batteries until they have recharged the cranking battery.  About the only time you will see them charging the TM batteries from the outboard is when making long runs.  I you fish like I do a lot, kinda like playing hop-scotch on the water with short runs to a bunch of different holes, you will see no benifit on having a charger that charges from the outboard.  Also, if you outboard only has a small charging system like the 15 amp many have, it would be a total waste of money.  Those small charging systems will not even keep a cranking battery fully charged, much less has some extra to charge TM batteries.

Get a Guest, forget the rest.

http://www.marinco.com/scpt/brandSearches.php?currentMarketName=Guest%20Recreational&currentSection=Trolling%20Motor%20Chargers&currentSubsection=Triple%20Battery%20Applications

Two boats, two systems: I fish at least 320 days a year and run the trolling motor just about 8 hours per day. I recharge daily, and no charger can compete or beat a Guest Charger.