The Yandina is pretty simple and works basically as a relay that opens below a certain voltage and closes above. Basically just connecting it to the cranking batt in parallel and it gets the "full" amperage.
I think the Minn Kota works a bit different - though still different than a "smart" battery charger that applies the various voltage curves to it when charging. Which method is better? Good question - time for a little more research. I like the very compact size of the Yandia solution for my rig.... And know several guys using these that fish HARD and have used them for several years now successfully. The MinnKota is about the size of another charger - and I wonder how much you get back in the batts with a max of 10A / bank without making long runs.
The key to these in my mind is as a few have said - it helps keep from from getting the battery too low. A long day/weekend of fishing (even when charge overnight, but short recharge times) can take a cumulative toll on the battery. Being able to get charge back in them when running up and down the lake can be helpful. If you are making short 5-10 min runs then It likey won't do much, but longer runs can help for sure.
Agreed!! The chart on the minnkota site says this: 3-5hrs recovery time for 15amps per bank, 4-6hrs for 10amps and 6-12hrs for the 5 amp bank. So yea, it takes some time running to charge em up. 5 amp on the water charger would basically be useless............