Please bear in mind that the type of charging device Minn Kota is talking about is one that is powered not by a 120V circuit through and extension cord, but via the alternator on an internal combustion engine (your outboard).

The warning from Minn Kota is to not use a non-Minn Kota Alternator Charger with the US/2 built in transducer.

Maybe Greg or Jody can explain what might happen if you don't use the Minn Kota Charger and why. I don't think its an idle threat or a marketing ploy. I think they make this recommendation based on real-world problems and they have modified their units to address these problems.