D/C charger qestion

I have been looking at the new(to me anyhow) D/C chargers by minn kota.very cool to be able to charge while on the water. from what I gather, the charger gets its power at the outboard battery. correct?? so will the outboard still charge the main battery while it powers up the d/c charger?
reworded, I have a 24volt t/m system plus the main battery. of course my a/c onboard charger is a 3 bank, but in order to charge all 3 batteries at once while using the d/c charger, do I need a 2 or 3 bank d/c charger? thanks in advance.

Many people struggle to keep their cranking batteries charged by the outboard with electronics and pumps. Trying to charge two more will be tough, especially with crappie fishing where you seldom make long runs with the outboard. IMO, they are a waste of money for most fishermen.

Also that is why it is a good idea to get an on-board charger that has enough leads to charge the TM batteries and the starting battery. Especially with older motors. Like Catfan said it is rare that a crappie fisherman makes long enough runs to keep the starting battery fully charged.

thanks for the reply's guys.