Very few crappie fishermen make the runs required to have much of an effect on their TM batteries with these chargers. They dependon big outboards and long runs at WOT to do any charging.
Remember, the charging system on the outboard was designed to keep your cranking battery charged, and they often struggle to do that on long days with livewells running. Only the extra charge goes to the TM batteries, and in a lot of cases there is no extra charge.