you want an Intelligent Battery Charger on-board or portable
it should have minimum 5amps, 10 is better, it drops the amps when battery reaches about 85% full to charge the way manufacturers want
a dead 100 amp battery at 5amps charge takes 20 hours to charge
the same battery at 3.3 amps takes 30 hours to charge
10 amp charger...10 hours, I like to fish all day and get up in the morning and fish again with a full battery