Automatic smart chargers do all the thinking for you....no need to over think the need. Last few years switched over to all Noco in the new boat from using several Minn Kota's for the last 25 years or so years...........couldn't be happier, love all the Noco products. Minn Kota certainly priced themselves out of the market. As far as charge rate/output, really depends on how much time you have to recharge between uses. A 2 amp will take about 30+ hours to charge a depleted group 31, a 5 amp 12+ hrs and so on. In the last 12 to 15 years, smart chargers have really evolved, I've had nothing less than 10 amps per bank........the charger regulates the output for optimized results, different modes depending on what type of battery, Wet Cell/AGM/GEL/LFP, etc. . Starts off at a higher rate and regulates towards the end, PERFECT! For even a more carefree install, go AGM!