Takes double the time to use one leg of a dual charger charging 2 batteries in series. Just use any charger to charge the motor battery and once it’s charged, it doesn’t loose that charge as nothing from motor is trickling usage from it. I use a minn Kota 10 amp portable charger 110PD on motor battery and it’s a awesome portable charger that will also minimize the battery as it charges just like on board chargers. Search the web for best prices.