We use the award winning CTEK D250SA Dual battery to battery charger with dual input capability which draws power from solar in parallel to the alternator. This fully automatic, 5 step charger supplies up to 20A of power delivering the optimum charge so your batteries are fully charged in the shortest possible time. With the ability to select normal or 14.7V AGM charge voltages and a temperature sensor to compensate for hot or cold conditions avoiding the risk of your battery overheating or overcharging.