10w Solar Panel 12V Car light weight Camping Caravan Battery Trickle Charger Why block diode is important When the sun shines, as long as the voltage produced by the panel is greater than that of the battery, charging will take place. However, in the dark, when no voltage is being produced by the panels, the voltage of the battery would cause a current to flow in the opposite direction through the panels, discharging the battery, if it was not for the blocking diode in the circuit. Blocking diodes will be of benefit in any system using solar panels to charge a battery.