A battery charger. It checks whether the battery has the minimum current needed for it to work and whether it still has electrolyte. If it does have electrolyte it applies a "first aid" voltage of up to 20V so that the battery can accept a normal charge. When it reaches the level wanted it carries on charging at 0.6A. Just before it is fully charged it goes into a lower-current mode in the form of pulses until it is fully charged. It checks for 30' and gives an extra charge if needed. Overcharging is avoided by specially designed circuits. It keeps the battery at the ideal state of charge.