Wa chargers are characterised by the charging current of the charger being reciprocally proportional to the counter voltage of the battery. The charger delivers high current at low battery voltages and low current at high battery voltages. To be precise, the charger delivers its rated current at 2.1 V/cell, 50% of rated current at 2.4 V/cell and finally 25% at 2.6 V/cell. 

Wa chargers are not regulated. The Wa curve is created by correctly dimensioning the transformer and the choke coil. The charging process is affected by variations in mains voltage. In order to compensate for major deviations in mains voltage, the chargers can be switched between low and normal mains voltage. Micropower’s product range does not include the simplest type of Wa charger with a fixed top charging time, but the STC is equipped with electronic control with variable top charging times that are automatically adapted to the charging depth of the battery. 

The charger is a Wa charger, which means that the dimensioning of transformers and choke coils determines the charging curve. The charging current is 100% at 2.1V/c, 50% at 2.4V/c and 25% at 2.6V/c. 

The ratio between main and top charging is determined by time circuits on the electronics board. Constructions give a recharging factor of around 15%, which is optimal in normal operation. 

Unlike simpler timer chargers, the STC provides correct recharging regardless of discharge depth, which reduces gassing and thus water consumption. 

There are three different safety systems. These turn off the charger if: