Overview
For some common traction battery chargers, such as electric vehicle chargers, designers sometimes arrange that the charger does not output voltage when its output is not connected to a battery. The following explains how that function is implemented, with reference to the diagram below.
Charger secondary when no battery is connected
The left-side input in the diagram is the secondary output of the charger power supply. When the charger is connected to mains power, the secondary typically provides about 70 V, although the exact voltage depends on the charger model.
When the charger output is not connected to a battery, there is no current path from the base of Q1. Q1 therefore remains off, the bidirectional diode DB3 does not conduct, and the thyristor Q2 is not triggered. Because the secondary output has no current path to form a loop, there is effectively no output voltage at the charger terminal. Even if a resistive load is connected at the adapter output, the charger still does not produce an output voltage for the same reason: thyristor Q2 is not triggered and no discharge current loop is formed.
What happens when a battery is connected
When a battery is connected, it has some residual charge and therefore a voltage at its terminals. The battery voltage can create a base current path for Q1 via the route: battery positive → Q1 emitter → Q1 base → R3 → battery negative. This current path causes Q1 to conduct.
Once Q1 conducts, the bidirectional diode DB3 will conduct, which in turn allows the thyristor Q2 to be triggered. The secondary output of the power supply then forms a current loop to the battery, and the battery is charged normally.
Conclusion
The key point of this circuit is that the residual charge in the battery provides the condition needed for Q1 to conduct. Therefore, the circuit assumes the battery is not completely discharged; the battery must retain some charge for the charger to detect it and enable charging.
