Like other simple, single-cell lithium-ion battery chargers, Microchip’s MCP73812 provides no means of indicating the charging status. You can remedy this situation by adding four components (Figure 1). Add one more LED, and you also get a charging-complete indication. This two-LED configuration has the added benefit that one of the LEDs is always on, providing an indication that the charger is powered.
While the cell is in the constant-current charging mode, 401 mA flows through the 1N4001 diode, D1. The additional 1 mA is the supply current of the control chip. Because the 1N4001 conducts before the base-emitter junction of Q1, it prevents Q1 from turning on until the forward voltage across it reaches about 450 mV. Q1 then starts to conduct and turns on D2, a red LED that indicates charging. Because the forward-voltage drop for a green LED is typically higher than that of a red LED—2.1 versus 1.7V—the voltage across D2 and Q1 is less than the turn-on voltage of the green LED, D3, and it remains off.
For the last part of the charging cycle, the controller switches to constant-voltage mode. As the cell voltage gets closer to this 4.2V terminal voltage, the current through D1 drops, and at 15 to 40 mA, both LEDs illuminate.
Tests measured this range for several 2N3904 transistors. Testing with 2N4401s gave a lower range of 4 to 18 mA. When the current drops below about 15 mA, Q1 turns off D2 . The voltage across D3 now rises above its forward-voltage threshold, and the green charging-completed LED lights.