All old laptops have battery of Li-ion cells, that can be useful for electronic projects. But these cells need to be checked. This charger circuit is assembled from vintage components. The RS-trigger turns on the relay, the cell (20 mF capacitor) starts charging. The red LED indicates the charging mode. When the voltage on the cell reaches 4.1 volts, the trigger returns to original state; the green LED lights up. Before start the power switch has to be turned off. Then start, turn on the power, press the start button.
