Button cells (also called watch battery) are not designed for charging, but they can be at least few times partially recharge. Charging current must be relatively small (about 1 mA). It is not necessary to construct charger with power transformer, so I used a circuit powered directly from the mains. The negative terminal is connected to the ground and line voltage is rectified using 1N4007 diode and current is limited using resistor. Capacitor is used to filter the voltage. The red superbright LED indicates a fully charged battery and also prevents overcharging of the battery (red LED voltage drop in the forward direction is about 1.55 V). for charging battery with a voltage of 3V charger can be adjusted by using two red LEDs in series or LEDs with higher voltage drop (eg blue or white with a drop of 3 - 3.2 V). The entire charger can be built into the plug with ground connection.