Hand tools and fabrication machines
I did not want to waste time to replace the battery. But the flashlight actually worked well. I did not want to throw it away and I decided to convert it to a rechargeable flashlight.
1. Small LiPo battery. I used 130mAh battery because it fit inside flashlight.. LiPo Charger board. I used SparkFun LiPo Charger Basic
3. Resistor 100 Ohm
4. 2 pin JST connector with wires.
5. Hot glue gun
Create a hole for Micro USB connector. I used a Dremel Rotary Tool and Rotary File Burrs. But I suppose you can use a hot nail.
Probably you will need to adjust charging current of LiPo Charger board. Default current of board I used is 500mA. It is way too much for 130mAh battery I used. Usually charging current should be below capacity of charged battery. I decided to charge battery with current less then 100mA. So I removed resistor marked by blue oval on photo and soldered 12 kOhm resistor instead. You can see the relation between resistor and charging current on attached graph.
1. Solder LiPO battery to LiPo Charger board. Pay attention to polarity.
2. Insert LiPo Charger board into flashlight so that Micro USB connector will fit inside hole created in previous step and glue LiPo Charger board to flashlight.
3. Solder 100 Ohm resistor to one leg of LED
4. Solder JST connector to other leg of LED and to power switch. Pay attention to LED polarity.
5. Connect JST connector to LiPo Charger board
Generally you can secure the battery with hot glue. But I wanted the option to replace it in future. So I used self-adhesive foam to secure the battery.
That is all. Assemble you flashlight and charge battery by connecting flashlight to computer or charger.