Pictures and explanations
this mod is hazardous as there is no more space and in case of battery failure it may lead to the destruction of the phone.
A. Disassemble the EP500 battery (same way as Gx3
B. Remove the original mini battery from its metallic frame and cut the 3 wires from their solder
C. Preparation of the EP500 battery
1. Remove the coating on those tracks
2. Cut the thermal protection and bent it so that it's become flat
3. Resolder the thermal protection
4. Tin the prepared tracks and positive pole of the battery
5. Solder the 3 wires of the battery connector
6 & 7. Insulate the whole BMS board (both sides) using tape
8. Cut a window in the tape to let the thermal protection be in contact with the body of the battery
9. Press the BMS against the battery
D. Insert the EP500 inside the metallic frame
E. Finished !