Step 4: Remove the carbon lining
A 12mm bit should fit inside the battery quite neatly.
Stop every now and again to empty the carbon dust out of the battery casing.
CAUTION : If you drill too fast, the casing will get too hot to hold, or the drill will clog with dust and snatch the casing from your hand.
When the casing is empty, wash it again and clean it with a piece of cloth on a screwdriver.
Carbon dust is electrically conductive, so you don’t want to leave any in there to short out the USB circuitry.