Turning a years-old iPhone 6 battery that only lasts half an hour into an external 20,000mAh pack. Teardown: open the cover plate, remove the battery connector bracket, pry the battery out with a spudger; drill a small M3 hole in the case, then widen and deburr it so it won’t cut the wire, and route the iPhone 6’s power flex through the hole to the back. First test with 1 L700 taped to + and −: no light the first time because the flex wasn’t clipped in tight; press it down and ‘3, 2, 1, on’, swiping works. Then the big battery: 4 × L700 (5000mAh each) in 4P = 20,000mAh; out comes the AWithZ spot welder, nickel strip joins the positives and likewise the negatives, with fish paper for insulation. The hot-glue gun first dabs the power flex so it can’t come loose, then the back of the phone is covered in hot glue to stick the battery on, both sides filled so it won’t wobble. A soldering iron tins the nickel strip and the wire ends, then solders + and −; it boots to the home screen, Wi-Fi works. Finally a 45℃ thermal switch (actual part marked KSD9700 50℃ 250V10A) is wired to a cooling fan, hot glue holds the switch and fan together, the switch’s + and − are soldered to the battery: once the case goes over 45℃ the fan starts by itself, no physical button. Camera and swiping work — done. Top-liked correction in the comments: charging without a BMS is very dangerous, a 21700 is 4.2V full while an iPhone battery is 4.45V; others worry the big battery will fry the logic board, and ask how to read the charge percentage.