Modding an iPhone battery to 20,000mAh — a week of runtime

iPhone 6 modded to 20,000mAh: 4 × L700 (5000mAh) in 4P, nickel strip joined with an AWithZ spot welder, hot-glued to the back of the phone, a 45℃ thermal switch auto-starts a cooling fan

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.

Outcome

Steps8 steps · click a frame to jump

1. 
   1. Teardown, remove the old battery

   Open the cover plate, remove the battery connector bracket, undo the screws one by one, gently pry out the battery with a spudger; the old battery was replaced years ago and only lasts half an hour.

   00:00 – 00:39
2. 
   2. Drill an M3 hole in the case and route the power flex to the back

   Drill a small M3 hole, then widen and deburr it so it won’t cut the wire; the iPhone 6 power flex goes through the hole to the back, where the L700 will be taped.

   00:39 – 01:20
3. 
   3. Single L700 boot test: flex not clipped in → lights up

   Tape + first, then −; no light, pry open and find the power flex isn’t clipped in tight; press it down, 3, 2, 1, on; swiping fine, power off.

   01:20 – 02:02
4. 
   4. 4 × L700 in 4P, AWithZ welder joins the nickel strip, fish paper insulation

   5000mAh per cell, 4P = 20,000mAh; AWithZ spot welder on, nickel strip joins the positives (fast-forward), negatives likewise; fish paper for insulation.

   02:02 – 02:49
5. 
   5. Hot glue fixes the power flex and the battery

   Dab hot glue on the power flex so it can’t come loose, close the cover; cover the back of the phone in hot glue, set the battery, fill both sides so it won’t wobble.

   02:58 – 03:32
6. 
   6. Tin and solder + and −, boot test

   Melt solder wire onto the nickel strip with the iron, tin the wire ends, solder + and −; boots to the home screen, Wi-Fi works, power off.

   03:36 – 04:16
7. 
   7. 45℃ thermal switch + cooling fan

   Thermal switch wired to the fan, starts by itself when the case goes over 45℃; actual part KSD9700 50℃ 250V10A; tidy the wires, hot-glue the switch and fan together, solder + and − to the battery, no physical button needed.

   04:16 – 05:02
8. 
   8. Function test, done

   Camera and swiping both fine, the experiment is a success.

   05:05 – 05:23

Bill of materials

Creator’s tips

• After drilling, widen the hole and deburr it so a sharp edge can’t cut the wire 00:53
• Tape + first, then −, so nothing lets you down at the key moment 01:20
• Tin the nickel strip and wire ends before soldering — much easier 03:36
• Solder the thermal switch straight to the battery; the fan starts by itself once the case passes the threshold 04:48

Pitfalls

• No light on the first boot: the power flex wasn’t clipped in tight 01:33
• Comment: charging without a BMS is dangerous; a 21700 is 4.2V full while an iPhone battery is 4.45V — different voltage systems
• Comment: too big a battery may fry the logic board; no way to read the charge percentage

Li-ion safety

• 20,000mAh in 4P wired straight to the phone with no BMS — high risk in charge and discharge (top-liked correction in the comments)

Quotes