No flashlight repair today — replacing the 18650s inside a Milwaukee 12V battery

A viewer’s Milwaukee 12V battery is weak: non-OEM build, pack voltage 12V is normal but the three cells read 55/56/57mΩ; swapped for domestic 2500mAh (IR 12.4x), fish paper on the positives, AWithZ handheld welds 0.1 nickel strip with no effort, 0.15 needs max gear (0.2 won’t hold), pack 10.7V IR 46.9, noticeably better speed once fitted

A viewer sent in a Milwaukee 12V battery that went weak after a few uses, slow speed and wrong sound in the tool; opened up, it’s not Milwaukee OEM but a cheap pack someone else built. Multimeter reads a bit over 12V total, normal; swap for domestic 2500mAh cells (the ones he uses himself); the Zoyi multimeter reads the new cells at 12.42/12.48/12.46mΩ, the old cells that came out at 55/56/57mΩ — IR that high is never going to work well in a power tool. Fish paper on the new cells’ positives for insulation. The AWithZ handheld spot welder is handier for the welding; 0.1 nickel strip, no effort at all; he had thought of building a big welder but doesn’t use one much so gave up, a small one is enough for occasional use; 0.1 welded on, you can’t pull it off by hand. The main negative reuses the 0.15 strip taken off earlier, and it needs max gear to hold; this machine welds at most 0.15, 0.2 won’t hold. Pack reads 10.7V, IR 46.9mΩ. Reassembled and charged a while, then tested: better sound, high speed fine too, not fully charged so not at its best yet but much better than before. Comments: just pick three JP30s and drop them in (that works); is a drill’s weakness related to the battery; multimeter recommendation / are you using it as an IR meter.

Outcome

Steps6 steps · click a frame to jump

1. 
   1. Milwaukee 12V battery weak, a non-OEM build

   A viewer’s Milwaukee 12V battery is weak; slow speed and wrong sound in the tool; opened up, not OEM but a cheap pack someone built; OEM is good but pricey.

   00:00 – 00:29
2. 
   2. Pack 12V normal; swapping in domestic 2500mAh

   Multimeter reads a bit over 12V, fine; swap for domestic 2500mAh, been using them and they’re good.

   00:29 – 00:43
3. 
   3. IR comparison: new 12.4x vs old 55/56/57; fish paper

   Zoyi multimeter reads the new cells at 12.42/12.48/12.46; the old cells 55/56/57 milliohms, IR that high is never going to work well in a power tool; fish paper on the new cells’ positives for insulation.

   00:43 – 01:25
4. 
   4. AWithZ handheld welds 0.1 strip with no effort; a small one is enough

   The AWithZ handheld spot welder is handier; 0.1 nickel strip, no effort at all; had thought of building a big welder but doesn’t use one much so gave up, a small one is enough for occasional use; 0.1 welded on can’t be pulled off by hand.

   01:27 – 01:50
5. 
   5. Main negative on 0.15 strip needs max gear; 0.2 won’t hold; reads 10.7V / 46.9mΩ

   The main negative reuses the 0.15 strip taken off earlier, needs max gear to hold; this welder welds at most 0.15, 0.2 won’t hold; 0.15 welded can’t be pulled off; wiring not shown; pack reads 10.7V, resistance 46.9 milliohms.

   01:51 – 02:20
6. 
   6. Refit, charge and test, better on both low and high

   Reassembled and charged a while; better sound; low and high speed tried; not fully charged so not at its best, but much better than before.

   02:21 – 02:41

Bill of materials

Measured on camera

Creator’s tips

• Weak tool battery — check single-cell IR first; 50+ mΩ means replace 00:58
• The small handheld welds 0.1 easily, 0.15 at max gear, 0.2 won’t hold 01:27
• For occasional use a small spot welder is enough, no need to build a big one 01:37

Pitfalls

• Non-OEM packs use poor cells with high IR 00:15
• 0.15 strip needs max gear, 0.2 won’t weld solid 01:51

Li-ion safety

• Fish paper on the positives for insulation 01:12

Quotes