Fixing a fan’s laptop battery that died in storage

Re-celling a fan’s old bank-terminal laptop: the original 6 × Samsung 2600mAh cells were badly unbalanced and dead, swapped for 2900mAh cells spot-welded with the handheld welder — no board lock, charging normal

An old bank mobile-terminal laptop (card reader, fingerprint) whose battery only showed 4% and would not climb. The creator had turned down re-cell requests before because “brand laptops lock the board when power is cut”, but this battery was already dead, so it was worth a try. Opening the pack: Samsung cells, total voltage looked normal, but per-group readings were 1.790V / 3.395V / 4.11V — left too long, badly unbalanced, scrap. New cells are 2900mAh (original 2600), internal resistance all around 19mΩ; the BMS has current sensing and will limit, so extra capacity would go unused anyway. With the new handheld welder (light) he tried gears 2 through 5 on nickel strip; the whole cell can is negative so no spacer is needed, the positive end gets insulating paper. Along the way: the layout shorts easily, the nickel strip is sharp, and with no jig it is awkward to handle — the three-cell junction was a struggle. Reinstalled, the laptop booted straight up, charge went 6% → 7%, and it did not lock the board after a power-off/on; he cleaned the old glue, checked balance (very consistent), fixed the temperature probe with 704 silicone, and rubber-banded it for a day while the glue cured. Later it showed 0% for an hour (suspected conflict with the BMS); he used the powercfg battery report to read the capacity history. A comment warned that re-celling may still need the battery-management chip (BMS) data reflashed.

Outcome

Steps8 steps · click a frame to jump

1. 
   1. The machine and the fault

   An old bank mobile-terminal laptop with card reader, fingerprint and camera; the creator had reinstalled Win10; the battery LED lights and the icon says charging, but it sits at 4%; the fan said the battery was already dead, so go ahead and try.

   00:00 – 01:10
2. 
   2. Open the pack, measure per group: badly unbalanced

   Replacement cells were ready; the original pack has Samsung cells, total voltage looked fine, but group by group it reads 1.790V, 3.395V, 4.11V; left too long, badly unbalanced, scrap.

   01:10 – 02:04
3. 
   3. New cells 2900mAh, about 19mΩ

   Bought 2900mAh cells (original 2600) so that after two years of fade they still count as full; much higher capacity would go unused because the BMS has current sensing and limits; all 8 cells measured at about 19mΩ. A chance to try the new welder too — it really is light.

   01:51 – 02:30
4. 
   4. Trial welds at gears 2 → 5; no spacer on negative, insulating paper on positive

   Two negatives lined up together; tried from gear 2 up to gear 5; the whole cell can is negative so nothing needs to go under it, the positive must get insulating paper; no jig prepared, better to clamp; result was decent.

   02:30 – 04:00
5. 
   5. Layout and welding: shorts easily, sharp strip, strip too short

   A bit crooked but not a big problem; three cells joined together are hard to handle; this layout shorts very easily; unknown whether the BMS will lock on power loss; the nickel strip is very sharp; the strip is not quite long enough.

   04:00 – 07:45
6. 
   6. Reinstall and boot: starts straight up, 6% → 7%, no board lock

   Boots straight up; low battery so plug in, shows 6%, rises to 7% after a while; power off and on again, no board lock; cleaned off the old glue.

   07:45 – 08:43
7. 
   7. Check balance, fix the probe with 704 silicone, rubber-band for a day

   Consistency is very good; apply 704 silicone, seat the temperature probe, rubber-band it; everyone says the BMS locks the board on power loss, tried twice and got lucky both times; leave it a day for the glue to cure.

   08:43 – 09:53
8. 
   8. Glitch: 0% for an hour; check the battery report

   Next day the battery voltage reading seemed to conflict with the BMS, showing 0% for an hour; Win+R and the powercfg battery report show the capacity history — this battery had barely been used; re-celled March 3, fully discharged March 4.

   09:41 – 10:55

Bill of materials

Measured on camera

Creator’s tips

• Measure each group’s voltage to spot imbalance instead of only looking at the total 01:25
• The whole cell can is negative and needs no spacer; the positive end must get insulating paper 03:00
• Much higher capacity goes unused because the BMS has current sensing and limits; go slightly higher (2600 → 2900) so it is still near full after two years 01:58
• Use the powercfg battery report to check capacity history 10:09

Pitfalls

• With no jig the layout goes crooked and shorts easily; the three-cell junction is hard to handle 03:24
• After re-celling it showed 0% for an hour, suspected conflict with the BMS / fuel gauge (comment says the battery-management chip data may need reflashing) 09:50
• Brand laptop batteries may lock the board on power loss; not locking twice here was luck 00:29

Li-ion safety

• Nickel strip is very sharp 07:00
• This layout shorts very easily 06:16

Quotes

Cost & time