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My laptop battery is composed of 6 polymer cells in the constellation: 3 parallel pairs in series.

All cells are Coslight CA653495: 3.7V, 2,4Ah. I'm considering replacing one of the cells, which is puffed. It's puffed hard, not like it's puffed with gas(Hydrogen).

I haven't found the exact cell to buy, due to size constraints. But I've found one that fits. only that it's 2,5Ah.

  • How would changing this cell affect the overall performance of the battery?
  • Will this unbalance the battery?
  • Or will this just use up the new cell faster?

I know I can get a new battery, but I want to understand how cells work together, and learn.

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There are several reasons why you shouldn't do this.

  • The battery pack's controller will probably detect that someone has messed with the batteries and shut the pack down permanently.
  • The old cells have a way lower capacity than the new one due to aging. This will imbalance the battery pack and cause stress to the balancer circuitry. The controller might also decide that the imbalance is too big for the battery to continue operation and shut itself down.
  • "Puffing up" is just one step away from "catching fire" with lithium batteries. If one of the cells already puffed up, another one might do this as well and possibly catch fire.
  • You might damage the cells or battery controller circuit during the repair.
  • Handling unprotected lithium batteries can be dangerous.
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  • \$\begingroup\$ Safer to get a new battery pack... \$\endgroup\$ – Solar Mike Dec 6 '17 at 23:19
  • \$\begingroup\$ Thank you for the feedback. I'll go with a new battery, even considering @jack-creasey 's answer. The cells are sport welded to the PCB, and it's very hard to disconnect them without breaking the leads. \$\endgroup\$ – Qsiris Dec 7 '17 at 6:32
  • \$\begingroup\$ 1) There is no stress to the battery gauge, the vast majority of this type of pack have no cell balancing. 2) The current cannot be unbalanced since the 3 pairs are in series. The pack simply has a lower runtime to terminal voltage as capacity reduces, the new cells are simply underutilized. \$\endgroup\$ – Jack Creasey Dec 7 '17 at 16:17
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I'd suggest that if you want to repair the battery pack you should replace both the cells that are in parallel.

Since you know one cell is bad (the swollen one) then you can surmise that the other cell in that parallel pair is damaged too. In fact it may be that the swollen battery is not the initial failure. If a cell fails open for example (and it's in a parallel pair) then the other cell is left supporting both a higher than normal charge and discharge rate. This is a classical overload mode that causes swelling.

You may also have a battery management IC in the pack and it can be tricky to replace cells once the battery gauge is initialized. You may have to jumper a cell into place before removing and replacing the chosen cells.

Once you've replaced the cells you need to complete at least 2-3 charge discharge cycles to ensure the battery gauge learns the new terminal voltages.

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