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My thinkpad 420 battery is not charging. So far I have done following.

  • Put another battery in my thinkpad and it is charging perfectly.
  • Opened the defective battery and checked all three sets of 2 cells each (it is a 6 cell battery) and I got reading on voltmeter between 3.95 - 4.05 on those sets and all 6 cells together read 11.8.

I don't know if I can suspect the board inside the battery. Please guide me. Although I have bought another battery but I want to learn something by repairing this battery. Please guide me.

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closed as off-topic by Chris Stratton, laptop2d, Finbarr, RoyC, mkeith Apr 9 at 3:07

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Chris Stratton, laptop2d, Finbarr, RoyC, mkeith
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ If you have not tested the board then do so... \$\endgroup\$ – Solar Mike Apr 7 at 4:41
  • \$\begingroup\$ there are a lot of components on the board and I am not an expert in electrical. Can someone please confirm what could be the problem for sure so I can focus on that specific part. Just some more info - If i plugin the battery and charge, windows show 100% charged and after plugging off the charger the battery dies within 2 minutes. I checked the percentage every 5 seconds and looks like it discharge gradually. This info might help to pinpoint the problem. \$\endgroup\$ – rajender sohi Apr 7 at 5:29
  • \$\begingroup\$ This problem has been addressed a number of times at Superuser, e.g. superuser.com/questions/1035641/… . Hopefully, your old battery can be used again. \$\endgroup\$ – DrMoishe Pippik Apr 7 at 5:38
  • \$\begingroup\$ I checked the provided link and if you read thoroughly my question you will notice that my problem is different then what is discussed in the superuser link. In my case one battery is working perfectly and another isn't so clearly the battery is defective. Now inside that battery, there is a small board and 6 cells. I have checked the cells and they are all in good condition that tells me that the problem is with the board. I need some guidance on what exactly to look for on the board. \$\endgroup\$ – rajender sohi Apr 7 at 5:56
  • \$\begingroup\$ Did you check if all cell pairs are really good, meaning that they hold the voltage under, say, 1 A load ( 4 Ohm load) for several seconds at least? Smart batteries are very smart, they have coulomb gauges, and count battery cycles and remaining state of health. If the battery software/firmware thinks that the battery is bad, the laptop will refuse to charge the battery. \$\endgroup\$ – Ale..chenski Apr 7 at 6:30
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In addition to checking voltage on the cell's pairs, you should try to load them with a resistive load (say, 4 Ω), and see if all sections holds any charge for longer than several seconds.

Your comment that "windows shows 100% charged" means that the battery has reached the termination voltage (4.2V per cell), maybe too soon, and the battery management electronics likely indicates the fault to OS. It usually happens when some section of the battery develops high ESR and doesn't hold much of charge.

Smart batteries are very smart, they have coulomb gauges, and count battery cycles and remaining state of health. If the battery software/firmware thinks that the battery is bad, the laptop will refuse to charge the battery. You might want to look into this answer and into an example of smart battery IC BQ8050 to appreciate the complexity of modern battery designs.

I am afraid your battery has reached EOL and needs to be thrown into recycling bucket. Some cells might be still good enough to be re-used in DYI projects.

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