Replacing Lenovo laptop lithium batteries

ClimbinElectronicsClimbinElectronics wrote 03/01/2014 at 05:56 • 5 min read • Like

The battery in my Lenovo laptop were slowly losing charge. Remaining capacity was 69% (good score after 2,5 years) but I need the really long battery time while travelling. 

The battery statistics according to Lenovo power utility:

So, I bought some Panasonic NCR18650B Cells (3400mah) for 6 dollar a piece (can be bought cheaply here in China, just watch out for fakes) and decided to put them in. Theoretically Increasing the capacity from 69% of 5.6 Ah =  3.86Ah to 6.8Ah, a 76% increase in battery life!

After breaking the casing I saw a really nicely designed and built battery pack. Being curious about the IC's on the PCB I searched online and found two trusty IC's from Texas Instruments doing all the hard work:

bq29330, a lithium battery protection IC

- bq8030, a lithium battery gauge, to keep track of the amount of charge left in the battery.

By chance I also  found a site where they tried to refurbish a battery pack from a Lenovo battery pack and it did not work because of safety features on the PCB (link). 

So checking the PCB again I found a curious small circuit which burns a non resettable fuse when problems are detected, trashing the battery. This is quite understandable, just another safety feature Lenovo build into the lithium battery I guess. Not knowing when the fuse would be blown, I had to proceed with caution. Thanks allaboutcircuits, for not letting me end up with a busted battery pack!

You cannot just solder wires to lithium-ion cells, they will just overheat and might even explode! Luckily at the moment I am living in Shanghai, so I went to a small electronics factory and spot welded some connections to the  batteries (I did it myself) and connected them together in a 3x2 batteries in series way. 

Back home, I first made sure both batteries had about the same voltage, the bad banana plug wires I used had enough resistance to keep  the balancing current under 1A.

After waiting for a night the cells from the old and new pack had the same voltage. To keep the protection circuit from detecting the change in batteries I would need to keep the pcb powered with the old batteries until the new batteries were connected. Luckily Lenovo provided some testpads right next to the places where the batteries are connected to the PCB. So I soldered wires between the testpads and the tabs on the old batteries.

Then I slowly started to dismantle the old battery pack for the solder tabs connected  to the pcb. It takes a bit of force, but you can get the tabs off in reasonably good shape. 

And I attached them to the new batteries by soldering them together with a really thin layer of solder. The plastic seperators from the old pack were also reused. 

Added some new double sided tape to the old plastic casing and put the batteries in:

I Soldered the tabs  to the PCB again, taking good care not to short circuit anything. Then desoldered the wires running to the old battery pack.

After sliding the battery back into the laptop and trying to switch the laptop on......... It booted! Luckily, Lenovo does provide a way to reset the battery gauge in the Lenovo Power Controls software:

At the moment the laptop is still charging (says there were only two minutes of charging left for about half an hour now). Hopefully Hackaday projects will let me post an update, don't know about that yet.

So, what this shows is, that it is possible to rebuild your Lenovo battery pack!

DISCLAIMER: Lithium batteries can be dangerous for a long healty life if you don't treat them with respect, so be curefull folks and do it at your own risk.



Jevro wrote 03/06/2019 at 04:19 point

Stumbled on this article when looking for possibly rebuild a 41++ for my T60 but found you're using the same 35+ as my x140e.  My only question is that any particular method when it comes to taking apart the pack so that it can be reassembled as I've not done it before?

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Marc MERLIN wrote 11/04/2018 at 20:43 point

Thank you for explaining why I can't seem to jumpstart my 78++ battery for a thinkpad P70 after replacing a bad cell (jumpstart = connect + from the last battery to the plus connector on the battery).  It seems from what you're saying that there is a blown fuse.

Question #1: can I replace or short the fuse? It's it a software fuse that I can't get to?

Question #2: how bad would it be if I connected + from the battery to the plus connector on the board, bypassing what's in the middle? I'm assuming this bypasses all protection and charging circuitry and would probably be a bad idea?

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jairbj wrote 04/10/2018 at 01:46 point

Hi friend, when you performed the "reset gauge", does the displayed battery capacity has increased?

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coon wrote 03/09/2018 at 20:23 point

Thanks for your answer! So it seems the battery is also limited to some total amount of cycles where the BMS refuses charging. Even if this is the case, it would still be worth buying dead battery packs for getting the charging / balancing electronics and then refurbish the battery using new cells. Maybe it is even better getting non original battery packs since such electronics might not have planned obsolescence circuitry in it...

Are you able to read out the total cycle count from the battery? Is it some even number? I will let you know when I am doing the mod. I will also play around with non original battery packs but first I have to get a spot welding machine I guess...

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ClimbinElectronics wrote 03/09/2018 at 13:59 point

Hi Coon, thanks for your interest. The battery kept working for a lot of extra charging cycles, but there was an end to it. After double the amount of cycles (I don't know anymore how much) it stopped recharging sadly. But it worked for quite a long time, was worth the effort. Let me know if you do the replacement and how it went.

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coon wrote 03/08/2018 at 00:24 point

Hey, nice tutorial! Currently I am playing around with 18650 cells and I am planning to do this on my T420 after I gained more experience and feel safe to do so. Does your replacement battery still work btw? Did it fully charge? Using Panasonic NCR18650B cells sound very promising since on my 9 cell battery it would mean ending up with 9600mAh, which is even 600mAh more than the original battery. :)

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ClimbinElectronics wrote 11/24/2016 at 12:27 point

Oh no. Well there is this fuse with 4 connections somewheron the PCB. You could try bridging this one (only the 2 high current pads). But don't forget the last sentence of my walkthrough ;) Good luck!

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hackyMan wrote 11/24/2016 at 09:24 point

I didn't know they detect the cells being replaced :/ I just replaced the cells in my thinkpads battery. The battery does communicate like normal, but it doesn't put out any power..

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