Replacing Lenovo laptop lithium batteries

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.