Rebuilding a Battery Pack (PC-9821Lt2)


Lithium battery pack is the most hazardous component of a modern consumer device. An attempt to tap into it can lead to an explosion or fire. What's worse: DIY battery packs are not thoroughly tested in labs and thus have no guarantee. Even without human error, it is possible to overheat and start a fire when charging or operating. Follow this article at your own risk.


A month ago, I got an Lt2 from a friend of mine. I immediately fell in love with that cute machine and use it to do all sorts of tests and play games. Now, an important Touhou event is to be held in Shanghai the next month (and ZUN himself will be there, woohoo!)  I decided to take that laptop with me to kill some time during the train ride.

However, just like any vintage laptop out there, her lithium-ion battery is long dead, rotten, and gives a strong odor. (Luckily, the battery is contained in its own plastic container, so it didn't contaminate anything underneath.) The battery pack is built with standard 18650 lithium-ion cells, which is easily rebuildable.

Since a long time ago, I came to know that soldering on a battery's terminal will significantly compromise its life expectancy and is quite dangerous. Spot welder - a tool that welds things together using short but very high power pulses - is needed. Thanks to the development of high-power, fast, and low-cost MOSFETs, it's quite cheap to produce digitally controlled pulse welders today. The one I'm using is available on Taobao (the domestic version of Aliexpress) as a kit. A disadvantage of this portable welder is the painfully slow charging cycle - a price to pay for portability and cheapness.

It works. Now it's time to clean up the battery compartment. Lt2 has a modern battery pack design, completed with all sorts of protections. After a brief examination of the structure, I'm confident that nothing is stopping me. Eww…that's what I meant by "rotten."

After removing the intelligent part of this battery pack, there's one extra component to take care of: a thermal fuse. I think it might be a better choice to include this protection feature in my work, so I desoldered it. It's important to photograph the internal structure for future reference - at least make sure the polarities are correct.

(Oh well, it might not be a good idea to touch it with your bare hands. But it's lithium-ion, which is kinda sorta harmless. I guess. Remember to wash your hands after doing this! )

It's never a good idea to buy unbranded cells. But branded ones (here I'm using Panasonic cells) come at a premium. After placing the order online, I suddenly realized that I could simply take apart some portable phone chargers and reuse their cells. Silly me.

Put everything together, and we're ready to go. Unfortunately, the new cells are still slightly larger than the original ones. As a result, I can't perfectly mate the two plastic enclosure parts together anymore. I decided to leave it as-is. (Luckily  the battery door still closes, a tight fit.)

When it comes to testing, at first the new battery pack doesn't work at all. On average each cell only produces 3.6V, much too low for a lithium cell (normally ~4.0V). The only sign of life is that when charging, it draws 1A continuously from my linear power supply. 13.5V times 1A is a lot of power, had it not been for the battery to absorb all the energy, something would have got really hot. So I'm confident that the charging circuit is working correctly, all I have to do is to wait. After an hour or so, the output power is enough to sustain normal operation.

As conclusion: not a difficult build. If wired up correctly, one can revive a totally dead battery pack with little effort. Up to this point, I'm still testing the longevity of my new battery pack.

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