Saturday, December 21, 2013

Dealing With a Stubborn Laptop Battery

I own an older model Panasonic Toughbook CF-28. I picked it up a couple of years ago for around a hundred dollars or so. It has a Pentium III 800MHz processor and 512MB of RAM, nothing to phone home about. What it lacks in performance it makes up for in its' rugged design. It is a military specification laptop that is capable of withstanding short drops, extreme temperatures and light water splashes. It also has a touchscreen. In other words, it is a fun toy.

This laptop was missing the battery when I acquired it. I decided to opt for a cheap replacement available on eBay for around fifty dollars. When the battery arrived, I was quite pleased with the quality and performance of the product. I was easily able to get more than 5 hours of battery runtime out of the laptop.

This laptop is far from my daily driver and I left it on the shelf for a few months. When I came back to it the battery was expectedly discharged completely. The interesting part is that it would not accept a charge despite the charge indicator being illuminated.

I decided to disassemble the battery to investigate and was able to bring it back to life!

Fixed Battery

First I disassembled the battery to expose the inner cells. My initial assumption was that one of the cells or subpacks had failed short.

CF-28 Battery Inside
 I started measuring the voltages across the various cells and subpacks only to find that there were no cells at 0V. At this point, I started to over-engineer a solution. I pulled the battery management circuit board off and started looking for datasheets to see if I needed to somehow reset the logic. I came across some sketchy websites containing vague traces of a datasheet translated from Chinese to English. Needless to say, I was not getting very far.

Power Supply and Multimeter
I decided to go back to the pack again and measure voltages. I noticed that every cell/subpack was reading approximately 2.95V. This is just 0.05V outside of the "safe operating limits" of lithium-ion cells. I decided to try my luck at giving them a boost using my bench supply. I gave the entire pack a constant voltage supply of approximately 12V for 2 - 3 minutes while monitoring the current. I stopped charging the pack once the cells reached 3.2V each.

I re-assembled the battery and am proud to report that it is now accepting a charge!

BIOS Battery Refresh Screen
This laptop is a really cool piece of gear. I like the "High Temperature" option in the BIOS menu. I imagine a laptop of this class might be used in desert environments. This concept just seems awesome to me.

CF-28 High Temperature Settings


  1. Hey just wanted to say awsome hack. I have a CF-71 and can not get a OS to install. I'm told that the "Save To File Option has been disabled". Any ideas on how I can get past it...

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