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...

  2. Wonderful!! this is really one of the most beneficial blogs I’ve ever browsed on this subject. I am very glad to read such a great blog and thank you for sharing this good info with us. what laptop is right for me

  3. On the off chance that your tablet is running moderate and drowsy and programs are setting aside a long opportunity to load, or solidifying up then you may require a memory redesign, or RAM.

  4. I could donate it to a recycling center, but if it is in a workable condition, I think I would like to at least make a few bucks on it? So if I have decided to sell my laptop, there are several ways I can sell it, but there is something I need to do before I try to sell it. Laptop Tube

  5. First You got a great blog .I will be interested in more similar topics. i see you got really very useful topics, i will be always checking your blog thanks.

  6. Oh great great , I have so many these kind of batteries , finally I got solution for it
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