Recelling IBM Thinkpad T40’s battery

Replacing the cells in laptops battery can be much cheaper than buying a new battery. I had a problem after recelling my laptops battery. Read along to find out what it was and how I fixed it.

Disclaimer: This is description of my personal project and not a tutorial.
Lithium batteries can be dangerous, if they are not treated correctly. I
take no responsibility if anyone decides to do anything described in this
page. Safest way to get laptop with dead battery to work again is to use it
without the battery, charger plugged in or to get a new battery.

Prologue

I bought a used Thinkpad T40 this summer and I love how it feels. I
was surprised when I took T40 out of the box and noticed how thin it
is. The design of the body makes T40 feel lighter than it is.
After initial testing I installed newest Ubuntu (11.04) in place of the
Windows XP. I wasn’t sure how long the battery lasts and how long
installation would take, so I kept the charger plugged in. So far so
good. I noticed one major problem when I tried to test the screen by
watching few DVDs. I was lying on the couch and T40 was on my lap. I
started VLC and opened the disc. Couple of seconds later T40 shut down
without any warning. Unlike I first though, reason wasn’t overheating,
but instead the battery. I took the battery apart and made few tests
with multimeter and 10ohm power resistor. Conclusion was that the
battery wasn’t able to output even 1A current, without the
voltage dropping below 9.5V. Circuit inside the battery
shuts off the battery when the voltage drops to about 3.1V / cell
or combined 9.3V. I found out one of the three cell pairs was bad and
rest of the cells kept almost the same voltage despite the 1A load. I
replaced all six cells with “new ones”, which might not have the same
capacity as when they were new, but I was sure they could give couple
of amps without problems.

List of materials used

  • 6 identical 18650 li-ion batteries, 2200mAh or more
  • Some wire, capable of conducting few amps
  • Durable, nonconducting tape

Tools needed

  • Soldering and desoldering tools
  • Flat screwdriver for prying the battery open
  • Multimeter

Recelling

Hardest part of this project was opening the battery. I used a
screwdriver to pry it open and I must admid it was hard. I had to use
some force in few spots. There might be good tutorials in the internet
on opening those batteries, but I managed to do it without any help
(even though the battery looks like it might have been hit by a train).
Tip for anyone considering on opening a T40’s battery: Find a tutorial
or buy a new battery, if you want your battery to look like new.

When the battery was opened it was time to replace the
cells. Before doing anything else, I draw a diagram of the battery:
There are four wires between the battery’s circuit board and cells: 0V,
3.6V, 7.2V and 10.8V. Tabs in the side of the PCB weren’t in the same
order as the battery tabs (0V wire came between 3.6V and 7.2V wires, if
I remember correctly. If you are going to recell your battery, check
the connections yourself, that is the safest way). I soldered new cells
EXACTLY the same way as the original cells
were wired. The case of the battery was very tight, so wires had to be
placed carefully (I used dremel to widen the inside of the battery
case).

Major problem after recelling

I took a risk when I started recelling my battery. There are many
sites in the internet stating that Thinkpads have kind of circuitry in
the batteries, that will make the battery useless if the cells are
removed. That was exactly what happened to me. I assembled the battery,
plugged it in the T40 and pushed the power button. Nothing happened.
Not a single blink on any of the leds, fan didn’t spin nor did the
screen’s backlight light up. I opened the battery again and measured
voltage from the internal PCB and there I got about 11.3V:  I’d
say about 50% charged. Then I measured from the battery’s output pins:
0V! I thought that was it and I had to throw the battery away and get a
new one. While the battery was still open and I had some load (10ohm
resistor and small fan) connected to the battery’s connector, I made
some test. When I measured the current running throug fan and resistor,
by connecting multimeter between positive end of the battery cells and
battery’s connectors positive pin, I was surprised. Fan kept spinning
even after I removed the multimeter. I believe there are two switches
(mosfets or something like that) in the battery: One between protection
circuit and the cells and one between protection circuit and connector.
When voltage of the cells drops below 9V (3V *  3), the switch
between connector and protection circuit is opened. That switch will
close when the voltage reaches about 10V or so.
The switch between cells and protection circuit might open when voltage
reaches for example 7V (not tested), but this switch won’t close,
because protection circuit is one that usually keeps the switch closed
and it also gets power through that switch. So when the switch opens,
protection circuit won’t get power anymore and therefore it can’t close
the switch, even if the cells’ voltage returns to acceptable 9.5-12.6V.

Here is a youtube video of the “jump start” process:

Jump starting a recelled IBM Thinkpad T40 battery

Result

After first full charge I was able to use T40 for 1hr 50min while a
youtube video running in the background and browsing the internet at
the same time. That’s about half of the original battery life that IBM
promises, but atleast it is better than before recelling and I’m able
to use the DVD-drive without a charger.

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One thought on “Recelling IBM Thinkpad T40’s battery

  1. […] is in the Toyota Prius, but that’s a story for another time. In this case, [Mika] wanted to resurrect the battery from his IBM Thinkpad T40. He identified the offending cell and replaced it, but couldn’t get any juice out of the […]

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