Gonzalo A. de la Vega

Fixing a Palm Tungsten's battery

This is just a description of what the autor did. Don't try to reproduce this if you don't know what you are doing and are not willing to take the risk to brake things. The author takes no responsibility for any damage resulting from reproducing this experience. Improperrly charging a Lithium based battery may result in fire. You do this at your own risk.

I recommend you have a look at Battery University to have a good idea about Li batteries.

When I "bought" my first Palm, Tungsten T, bluetooth enabled (see the linux part), I did so by exchanging some of my about to expire American Airlines miles, and found a quiet cheap one in Amazon.com. It arrived home , I charged it 3 hours, only to find out that it didn't charge at all. The battery was "bad". Actually, based on some knowledge about batteries and some info form Li+ charging ICs, I found out the battery was charged bellow 40%, and wouldn't  charge with the integrated charger.

After fighting PalmOne's support service (IT SUCKS by the way), for a week only to know that they would not cover the warranty because I paied it to cheap! I decided to fix the thing my self. Two hours later it was working great. Here is how.

What happened was that the device was probably stored for a long time, and the battery charged droped too low. When that happens, the usually in battery chargers can not charge it. I'm not sure why, but I guess these small chargers can't deliver enough current, to widthstand the short circuit the battery represents when completely depleted.


The thing is that I needed to charge the battery somehow, over 40% (~2.7V) of total charge, that is called "conditioning" or "preconditioning" a Li+ cell.  So I did this.

Let's get to it

First of all took the device apart following this intructions.

Note that the battery leads (red and black), are actually the charging circuit leads, so you won't be able to measure voltage with the battery depleted, and it is not possible to condition the cell because the charger is still on the way. I Took out the battery and took it apart. That means removing the sticker that seals it. The circuitry cover will come out easilly. The sticker has enough glue to stick it again, but if you want to do this repair, you may want to have some tape at hand.

Once I could see the actual battery leads (metal strips soildered into the board), I was able to charge the battery. But that has to be done with limited current  (90mA), and a voltage source of about 4V (less that 4.2V). To make a constant current source I used a 2N2222 transistor a resistor and potentiometer, to calibrate the current (the 2N2222 current gain is suposed to be ~200, but the one I used was ~280).

The total base resistence should be:

Rt=hfe*4V/90mA=280*4V/90mA=12.9kOhm

So a 10k resistor with 5k pot was fine.

To calibrate the current source the battery was replaced by a cable. With a power source connected (3.5V or maybe 4.5V or 5V with a series diode), and a multimeter as a current meter, I calibrated the current to 90mA with the potentiometer.


Then it was time to plug the battery, with two cables (red and black, for standard positive an negative), twisted arround the battery leads (metal strips) like in the picture. Note the polarity, I haven't tried, but I understand, Li ion batteries break inmediately with polarity inversion.