I originally came across Traxxas cars by searching for information on lithium batteries. I have accumulated quite a few "Bad" laptop batteries and a treadmill motor that I was planning on building a go-kart out of. After finding this forum, I decided to buy some Slashes and start having fun. I didn't give up on my batteries though. Here are my findings.
This is not meant as a recommendation to do what I have done. This can be dangerous. I came close to starting a fire more than a few times and smoked several cells. This is just informational to save someone time, who was thinking about doing this.
If you take apart a laptop battery, you are likely to find 9 to 12 LiIon cells. I have cells from a few different years and models so the cells are from different manufacturers but all roughly the same. All of the cells were rated at 2000Mah or 2200Mah, 3.6v, 1C charge and 2C discharge.
I experimented with putting 3 of the batteries in series and charging them with a 12V car charger. I built a rig for this so I could pop cells in and out easily. I would "balance" the cells by stopping the charge when the total voltage got to 12.3 volts and then checking the voltage of each cell. This did not work out well because sometimes one cell would get charged to 4.5V while the other 2 were less than 4. Lesson learned.
I then started measuring cells before I started and put 3 together that had a similar starting voltage. I would then charge to less than 12 volts and pull out any cells that had reached 4.2V and replace them with cells that had not reached 4.2V. Using this method, I was able to charge a pile of Cells to 4.2V.
I used coils of wire and an old automobile coil resistor as loads to test the cells. I found that most all of the cells would hold the advertised 2C or 4 amps all the way down to 3.6V.
Since Traxxas is selling 25C batteries, I started with a guess that 2C wasn't going to cut it. So I soldered 2 cells in parallel then put 2 of these in series. This is where I had my first problems. About half the time that I soldered 2 cells in parallel, one of them would short out and get very hot. At first I thought I was bridging the positive to the negative. Later I would find out that my problem was soldering 2 cells together that were not at the same voltage charge. So with my 2S2P homemade LiIon battery pack I could make my Slash move, but not for long. I had to run the car with the LVC disabled and any full throttle pull, would make the car start, but immediately cut out. This jives with what I found in my load experiments. If I tried to pull 6 or more amps from a cell, the voltage would drop below 3 volts in a few seconds.
Next experiment was to try 3s3p pack. I did this mainly because when I broke down the laptop packs they were configured with 3 cells welded in parallel, connected in series to 3 more cells in parallel. Less soldering more play time. This worked very well for the limited time that I played with it. The car ran the fastest that I have ever seen it run, then I lost sight of it behind a parked car and it went down the storm drain.
After retrieving the car I had no more time to test. I dropped the testing for a while and never went back to the 3s3p configuration, because I could not come up with a way to make it fit cleanly in the Slash. The best configuration I could come up with for fitment was a 2S5P configuration. 2 cells lay in parallel in the tray and three cells sit on top of the 2 in the tray. The Traxxas battery hold down for 3S Lipos work well for this configuration. This makes a final config of 10000Mah, 10C. From my testing this pack performs simmilarly to a 5000Mah NiMh pack, except that it runs much longer. Playing side by side with my son. I never fully discharged this pack while he went through 2 5000Mah NiMh packs and one 3000Mah pack. The weight is slightly less than a 5000Mah NiMh pack.
Things to note.
Batteries were free.
Spent $5 on TRX connectors.
Spent about 40 hours of labor and trashed 6 cells getting to this point.
If you try this discharge your Cells to a within .01V of each other before soldering them together or you will ruin one or more of the cells and may burn your house down or get injured or both.
I charge the cells using a normal peak charger at 4 amps and monitor the cells using a DVM. I stop charging when one cell reaches 4.25V then discharge it to match the lower cell then repeat untill both cells are at 4.25V.
Hope this resolves some curiosity and prevents someone from wasting a bunch of time and batteries.