I was able to get 10 large used laptop batteries for only $8. I was able to build a 190 watt hour battery pack very easily.
Step 1: Removing the Cells and Charging
First pop open the batteries and take out the cells. You can open the plastic cases using a flat head screwdriver. Sawing the corner a little bit helps get the screwdriver in. Keep the parallel groups of cells together.
Measure the voltages and write them on the packs. Arrange the cells by voltage, less than .5, .5 to 3 and over 3.
I charge with cheap transformers I can find. This is very dangerous as you can over charge the cells. Never charge the cells past 4.2V.
If the cell is less than 3 volts you need to charge the cells at low current, less than 50mA. I use walkie talkies that usually charge 3 AAA batteries each. Charge until 3 volt and then switch to high current. After 3 volts you can charge up to 1A per cell. I use a 12 volt 4 amp charger and charge 3 cells on series.
Cell under .5 often have problems.
Step 2: Connecting the Cells
Lithium cells are great But you have to keep them between about 3 volts and 4 volts.
When you build a pack arrange the cells in parallel and series groups to get the amps and volts you need. Most laptop cells are 2200mAH and can be discharged safely up to 1C (2.2A for one hour).
For my pack I have 3 sets of 8 parallel cells in series. So my voltage is 12 to 9 and my max current is 17.6Amps. This works well for powering cheap car 12V motors. If you want more power you can add another series set and get 16 to 12 volts.
It is important that each series set has the same amount of parallel cells.
First tape your cells together in the parallel sets. Then tape the sets together alternating positive up positive down and tape it all together. I used small nails to connect the packs. Cut off the nail head, sand it and the top of the cells and solder the nail to the packs. I kept the 4 cells together from the packs. When soldering on the positive side it is super important not to get solder on the sides of the battery top. The outside of the battery is negative so the cell will short out.
I also soldered 2 14 gauge wires to the positive and negative sides of the battery. I taped the wires down so they couldn't be pulled of the battery.
Step 3: Battery Box and Padding
I glued some fibre board pieces together to make a box. I folded some padded fabric and laid it in the bottom of the box.
I rapped the battery in fabric and placed it in the box. I stuffed more fabric in the corners so the battery was secure.
The fabric helped with reducing shock and vibration but the battery can get too hot because its insulated.
Step 4: Measuring Cell Voltage
I wanted to be able to measure each parallel set of batteries without taking the battery out of the box. The first set can be measured using the negative wire and the top of the battery. The last set can be measured using the positive wire and making a small hole in the batteries plastic. You can stick a prob in this hole to measure the negative. If you measure the total voltage and subtract the two cells you will get the middle voltage.
Step 5: Testing in the Prop Bike
I installed the battery in my prop bike and it works great.