At my work, we have a recycle tub for used batteries. I decided to try to make use of the used laptop batteries for a little stand up electric scooter by sorting through them and finding the cells that were still in good shape.
There are many types of Li-ion rechargeable cell chemistries. (scroll down this link)
Typically, laptops (and the tesla roadster) use Li-ion cells with the CoO2 cathode. LiMn2O4 cathodes are used in the "X-treme" Li-Ion bike. LiFePO4 batteries are quickly gaining momentum as the battery of choice for scooters because they are inherently safer. LiPo (lithium polymer) is some of the most powerful, but also the most dangerous.
OK, so, the most common size of the CoO2 cathode Li-Ion cells is called an 18650 which is simply the dimension of the cylindrical cell (18mm diameter x650mm in length). Here you can see how they are laid out in a laptop battery.
These cells do not last for ever. Their capacity diminishes over time even with out going through charge cycles.
That said, some cells will degrade faster than others, so in each laptop battery, there is likely some cells still in good shape. You can see that to make this work, you would need to start with a lot of cells.
These cells had to be taken apart one at a time and remove the BMS. If the voltage of the cell was below 2V, dont bother trying to salvage the cell. It has probably become defective. Any of the cells that have over 3V have a good chance of recovery.
A new 18650 cell has ~2Ahr capacity, and can discharge 4Amps.
I decided to put 6 in parallel so that I can discharge as much as 24A, and if the cells were new (which they are not) I would have a 12Ahr battery pack.
I did not have a spot welder, so I decided to solder the cells into a 6 cell parallel battery.
I then used 8 batteries to make a pack.
Now for the safety part.
I found a really cool BMS by ecity power to use for the pack.
You can order them off ebay for $40 shipped. (search for BMS li-ion)
Software and diagrams can be found here.
It can handle 8-12 cells, up to 40A, and the voltages can be programmed by the computer for your specific battery chemistry. For the CoO2 Cathode, the cut off voltage should be 4.2V.
Unfortunately, I had trouble getting the BMS to recognize all 8 cells. While messing around with it, some wires touched and zap-pop, melted the wires. So now I am ordering another one.
Until then, I charged up each of the 8 batteries separately and hooked it up and rode it around.
None of the batteries got hot, nor did the voltage seem to drop on hills. When I get my new BMS in, I will be a little more brave on how hard I push it, and update the blog at that time.