Hello MacTech,
Li-Ion cells are considered "dead" when they no longer can provide 80% of their initial capacity. Once you pass this point, strange things can happen and aging becomes non linear. This can be a problem in normal use and is a problem if you use more than one cell in an application.
The first step is to do a visual examination of each cell. If the cell is dented or damaged, recycle it.
Next you need to determine the initial capacity of the cell. The cells may be marked, or you may be able to get the information from the label on the battery pack.
The next step you have already done. Measure the voltage. Any cell below 3.0 volts is probably not worth the effort. 3.4 volts or higher is better.
Now it is time to charge each cell individually.
This is where things can get dicey, so take the normal safety precautions. Make sure you set aside a few hours to attend to the charge. Charge in an area that is free of combustible material. Finally, survey the area and ask yourself "If something goes wrong, what do I think will happen?" The picture I keep in mind is lighting a road safety flare. Sparks and smoke will be abundant and your charging area should be designed with that in mind. A fire extinguisher, or a bucket of sand, or a bucket of water handy is also a good suggestion.
On to the actual charging process. If you have access to a regulated power supply you can make up some leads and charge the cells. Using an accurate voltmeter, set the voltage on the power supply to 4.200 volts. The charging current is dependent on the cells capacity. The first charge should be done at around 0.5C or maybe a little lower. During the first charge, monitor the cell for heat. A damaged cell will heat up, a normal cell will not. If a cell heats up, immediately stop the charge, discharge the cell to 0 volts and recycle the cell. If the cell remains cool, you can increase the charging current up to a maximum of 1.0C. I find the shortest charging time for Li-Ion cells is at a rate around 0.7C. When the current drops to around 50 - 100 mA, the charge is complete. Remove the cell and check its voltage. A good cell will be very close to 4.200 volts. If a cell has aged and developed higher internal resistance, it will end up with a lower voltage. If it drops to 4.0 or lower, it is generally considered "dead."
Note that this voltage measurement is a resting, open circuit measurement with a rest period of around 15 minutes.
Now you have some cells that are charged up and ready to go. I do a capacity check on the cells at this time. I discharge the cells and compare their capacity now with their initial capacity. I have the equipment to do this, and log the data for future reference. If you don't have discharge test equipment, you can do a runtime test with a light that the cell works with, and test that way.
Once you have confirmed that the cells are "good" and have gone through a couple of charge/discharge cycles without problems, you then can consider trying to match the cells for multi cell applications.
As others have mentioned, Li-Ion cells have a very slow self discharge rate. Any cell that you notice a voltage drop over a few days of storage probably is developing internal shorts, and should be recycled.
Good luck, and remember to charge safely. Li-Ion chemistry is very nice in that there is no problem shutting the charger off and interrupting the charge. If you have to run an errand or leave the area, shut the charger off. When you get back, turn it back on and continue the charge. The cells will charge normally in spite of the interruption.
Tom
