Hello DonShock,
While series charging of NiMh cells is good, I would not recommend it for Li-Ion cells, unless you have a way to balance the cells at the end of the charge.
A series Li-Ion charger charges to a maximum voltage. It is possible for your cells to age at different rates and develop different internal resistances. Since the maximum voltage is the sum of the voltages of each cell, you could end up with cells charging to different voltages.
Ideally, when charging 4 cells in series, all of the cells would end up at 4.20 volts and the maximum voltage would be 16.80 volts. However, what happens if there is some additional resistance in a connection or cell? If we have 4 cells at 4.0, 4.1, 4.2, and 4.5 volts, we still have the maximum 16.80 volts, but the cell that is at 4.5 volts will most likely "rapidly vent with flame."
The RC people have burned down houses, garages, and automobiles charging like this.
The solution is to charge Li-Ion cells individually, in parallel, or to use a charger that monitors each cells voltage during the charge and balances each cell so none of them ends up overcharged.
Protected cells will work, but you still may end up with an imbalance that will hurt performance.
CPF people have an excellent record using Li-Ion cells. Please, think through your Li-Ion charging so we can continue to enjoy that excellent record.
Tom
