Protected Lithium Ion cells in series

[Burncycle]

Out of curiosity, how paranoid do I need to be that cells are matched very closely if they're going to be used in series? I have two 16340 that are protected and normally charge them at the same time but they're not always spot on to each other voltage wise after resting.

 

[sbj]

I use unprotected 18650 in my 4s lamp. I balance them from time to time when discharged! at approx. 3.0V/cell. Then I charge them serially without balancing with an RC charger up to approx. 4.05V/cell.
In this way I hope to prevent a cell from going into deep discharge prematurely.

However, when I bought them new, I sorted and labeled two sets according to capacity. But the differences weren't very big.

If you have a good single-slot charger with an adjustable end-of-discharge voltage, you could do this also manually.

Then charge the batteries at the same time and switch them off in good time before the charger starts charging more into one cell than the other.

On the other hand, your cells are protected and would only switch off very late, but at least.

It's important to always check the battery voltages. On the go you can briefly switch to high or turbo mode. If the lamp goes out after a short time, I would then stop using the lamp further if possible.

 

[Kestrel]

I wouldn't worry about in the least; the cell protection is another safety layer.

A good search of the CPF archives will find discussion on the observed behavior of cells becoming mis-matched during series operation (no, seriously). The going theory is that the battery chemistry is operating at a different efficiency for the cell that is closer to the heat source.
If that is occurring, I doubt that a few hundredths of a volt from typical charging variations will make much difference. Furthermore, both cells will be very close to expended by the time one is starting to reverse charge. No energy = no explosion.