Even WITH appropriate equipment it is almost impossible to check for internal damage to a cell that has been dropped.
Dropped or physically damaged cells may have two problems:
If it is a "protected" ICR cell (xxxFire, cheap stuff with high mAh ratings) then the protection circuitry can become damaged. I would expect such a damaged cell to not charge at all or not hold a charge.
To protect against that, measure the voltage of the cell right after the incident - 0 volts or unstable volts -> bin it.
Then try to charge it. If the cell gets warmer than usual -> bin it.
Then leave it for a day in a safe place. Measure the voltage. If it is below, say 4 Volts -> bin it.
The other problem is the buildup of so called dendrites inside the cell. Dendrites are tiny bits of solid material that can grow over time because of chemical reactions (and other things, but I'll leave that out).
Dendrites that touch other dendrites or electrodes MAY lead to an internal short inside the cell - and if the cell is of the Lithium-Cobalt based type, then may lead to catastrophic failure.
The shock of a drop on a hard surface can lead to dendrites moving inside the cell and touching each other. See above.
We are all using "safe chemistry" cells, either IMR (AW brand) or NMC (Panasonic CGR...CH, BAK...CC, Sony US...V..). These will not "explode" when dropped, but rather "fizzle". But it is rare. I am not aware of an actual incident of a dropped cell exploding or fizzling, but it could theoretically (or even in a lab environment) happen.
Of course, if a cell is actually pierced open by a nail or screwdriver, or run over by a truck or some such, weird stuff can happen. But you wouldn't want to put that inside your REO anyway =)
Basically:
- If a cell gets warm in normal usage or charging -> Throw away
- A drop on a hard surface shortens the lifespan of a cell, because of the dendrites.
- Load dropped cells in a safe place, mark them, watch them.
- Use "safe chemistry"
