Condition - Capacity Testing
Acquire a batch of identical packs!
Remove cells from packs but leave the connecting tabs-straps.
Stage #1 - determine optimal voltage
"Map" a representative cell ... determine optimal peak charge.
ex - Charge to 4.25V, (3.7V cell)
discharge at .5C for 1min, (shorter time for greater accuracy)
let sit 10sec then measure voltage,
record voltage in 100th V,
repeat.
You are looking for the point where voltage drop "stabilizes", this should indicate the relative capacity at various voltages.
You would want to limit your peak charge to within the "beefier" capacity region of the cells.
DrkAngel said:
3.7V - 4.05V takes advantage of the "beefiest" area of the discharge profile.
It also leaves a healthy "emergency reserve", for "limping" home.
Be aware! These results are for a specific cell type-formulation and might have no relation to other brands-types-formulations!
Stage #2 - charge all cells to test voltage
I charge a large quantity at less than .5C, in parallel, to somewhat above working voltage (.05V?), 4.25V for 4.20V targeted - 3.7V, 4.15V for 4.10V targeted - 3.6V.
I use a S-150-5V Meanwell modified for 3.5V-5.4V adjustable.
Monitor charging and remove any cells showing heat production.
Stage #3 - self-discharge - (bleed down) test
Cells are separated and left to sit, I leave paired cells connected.
I measure daily and segregate by retained voltage, the more days the better.
Any cells with notable drop are removed.
With drop slightly below target peak voltage are retained for other projects?
Stage #4 - quick capacity & IR test!
Apply a ~.5C drain to paired cells.
Note and record immediate "sag" voltage - this gives you a comparative IR (Internal Resistance) measure.
(Any pair with 1 bad cell will show a drastic drop and cells should be separated and bad cell trashed.)
Monitor voltage drop, at 15min intervals, 1 hour test should give a nice "comparative" capacity reading - as indicated by resulting voltage.
Run multiple simultaneously using identical discharge devices, light bulbs?, small fans?, etc?
Most multimeters have a <10A function for determining actual discharge amps.
(Not as accurate but faster-easier.
I rig cells as 3s (11.1V) and test using a 12V discharge.
Initial sag for each cell important in determining bad-weak cells - to be replaced before continuing test!)
Resulting voltage, of each pair, after timed test completed, used for comparison with total batch of identically tested pairs.