2015 - My, presently, favorite developed best-fastest cell testing method.
- Charge cells
Charge all cells equally to a voltage above preferred use voltage. ... (4.20V?)
I charge 40p ... or 56p using modded 5V MeanWell (combine cells when of nearly equal voltage)
I am testing combining using steel wire and 8mm round Neodymium magnets during testing, rather than soldering.
Keep eye, or finger, on cells, remove any that start getting warm, 40p 2600mAh = 104Ah so 30A Meanwell will not create heat while charging unless cells are bad.
- eliminate leaky cells
Separate and allow cells to set and self discharge - the longer the better. (Minimum of several days )
Eliminate all with substantial voltage loss - keep only cells that maintain above your preferred use voltage.
I used to charge to 4.20V but have begun charging to 4.05V for certain cells and 4.15V for others.
(Different variations in formulation produce different optimal charged voltages)
Test cell pairs for a comparative internal resistance.
While monitoring cell voltage, apply modest drain (2A?) and note voltage sag.
Remove any with notably greater voltage sag. These are likely very poor capacity cells or one of pair is bad. Separate and test for alternate use, flashlight etc.
- Capacity rate cells
With all cells at equal voltage, discharge at a measured rate.
2014 - I began using 28s2p, discharging with 2 - 60w light bulbs (~120V DC discharging 120w = 1A=1000mA per hour)
2x2600mAh cells = 5200mAh, 1000mA discharge = ~.2C. Capacity metered by time discharging till "empty" 3.60V*
2015 - $12 90V V-A-W-mAh meter, (w/external power to maintain mAh reading), and various AC devices to provide ~1A discharge rate for 20s2p cells.
Monitor each cell voltage, remove any that fall below 3.60V*
(voltage will drop suddenly at this voltage, so monitor carefully) and mark mAh from meter.
Discharging for 2.5 hours(50%+ oem rated capacity), 3 hours(60%+ oem rated capacity), or, if very good cells, 4 hours(80%+ oem rated capacity). Then rating cells by residual voltage, works nicely.
If cells don't last 2.5 hours, less than 50% capacity, probably not worth building into pack? (unless large bulk pack?)
Mark rated capacity on cells-pairs. (eg "3H 3.82V" or "3000mAh + 3.82V")
Method provides a fairly accurate comparative capacity ... 40 cells capacity tested in 3 hours.
- Build equal banks
Recharge all cells to equal voltage.
Build banks of equal capacity.
1. Using mAh rating of cells, build banks of equal total mAh.
2. Line up all cells, best to worst. Shuffle into banks.
7s = 1234567765432112345677654321 etc **
Should provide reasonably well balanced capacity banks.
- In Service test
Test full pack discharge, if not perfectly balanced at deep discharge, reshuffle cells to equalize, or add cell-cells to any weak bank.
Quick and easy and reasonably accurate method to test cells.
See - Capacity Mapping (Optimal Charge-Discharge Voltages)
See = Cell interconnections for cans - ES Wiki