Testing voltage drop under load to compare C Rate

[rg12]

Hey Dudes,

I wanted to know the C rate of some unbranded cells that I have and someone in this forum suggested to test the sag by topping the cells and then discharging them under the same load and compare the voltage drop to the voltage drop of the cells with the known C rate.

Here are the results, starting from 4.21v under load of 1A through the discharge mode on my Turnigy Accucel 8 charger and waiting for the voltage stabilize for about 20 seconds):
Unbranded 2000mAh = 3.77v
Panasonic 18650B (2C) 3400mAh = 3.81v
Samsung 26F (2C) 2600mAh = 3.72v

I'm 99% sure that the Panasonic and Samsung 26F are legit, tested their capacity and their weight (26F is 44g, NCR18650B is 46g (unbranded one is 42g)).

The problem is that I'm pretty sure that the unbranded one is around 0.5c (came out of a brand new generic laptop battery) while the Panasonic performed only a little bit better than the unbranded.
Let alone that, the Samsung dropped to 3.72v which is lower than the unbranded cell!

Am I missing something?

EDIT:
Just tried this with a simple small light bulb as the load and the results are different but still weird:
Unbranded 2000mAh = no load 4.21v, load 4.14v
Panasonic 18650B (2C) 3400mAh = no load 4.21v, load 4.15v
Samsung 26F (2C) 2600mAh = no load 4.22v, load 4.15v

So it seems that they are all the same which can't be in terms of C rate as I have a small 8Ah 10s pack made out of the unbranded cells and with my 36v ebike (13A at 42v=550w) got pretty hot (almost hot to touch) just from riding a few km around town (unpacked, just the cells zip tied to the frame) so I'm pretty sure they are about 0.5c

 

[Hillhater]

Just a few comments..
Whilst its a fair suggestion to do a comparison to a known (??) C rate cell,...it may not be that simple.
Remember that C rate is a ratio of Continuous discharge Amps, to Amphrs ( current flow to capacity)
You kept the same current flow for each of the different capacity cells, so you did not test at a similar C rate and so cannot expect equivalent results .
Also, there is no set "test standard" for measuring C rate, or for that matter "continuous discharge" rating...
..IE one manufacturer may be happy for a cell to discharge at 50deg C, whilst another migh limit the allowable temperature to 40deg C during a discharge test. Likewise for the maximum voltage sag allowable during a continuous discharge test.
Add to that , different chemistry cells will have different voltage profiles during discharge....some minor, others much more obvious ( A123 for example)
C rate is a "sales" figure, if you want to know the real potential of your cells , measure their DC internal resistance.

 

[rg12]

I see, so I will go look for a fat resistor then and run some tests...
Thanks alot

 

[redilast]

Almost all laptop grade cells will have a max C rate of no more than 2C. So I think its safe to assume that the cells you have are probably 2C rated. But that doesn't mean much as its never recommended to run cells anywhere near their C rate especially when they are packed into an ebike or similar pack with little to no ventilation. <1C for these types of cells is best IMO.