Ultrafire 2400mAh 18650 internal impedance

[HumanLumen]

All,

Just thought I would share some test results.
Using a dummy load (resistor) I measured the terminal voltage of the above fully charged cell (whilst monitoring the current through the load) and compared with it's no load voltge (both before and application of the load). Result - internal impedance of the cell was 360mOhms cold rising to 481mOhms after aprox 20 seconds. This was repeated on a second cell at a different state of initial charge (3.95v), with similar results. This explains why I was getting high current draw from the batteries when I was trying to drive a C1H dereelight - the terminal voltage was sagging.

Tried the same test with 2 AW2200mAh 18650 cells at full charge - internal impedance was 115mOhms - far better!

Cheers,

HL

 

[HumanLumen]

Ooops - missed a word out - should have read 'both before and after application of the load'

HL

 

[SilverFox]

Hello HumanLumen,

Wow, almost 500 mOhms of internal resistance is a lot. I hope you have gotten a lot of use from those cells. They appear to be nearly completely used up.

I just measured some new Wolf Eyes protected 18650 cells. They came in at around 100 mOhms.

Tom

 

[Probedude]

Was the Ultrafire a protected cell?

How much resistance does the protection circuit add to the overall cell resistance? With the low voltages being used there isn't a lot of gate voltage to drive the FET hard enough to get really low on resistances.

Dave

 

[rizky_p]

In my experiences Ultrafire 18650 protected drop its voltage even under light load.

how do one measure internal resistor by using dummy load and DMM? using some ohm laws formulas? please teach me.

 

[MatajumotorS]

In my experiences Ultrafire 18650 protected drop its voltage even under light load.

how do one measure internal resistor by using dummy load and DMM? using some ohm laws formulas? please teach me.

If i am right, you need to measure voltage without load, then sustract voltage under load, and then divide it by the current.
please correct me if i am wrong.

 

[MatajumotorS]

I have some Trustfire 18650 from DX. Measured and calculated - i got 0,168 ohms @ 0,83A load.
Tested also AW 18650 cell - got 0,129 ohms @0,85A
(acuracy i think is +/- 10%)

 

[SilverFox]

Hello Dave,

My unprotected 18650 cells come in at around 50 mOhms.

Tom

 

[Probedude]

Hello Dave,

My unprotected 18650 cells come in at around 50 mOhms.

Tom

Thanks Tom!

Dave
(also Probedude on RCG)

 

[HumanLumen]

Note that the Ultrafire cells were new - 1 charge each! Looks like it is a bad design of protection CCT (unless there are are loads of duff cells out there)

HL

 

[rizky_p]

Ok, tested my Ultrafire Protected 18650 and its capacity only 1600mah ? they sure overated their capacity. :devil:

 

[Fallingwater]

My unprotected 18650 cells come in at around 50 mOhms.

Are you talking about AWs or cheapies?

 

[SilverFox]

Hello Fallingwater,

Those are for LG and Sanyo cells.

Tom

 

[MatajumotorS]

Ok, tested my Ultrafire Protected 18650 and its capacity only 1600mah ? they sure overated their capacity. :devil:

How did you test the capacity?

 

[rizky_p]

How did you test the capacity?

I did a discharge with Hobby charger down to 3.0v @ 0.5C because any higher discharge the voltage will sag too low and terminate prematurely.

thanks.