Lithium Ion 18650 cell capacity

[Ronnie_Space]

Hi,

I am looking to confirm the capacity of powerbank I am looking to buy in.

The stated capacity is 19200mAh and it contains x6 18650 Lithium Ion cells.

I have fully depleted pack and charged using a USB multimeter to measure capacity, repeating four times I am measuring an average 16523mAh. This suggests approx 85% conversion rate.

I then fully charged the cells and individual discharged each cell using a Lithium Ion discharger (see photo below). I am measuring 2750mAh per cell. From what I understand this takes the conversion rate out of the process.

My question is, if the inividual cells are measuring 2750mAh, how can this claim to be a 19200mAh powerbank, as 2750x6 = 16500mAh. Surely for each cell I should be measuring 19200/6 = 3200mAh.

Any help and advice much appreciated.

Ronnie

 

[Harald Kapp]

My question is, if the inividual cells are measuring 2750mAh, how can this claim to be a 19200mAh powerbank, as 2750x6 = 16500mAh. Surely for each cell I should be measuring 19200/6 = 3200mAh.

The measured individual capacities sum up nicely to the measured total capacity. I think nothing's wrong with your measurement, something's wrong with the bold 19200 mAh statement. This is very likely a marketing issue, not a technical one.

 

[Arouse1973]

Yes the stated capacity of the cell would have been tested at a particular cell cut off voltage and cell charging voltage 4.1 or 4.2 volts. This can make a difference and as Harald has said it's sometime in the hands of marketing
Adam

 

[Audioguru]

The 19200mAh number might be the typical, some are better and some are worse since they are all not exactly the same. I agree that the cutoff voltage and charging voltage make differences.

 

[Ronnie_Space]

Thank you all for your replies.

The vendor is tell me that the cells are 3200mAh each, which is 19200mAh/6 cells.

Therefore, I am surprised with my Lithium Ion discharge circuit I am not seeing 3200mAh and only 2750mAh.

I appreciate in use the powerbank increases voltage from 3.7V nominal to a regulated 5V and there is heat lost in the process, hence a conversion rate of 85%, which I understand is pretty typical.

I just can't get my head round why when I take the powerbank circuitry out of the equation I am not measuring the full 'stated' capacity of the cells.

Ronnie

 

[Arouse1973]

To further answer your question tell us what voltage you are charging the batteries to and what cut off voltage you are terminating the discharge at. How are you discharging the cells to arrive at your calculation of capacity. Also please post a data sheet of the cell or any information you have from the manufacturer.
Thanks
Adam

 

[Ronnie_Space]

I charged the cells in-situ of the powerbank, via a 5V 1A power supply, from flat to fully charged.

I then took them out of the powerbank and used the discharging circuit pictured to discharge down to a cut off of voltage 3V.

Sorry, this is the only information I have and my understanding is limited.

Ronnie

 

[Arouse1973]

That's your problem. The cut off voltage is too high. Have a look at this data sheet as an example. The capacity graph should explain all

Thanks
Adam

http://www.elumeen.com/media/wysiwyg/produits/Elumeen_panasonic_NCR-18650B_en.pdf

 

[Audioguru]

Discharging an ordinary Li-Ion cell (Panasonic's might be different) to as low as 2.5V will destroy it. When the voltage of the Lithium batteries in my RC model airplanes drop to 3.0V then the motor pulses as a warning. The manual says that repeated flying to this low voltage will damage the battery. The battery University also says a 3.0V cutoff should be used.

We do not know the fully charged voltage that Ronnie used. 4.1V or 4.2V? The battery University says there is a 20% difference in capacity.

What about the tolerance of the capacity? Is 19200mAh the guaranteed minimum or the typical?

 

[Arouse1973]

Yes you are correct with 4.1 volts versus 4.2 volts. 4.1 volts charging voltage gives approx. 80% capacity. The extra 20% versus reduced battery life if charged to 4.2 volts is not worth charging at 4.2 volts unless you really need that extra capacity and are not worried about the reduced battery life.

What batteries are you using in your RC planes? Lithium polymer? Lithium Ion? The Panasonic cells are no different to other high quality cells that I have designed in over the years. You will not destroy a battery by discharging it to 2.5 volts. Some of the cells I have used have a capacity cut off at 2 Volts.

Thanks
Adam

 

[Ronnie_Space]

Hi Adam,

Thanks for reply and help.

OK, that's interesting, I was under the impression that going below 3V would be damaging/dangerous. The Panasonic example you have shown does have a low self discharge.

Full charged voltage of cells was 4.2V.

Any ideas for a place for LG battery specs.

I am basically just trying to verify are the cells in the powerbanks I am buying actually the capacity claimed, as commonly they are switched for lower capacity and the difference blamed on the the conversion process of the powerbank.

Ronnie

 

[Arouse1973]

All qualified commercial battery packs, well the ones I have designed should incorporate battery protection. This includes overcharge, over discharge and short circuit protection.

You will however have a cut off voltage which is what the manufacturer of the pack has decided to put in their design.

What LG battery do you have?

 

[Ronnie_Space]

All qualified commercial battery packs, well the ones I have designed should incorporate battery protection. This includes overcharge, over discharge and short circuit protection.

You will however have a cut off voltage which is what the manufacturer of the pack has decided to put in their design.

What LG battery do you have?

HI Adam,

The stated powerbank capacity is 19200mAh and it contains x6 18650 Lithium Ion cells. Cells are LGABE11865 (purple). 3200mAh apparently.

Ronnie

 

[Arouse1973]

Not sure Ronnie. Never used LG batteries. I guess you would have to Google for a data sheet if you haven't already.
Thanks
Adam

 

[Audioguru]

Here is a "good" Li-Ion battery for you:

 

[Ronnie_Space]

Ha!!!! Oh my.... I don't think what I have is THAT bad! Yikes.

 

[Arouse1973]

Maybe you have a slightly bigger version of battery inside lol

 

[BobK]

Discharge rate also affects the capacity. The stated capacity might actually be achieved only at a lower discharge rate.

Bob

 

[Arouse1973]

That is mostly for primary cells like Lithium Thionyl Chloride, where this effect is very noticeable as well as orientation of the cell. Lithium Ion cells, not so much. I haven't done a study on this but to all accounts it's not a big deal.
Thanks
Adam

 

[Ronnie_Space]

Thanks for the replies, I requested the cell spec from the vendor.

LG Rechargeable Lithium Ion Battery Model : INR18650 MH1 3200mAh.

The nominal specification tells me:

2. Nominal Specification

Item Condition / Note Specification

2.1 Energy ( Power ) Std. charge / discharge Nominal 3200 mAh

Minimum 3100 mAh

2.2 Nominal Voltage Average 3.67V

2.3 Standard Charge

(Refer to 4.1.1)

Constant current 0.5C (1550mA)

Constant voltage 4.2V

End current(Cut off) 50mA

2.4 Max. Charge Voltage 4.2 ± 0.05V

2.5 Max. Charge Current 1.0 C (3100mA)

2.6 Standard Discharge

(Refer to 4.1.2)

Constant current 0.2C (620mA)

End voltage(Cut off) 2.5V

2.7 Max. Discharge Current 10A

2.8 Weight Approx. Max. 49.0 g

2.9 Operating Temperature Charge 0 ~ 45°C

Discharge -20 ~ 60°C

2.10 Storage Temperature

(for shipping state)

1 month -20 ~ 60°C

3 month -20 ~ 45°C

1 year -20 ~ 20°C


"4.1.2 Standard Discharge

“Standard Discharge” shall consist of discharging at a constant current of 0.2C to 2.50V. Discharging is

to be performed at 23 oC ± 2 oC unless otherwise noted (such as capacity versus temperature)."


Therefore, should I perform the same discharge test on the cells down to a cut off voltage of 2.5V?

My discharging circuit uses x2 5 W 7 ohm resistor. How can I discharge at a current of 0.2C? I am guessing that means:

3200mAh = 3.2A
3.2A x 0.2C = 0.64A


Thanks

Ronnie