How to measure 18650 capacity with balance charger?

[rg12]

Hey Dudes,

I have a few 18650 cells that I want to measure their capacity.

I thought about using the discharge method on my Turnigy Accucel-8 Balancer/Charger and then charge to full and see how many amps went back in, BUT if I remember right the end voltage for the discharge mode is around 3.3v and the advertised capacity on lithium ion cells is measured from 4.2v down to about 2.5v or 2.7v or something like that which I don't know how to discharge to this voltage with the balancer/charger and also don't think it's within the healthy range for a lithium ion to go down to.

So, is there a way to do this without using some random load and measuring stuff with a multimeter while timing the thing and making excel charts to figure it out?

 

[izeman]

my icharger can charge and discharge to free chooseable voltages. could be that your charger can't do that.
if you need to test individual cells you may try that: https://www.fasttech.com/products/1883206
it's good for single cells. just get a 18650 cell holder as well.

 

[rg12]

I will fiddle around with it...
If it can discharge to desired voltage, to which voltage should I discharge? what is the standard that is considered dead empty in order to measure full capacity while charging and will it damage the cell going so low in voltage?

EDIT: looked up a discharge curve chart for lithium ion and it seems that it's useless to go under 3v so I just checked my charger and it's discharge volt is 3.0v so I can now safely and easily test with it.

Thanks alot,
Roy

 

[Wheazel]

Every discharge and charge will "damage" a cell. It is a matter of how much. The upper and lower voltage regions accelerate this process.
If I were to test a cells capacity, I would likely chose different settings depending on what information I was after.

If I wanted to check how much the cell would give in my desired application, I would choose a cutoff relevant to that application.
For example 3.0V. I would also discharge with a current that somewhat represent real world drain to get an idea of expected usable capacity. Nothing silly like 0,05C.

If I were to do a test more towards the theoretical aspect, I would choose a very low dischargecurrent and discharge down to 2,5V.
Then see if my test would align with the spreadsheets.

It is probably no big deal to discharge a cell down to the 2,5-3,0V span once. But there is almost nothing to gain past that 3,0V point.
Also, remeber that if you load the cell harder, you will hit the cut-off sooner, and the restingvoltage when the load is released will be higher.
Meaning the cell isnt as discharged as it would have been with a lower current.

 

[izeman]

I think you will get some relevant information with the tester i posted. It will not be able to give 100% real live values as discharge current will be probalby around 1-2c which is less than real world.
BUT what it will provide is a way to compare cells. So if one cells shows significantly different results from others than you know that it's a bad cell.
Only testing in your bike will tell you how much capacity exactly your pack can deliver.
Edit: 3.0-4.2V will give you almost 100% capacity. Your results may vary with higher discharge current.

 

[rg12]

Yes, that's what I did, discharged to 3.0v since the all lithium ion discharge curves have almost no capacity gain between 2.5v to 3.0v and then charge all the way up.
I just want to make sure that my cells are real capacity as advertised, that's it.
So far the charger works perfect for that.

EDIT: DAMN CHARGER! it seems that it doesn't discharge right, it just takes a battery that is like 50%-80% full, shows that the voltage is dropping from 3.6v to about 2.88v and then says it's finished, BUT, the thing is that from 3.6v to 2.88v it took about 10 seconds, and then when I measure the cell it's still 3.6v, so it just lowers the voltage for a moment which doesn't sound right and due to the voltage dropping below 3.0v it then cuts the discharge and says it's finished although it discharged about 8mAh in 10 seconds.

EDIT#2: Ok, limited the discharge amps to 1A and it stays at 3.08v and keeps discharging...FIXED!