How can I know the C Rate of my Chinese 18650 cells?
- Thread starter rg12
- Start date
wesnewell
100 GW
If you know the capacity and the max continuous amp rated output, divide the amp rating by the ah capacity. IOW's a Samsung 25r is rated fro 20 amp and capacity is 2.5ah. 20 / 2.5 =8C.
rg12 said:
Do you have any pictures of the cells? Or can you at least list any markings on the cells?
If you have the ability to measure the internal resistance of the cells that would be helpful. Alternately you could do a 4-5A discharge to see what the voltage sag is like and that will give us an idea as to the rated discharge.
Also by far most 18650's on the market are laptop grade cells and are rated at 2C. Power tool cells make up a lot less of the market.
kdog
10 kW
if you know nothing about them and they have no markings, then you can only estimate based on a discharge test of your own.
I would start with .5c (1amp) then try 1C (2amps). do a continuous discharge at these rates with cells somewhat thermally insulated ( to simulate being in a pack).
when they plateau at a warm state I would consider that to be a reasonable estimate of the upper limit of continuous discharge. maybe x1.5-2 for peak rating. it might take 15-20 mins to get to this temp.
its all very aproximate but itll give you an idea for the pack you might build (totally my opinion and not to be taken as battery gospel :wink: )
I would start with .5c (1amp) then try 1C (2amps). do a continuous discharge at these rates with cells somewhat thermally insulated ( to simulate being in a pack).
when they plateau at a warm state I would consider that to be a reasonable estimate of the upper limit of continuous discharge. maybe x1.5-2 for peak rating. it might take 15-20 mins to get to this temp.
its all very aproximate but itll give you an idea for the pack you might build (totally my opinion and not to be taken as battery gospel :wink: )
spinningmagnets
100 TW
Start raising the amp-draw until a steady state is achieved where they are a little warm in your hand, but not hot. I would call that the peak, if you want them to live for any length of time. If they are too hot to touch, that is beyond peak amps.
redilast said:
Here are the markings: ASO GI10530EG 601036 and below that written NZS or N2S (can't understand the middle letter).
I don't know how to measure internal resistance, all I have is a multimeter and I just measure volts with it when I need as I'm not an expert at ohms stuff and such...
They were pulled out of brand new generic laptop batteries and I doubt that they have 2C.
What is the simplest way of putting a constant load on the cell to measure the amp draw? I don't have any ideas for things at the house that I can run on a single cell.
dogman dan
1 PW
Likely they will not perform well till you get your actual c rate in use below .5c. That has been typical of laptop cells in the past.
So 20amps, that will take about 40 ah of this stuff. You might get away with less, since a 20 mph cruise only takes about 15 amps.
With a half decent RC charger, you can test voltage drop during a discharge, then calculate IR.
So 20amps, that will take about 40 ah of this stuff. You might get away with less, since a 20 mph cruise only takes about 15 amps.
With a half decent RC charger, you can test voltage drop during a discharge, then calculate IR.
dogman dan said:
How do I measure discharge voltage drop? Do I measure the voltage of the cell before the discharge and then when it starts discharging do I need to watch the voltage on the screen right when it stabilizes on a certain voltage after a few seconds?
if so, how do I calculate the C rate from there if I get how much of a voltage drop it produced?
One little question, my charger (Turnigy Accucel-8) is not discharging properly.
For example, if I set the charger to do a single cell discharge to a set voltage of 3.0v then for some reason it stops alot before (around 3.4v) and then if I want to go lower than that I need to slow the discharge to about 0.1A to get to 3.25v and that's it, that is the max that it will go on any cell I tried including Samsung 26F, Panasonic 3400mAh and some cheap cells.
It's just that it gets to the 3.0v by sagging and not by actually getting the cells there and then it stops.
Any ideas what to do?
rg12 said:
Are you sure your charger is not discharging properly and that the voltage just isn't bouncing back up from 3v to 3.4v? A decent voltage rebound at higher currents especially is normal. At say 3A the cell would voltage rebound 0.3-0.4v in maybe just a few seconds after after the load (discharge) is removed.
There is no real "formula" for calculating C rate. The only thing you can do is compare the cells performance to other known cells with similar C rates in order to determine this cells approximate C rate.
You can take a samsung 26F (2C rated) and fully charge it and then measure the voltage of that cell under a 3A load after say 30 seconds after the start of the discharge. Then take your other generic chinese cell fully charge it and check the voltage after the same 30 seconds. If it is significantly lower, then you can assume the cell has higher resistance and can't do 2C and would be rated lower. If the voltage after 30 seconds is higher than the 26F then it is quite likely an IMR power tool or hybrid cell that can do higher loads.
About the charger, yes that rebound is the "sagging" when under loads the volts drop a bit.
The problem is that even with original high quality cells it drops about 0.3v and stops the discharge 0.3v before it needs to.
That way I can never discharge to 3.0v and even discharge to storage voltage stops at 3.88v which is not low enough for storage.
About the C rate, will do, thanks alot.
Roy
The problem is that even with original high quality cells it drops about 0.3v and stops the discharge 0.3v before it needs to.
That way I can never discharge to 3.0v and even discharge to storage voltage stops at 3.88v which is not low enough for storage.
About the C rate, will do, thanks alot.
Roy
rg12 said:
When discharging does the charge indicate a cell voltage of 3v under load or 3.3v when it cuts out? I didn't mention this before, but its possible you are dropping 0.3v across the leads you are using connected to the battery.
Another thing you can do is tell the charger you are discharging another chemistry like Ni-mh or something, then it may let you set a lower voltage point, such as 2.5 or 2.8v or something.
dogman dan
1 PW
Just look in you tube, for tutorials about how to measure internal resistance of cells. you can use the RC charger as a discharger, in order to test.
The formula used voltage before, then voltage under load, to calculate resistance. The RC charger should show the load as it is discharging, and voltage.
This won't give you the c rate. But it will give you an idea of this cell sucks, that cell is better, or,, likely in this case, the best cell in the batch still sucks.
The formula used voltage before, then voltage under load, to calculate resistance. The RC charger should show the load as it is discharging, and voltage.
This won't give you the c rate. But it will give you an idea of this cell sucks, that cell is better, or,, likely in this case, the best cell in the batch still sucks.
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