Cell testing help please

Ok. So Ive begun tearing up lots of old laptop batteries and testing the cells. I will test even the 0 volt ones as from what I have read here a few people have found good cells from the low voltage ones.

Any way my question relates to testing the cells and if I am doing something wrong.

So far I have tested over 40 cells and not found one that was high enough on capacity to use if I use the standards most of you here use. My best cell is only 1545 mah. So either Im just damnedunlucky or Im doing it all wrong.

Ive been charging the cells on a couple of imax b6ac chargers I have and Im charging the cells at .5 of an amp and then discharging them at 1 amp.
The thing that makes me think Im doing it wrong is if I take a cell Immediately its finnished discharging down to 3 volts at 1 amp I find that the cell recovers to around 3.77 volts which to me means its not properly discharged.

So tonight I retested a cell which had previously only tested at just over 450 mah but this time I discharged it at only .5 of an amp and it came out at over 1100 mah then recovered to 3.5 volt.

Now if my calculations are correct once I get my next 5 kw battery built I will have 15 kWh total capacity. The inverter in my Victron multi can draw a max of 100amps so the absolute max any of my cells should ever see load wise I think would be about .5 amp and that would be the absolute ax.
So am I testing my cells correctly or should I be testing them at a lower load.
Any advice would me much appreciated.

You charge and discharge at mA not mAh. mAh = the capacity you get afterwards.

If the cell goes fast down to 0 and bounces to 3.7 that means the cell is junk in my eyes. The IR is so high that It cannot cope with the 1A discharge current.

Its normal on old worn cells. You could potentially use them for other stuff where you dont need 1A current.
Imax B6 also gives lower numbers than Opus since it tests alot differently.


Note that if you use those cells in your powerbank and you run 1A discharge.. Then your number you get at 0.45A is useless right

So its important to test at your max current because thats what you want to have in worst case scenario.

Ok thanks so basically all the cells I have tested are no good so far :

I have ordered an opus so will see how things go when it arrives.

Ken140tdi said:

Ok thanks so basically all the cells I have tested are no good so far :

I have ordered an opus so will see how things go when it arrives.

Click to expand...

My imax clone tests very low. My FlySky tests are comparable to the Opus. However, my imax clone only charges to 4.1v, even on the 4.2v LiPo program. So there are a few reasons why you might be getting such low numbers.

I use my RC chargers to cycle and charge batteries, then I check for any self discharge, then finally my cells get fed to the Opus. As I do only have one Opus at the moment. Interestingly enough, how much charge gets fed back to the battery seems to be a reasonable indication of capacity. I charge in 4p packs, at 2a. If a pack takes 4000mah to charge on the imax, after a discharge, then they usually test 8000+ onthe Opus. The Opus does a full cycle 4.2v down to 2.8 and back again.

The voltage they recover to is called the resting voltage or idle voltage, it is the voltage they have based on their state of charge without any charging or discharging current. The difference between the idle voltage and the loaded voltage can be pretty big on worn out cells. That means you charge them to 4.20V, they will drop on their own to 4.05V, you apply the load of 1A, they will drop from resting 4.05V to loaded 3.50V and will reach the cutoff of 3.0V very soon and return to their idle voltage which might very well be in the region of 3.7V. Just a moment ago I had a cell that had a capacity of 74mAh at 1A. Its idle voltage after "discharging" was around 3.9V.

Thanks guys that makes a lot of sense.
First of all I have discovered that both of the Imax chargers I have are indeed copy ones and not the genuine item .
My chargers seem to charge to around 4.15 and 4.1 so both charge slightly differently.
I also thought we were not supposed to go below 3 volt on the discharge ???

DarkRaven. What you describe is almost exactly what I am seeing so even though my chargers may not be that accurate it looks like so far I have just been very unlucky and not found any good cells.
Perhaps it might be a good idea for me to get a brand new cell and cycle it on both my chargers then compare the capacity readings from both chargers with the new cels rated capacity to get a rough idea of how well the chargers actually test my cells.

3.0V is the general cutoff voltage that can be used with all cells easily. You can discharge most cells further without any problem, but it makes no sense since they have very little capacity in this voltage range.

Are you testing the cells with the fake B6 as well? If so you have to figure out how they work exactly. The original B6 don't do CC discharge, do your clones? Also you could test a cell with a known capacity, for example a new cell, to test how accurate they are, just like you said.

Yes I have been testing my cells with the fake b6 chargers. But the opus charger I ordered arrived today so I think I will re test the cells using this.
Perhaps I would be better to use the b6 chargers just for getting the cells up to the 4.1to 4.15 lit range then use the opus to Finnish them of and do the drain test ????

A thought I had today after reading your replies was. If a cell is charged and then gets put on to a discharge cycle and drops its voltage down to 3,5 or 3.6 volt as soon as the load gets up to 1 amp would this not mean that there will be no point in continuing with the test as the capacity will be very poor. ??

Yes, this is usually the case.

Ok. So it looks to me like the fake b6 chargers are absolutely useless for anything other than giving the cells a basic charge.
As I said earlier my new opus arrived today, so I took three cells which had been charged on one of my b6 chargers and one cell which had been charged on the other. Placing all 4 into the opus the single cell said full and the other three that had been charged on the other b6 all were only at 3.8 volts. So I checked what voltage each cell was on the opus then checked them against my multimeter. All the voltages read the same on both my multimeter and the opus which has never been the case on the fake Imax chargers.
So I finished of charging all 4 cells on the opus and then set them to run a discharge cycle. They have now been running for about one and a half hours and the lowest voltage reading cell is still 1.54 volts and all 4 cells have passed the 1500ma which is at leat double what the Imax chargers said when they had finished.
One more thing I have noticed is that all the cells get fairly warm. Where as when they were discharging on the Imax they all stayed cold.
One cell is slightly hotter than the other three but I can still hold my hand on it comfortably.

Can any one tell me how hot these cells should get while discharging and 1amp please.

Also on the internal resistance test I get pretty consistent readings but not sure what constitutes a good ir and what is a bad ir.

Note that some cells self-discharge. Before you throw out your iMax check that out
1.54V?!?!?!? That is WAY to low for Lithium cells. Also note that the Opus give alot higher numbers than any other tester. Generally 200-400mAh above real numbers but it is a number.

They should not get hotter than you can hold your hand on them comfortable...

Internal resistance test on opus can vary from 100 to 200mOhm easily!!

Check out the end of my video here and you see how easy it can diff on the opus

I guess it is a typo and he means 3.54V

It is normal for cells to get warm. As long as you can hold them in your hand and don't get the feeling that they are hot (instead of warm) then they are fine. Everything up to 40C is pretty normal I would say.
As for the internal resistance test, don't count on it too much, it is not very accurate. New cells and as good as new cells will usually measure under 100 mOhm easily, used but still good cells around 100-200, pretty much worn out cells 200-300 and the ones that have been ridden hard and put away wet will be above that, possibly even far above.
But, as I said, there is a margin so these aren't hard-and-fast rules.

Yes sorry that was a typo and I meant 3.54 volt.
I think the cell that was warmer than the others was because of the internal cooling fan of the opus as its on that side so all the heat from both the opus and the cells is drawn across the cell on the right hand side.
That first batch of 4 cells have now finished testing and one of the cells which is a Samsung originaly rated at 2600 ma came out at 2275 so Im thinking thats a good cell lol.

Its a decent cell. It got somewhere around 2Ah i would say. (Opus rates it a bit high)
At lower current it most likely give a bit higher capacity figures.


Edit: The capacity given changes due to how the battery is hmm what to say. Performing? Including how the charger is testing. If the charger cuts of at CC then it will cut of earlier at higher current for cells not being able to deliver the current needed.

^^ not trying to be clever... did you mean lower current draw will give higher usable capacity?

That's exactly what he meant

Daromer. Until you mention about your video I did not realise that all those videos were you. I have watched quite a few of your tube videos.
I tried the IR test like you said in your video but I get quite consistent results with out adding extra pressure to the connections. Perhaps this is because my opus is brand new and the springs are still strong.

At least now I have a charger that seems to produce consistent and realistic results. Good job I havent thrown away the 40 odd cells I have already tested as bad on the Imax chargers.

Another small question I have about the cells you all test as good is, when you start to group the cells together to build your packs how closely do you match them.
So lets say I want to use 2000 ma capacity cells to build my packs. What tolerance do you alow above and below the 2000 so u keep the cells as closely matched as possible. Eg. 1800 to 2200 or an even wider range ??

Yeah unfortunately i have not the same name so it may get confusing. Need to look into that

It may be that some Opus chargers have better contacts. Its worth to beware of above so you know that the numbers can change alot.

I match mine like this:


I dont know where the limit should be. Currently i have a diff that is at most 250mAh between cells in same string but thats because i have been lucky with cells. I would bet that it would work with even 500mAh diff or even 1000mAh when talking about LiIon. They will match each other out due to the voltage vs SOC.

LiFePo4 works differently so those I would be very carefull mixing. But I have NOT tested this long term and its just how my brain works in terms of the technical parts.

Ok so you dont do it how I thought it would need to be done lol.

Im using a 14s set up and my first battery was built with brand new 2850ma cells, 36 in each pack. So as my nex battery is going to be built from second hand lap top cells I assume that I should build this next battery to as close as possible the same capacity as my first one. So if I calculated correctly just over
5Kwh.
I assumed that each pack in this next battery should be as close as possible the same capacity. So in my case as close to 102600 mah in each pack.
Now to the cells with in each pack. Surly if you have some 2000ma cells and say some 1700 ma cells in a pack wont the weaker cells pull down the other cells as they would drop their voltage sooner and then the others would drain into the lower cells. ??????

I was thinking that I would need to build each of my 14 packs out of cells that had a capacity on no less than say 1900 and no more than say 2100 then build each oack so that they were all very closely matched for capacity.
Then on the next battery I could use say the 1700 to1900 cells but obviously use more cells in each pack to bring the battery capacity close to what my first batter was.

Hope you understand what Im trying to say.

I think you mix it up a bit


If you build 1 string out of 14 packs of cells.... Each of those packs should have same capacity and same amount of cells. Preferable same amount of cells in each category

If you do 14s10p then you should match so in each pack you have 2 cells of 1700. 5 cells of 1900 and then the last 3 cells of 2400. (Or whatever capacity you have)
This will end up with the samce capacity in each pack. Same amount of cells in same range in each pack.


Then what total capacity you have in next string doesnt matter that much since they will balance each other out.

And no.. If you drain 10Ah out of the pack its not split 1Ah per cell... The weaker cell wont deliver more than that they will keep the voltage the same. So you wont loose any energy going back and forth if thats what you are wondering aboug Thats the thing with LiIon that have a rather straight curve compare to SOC and will work. LiFePo4 works totally different..... But thats another story.

You can have your first string with new cells and the second with 2nd hand. The string that can deliver the most will give the most and when it settles they will all balance each other together again.

This is one way to do it. If you would do the perfect or best way you should match all strings with same type of size. all packs with same type of cells both in capacity, brand and internal resistance.. But not many of us have that luxury and have that amount of cells.