Li cell discharge tests

Still waiting for my SE cells to arrive, hopefully tomorrow, so in the meantime I built a test rig based on the water cooled load I saw on here somewhere!
I hooked up a Thunder Sky LFP40AH cell and did a 120 A discharge test. I used a Nangfeng contactor, courtesy of Zeva to keep it all safe, a Radio Shack 4 1/2 digit DMM to measure the cell voltage (4-wire) and a Jaycar Clamp Ammeter to measure the current. Here's the numbers for those who are interested:

Thunder Sky LFP40AH Discharge test               
Time     Volts          Amps
0     3.571          132
1     3.01          125
2     2.995          123
3     2.99          122
4     2.988          122
5     2.988          121
6     2.989          120
7     2.987          119
8     2.984          119
9     2.98          119
10     2.976          119
11     2.967          120
12     2.964          119
13     2.954          118
14     2.948          118
15     2.941          118
16     2.932          118
17     2.919          118
18     2.9          118
19     2.881          117
20     2.85          116
21     2.802          114
22     2.728          112
23     2.591          107
24               
25               

Had to keep adding water to try and keep the current fairly constant - it got pretty warm.
The cell itself felt barely warm to the touch at the end of the test.
As soon as I get a SE cell, I'll set it up on the same rig and give it a try.
Tried to upload a photo, but got "Permission Denied"

Originally posted by Nevilleh Had to keep adding water to try and keep the current fairly constant - it got pretty warm.

I think you might need some more water; we use about 1.5 litres. I guess we could use a bit more. We find that stirring the water also keeps the current up a bit. I guess the ultimate would be a 4L ice cream container with a small motorised stirrer.

Originally posted by coulomb Permission denied: check this post of mine from a week or two ago: http://www.aeva.asn.au/forums/forum_posts.asp?TID=1298

Thanks for the tip. I did take a long time as I had to download the picture, open Photoshop to squash it up and then try and upload it. I'll get everything ready before trying next time.

Here's a picture of my test setup and the water-cooled load! (Thanks Coulomb)



The Power Supply on the right is 12v for the contactor which is just behind it.



Edited by Nevilleh - 17 July 2009 at 6:08am

Originally posted by coulomb If you did a test and it really went from 3.2 to 3.0 at 120A, then indded you have 1.7 mR, or 67 mV/C. But this is about 40% (at 18°C) or half (at 30°C) what we have found, so I doubt that you would find a pair like that.

The advantage of using a contactor was that I could switch the load off and on at will. I turned it off for a few seconds at each minute and observed the cell voltage rise which is why I said that it stabilised around 3.2v (after 3 minutes). This technique also means that I am measuring the voltage before and after applying the load, as you said.
It does seem to me that this figure is realistic, reflecting the actual voltage drop under load whereas using a figure of 4.3 milliohms does not give the observed performance.
The cell was charged to 3.65v only, not the 4.2v recommended by TS.
If you look at their published discharge curves, you will see that around 3.2v at .3C discharge is the plateau which is reached after 20% discharge and this correlates quite closely with what I measured.


Edited by Nevilleh - 17 July 2009 at 6:35am

Oh, right! I thought you just used 120A as the current. Well done.

weber, can you short your load and see if the CB trips ?   
Would a fuse blow in this application ?

Just thinking on the relevance of these tests to the EV....
Nevilleh method (resistance measurement at turn off ?) is what I would want to know since my cells (other than first few minutes of the drive) would likely be in Tamb(internal)(nev) condition.
Perhaps there is common ground.

weber, you will probably explode back ! but any tests you guys end up with comparing cells need to be very EV application appropriate. (App.ev.) That is not to say tests so far aren't. Just kept it in mind.

It's all interesting stuff, isn't it?
I am trying to simulate the load an EV would put on the thing by running it at 3C and then "lifting off" monetarily with the cells at "garage" temperature, ie 17 deg C. Now I stand by my internal resistance measurement of 1.67 milliohms and the amount of heat generated in the cell then is 120 x 120 x 1.67 x 10^-3 = 24 watts. Don't know what the Specific Heat of the internal chemicals is, but if it were water and the weight is say 1.5 kg, then 24 watts applied for 23 minutes will raise its temperature by 5.26 deg C. Not a huge heat rise, is it?
Solid Li and Ethanol (might not be relevant, but we can always drink it )both have specific heats of about half that of water, so if the thing is built out of that sort of stuff, the temp rise would be about double, or say 10 - 11 deg C. Enough to have quite an effect, according to Coulomb. But it does drop the internal res, so that can only be good.
Still no sign of my SE cells, can't wait to do a comparison.

Just as an aside, I am using 135 cells wired as 3 parallel cell blocks with 45 such blocks in series. ie 45 x 120 AH blocks. I am doing this for 2 reasons: 1) the 120 AH cell is too tall to fit in my available space, and 2) I hope one day to shift to an ac motor and I can then use my 135 cells in series to get most of the way to the required voltage.

If these things deliver 3C with only a .2V drop , I can suck 360A at 3V out of my cells, ie the output power is 3v x 45 x 360A = 48.6 kW. That can be drawn for 23 minutes!

OOps leap frog post there. The 3V applied to weber's fuse blow topic.
I know, I should have quoted.

Maybe "hard to reconcile" rather than utterly meaningless ?

k type thermocouple multimeters are cheap.

Originally posted by Nevilleh Of course, I forgot that weber is "always right". He says he is, so he must be, or he wouldn't be? Almost an Einsteinian paradox?

I'm a bit worried that there was no smiley or winky after that. I hope you are aware that it was ACmotor who joked recently that I make him mad because I'm always right. This was in the thread on rewiring an induction motor. I was just playing along with his joke and making fun of myself. I guess I'd better stop that particular line.

See what trouble you've got me into now, ACmotor.

Originally posted by Nevilleh Originally posted by weber 1. A voltage-sag or internal resistance figure is utterly meaningless if you don't know the internal temperature of the cell at the time. The figures can almost halve with a temperature increase of only about 10 degrees Celsius. One day we'll have the relationship worked out so we only need figures at one or two temperatures and we can predict it at any other. Self-contradictory! How can it be "utterly meaningless" in one sentence and "almost halve" in the next?

Good point. Replace "utterly meaningless" with "fairly meaningless".

Sorry I'm addressing your posts in reverse order. I'll get there eventually. My point above is that internal resistance changes quite significantly with temperature, but I don't know the relationship yet. So you need to know the internal temperature for the measured result to be meaningful. More to come.
[Edit: formatting quotes properly]

Edited by weber - 17 July 2009 at 4:49pm

Yes, the temp relationship would be handy to know.
I think I read somewhere on here that a guy with an operating vehicle found that his BMS gave a low voltage alarm on cold mornings. He also said the alarm point didn't change very quickly as he used the vehicle, implying that the cells were not warming up all that much with quite heavy current drains. He would have been sucking at least 100A to even drive the thing and that didn't seem to create a great deal of heat in the battery.
When I did my test I stated that the garage temp and hence the cells (been there for two weeks or more) was 17 deg and it does not change much even on cold nights as it is insulated and part of the house. The cell I tested did not get appreciably warm to the touch. Not scientific, I know, but enough to tell me that extreme heating wasn't going on. Next time I do it, I'll drill a hole in the plastic case and stick a thermometer in there! I'm not prepared to open the case and put a probe inside.
But if my internal res figure is near right, and I reckon it is, the internal heating from 24 watts for 23 minutes is not very great.

I appreciate all the great ideas and the work that is reported on this site. I am working here in quite an intellectual vaccuum and am entirely on my own as I don't know anyone else interested in this stuff, so this is the only way to absorb/express ideas!

I just want to add the

that I omitted previously!
Also, I set out to try and measure the internal res of an SE cell under a typical EV load and measured a TS one instead as the SE hasn't arrived yet. The comparison will be interesting and it will be done at the same Temp!!!!!

Edited by Nevilleh - 17 July 2009 at 1:09pm