Thundersky Impedance values

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Richo
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Thundersky Impedance values

Post by Richo »

Does any one know the impedance of the different LFP cells?
There seems to be a void in this information on the net.

I would like to complete the table below:

TS LFP 40Ah 4.8mR 6C 2.5V 600W
TS LFP 90Ah 1.5mR 5C 25DegC
TS LFP 160 Ah xx mR
TS LFP 200 Ah xx mR

Edit: Update Table
Last edited by Richo on Wed, 24 Jun 2009, 19:08, edited 1 time in total.
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Post by coulomb »

I would say for our one TS LFP 40 AHA: 3.3 mR when warm and the surface charge is blown off. I calculated 33 mR for when it's just charged; it probably goes way higher than that if charged to say 4.0 V.

(Edit2: But of course that high internal resistance only lasts a few minutes, and probably disappears completely if you give it a quick 5C or higher discharge. It's just an anomaly just after charging that gives the cell an "artificially" high terminal voltage.)

Towards 100% discharge (2.5 VPC @ 6C), I measured 4.8 mR, but that was with about a 30 second delay in measuring the voltage after the load came off.

This seems to agree with the 3-4 mR generally accepted for these cells. They probably vary a fair bit from individual cell to cell, and with temperature.

We may have more on this tomorrow, particularly at low temperatures. The cell and the cable are in separate freezers tonight. Image

Edit: as a first approximation, the other sizes might be inversely proportional to capacity. So 200 Ah might be 40/200 * 3-4 = 0.6 - 0.8 mR.
Last edited by coulomb on Wed, 24 Jun 2009, 18:59, edited 1 time in total.
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Thundersky Impedance values

Post by Tritium_James »

We measured 1.5mR at 5C discharge from about half full for the 90Ah cells at room temperature.
Last edited by Tritium_James on Wed, 24 Jun 2009, 18:59, edited 1 time in total.
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Thundersky Impedance values

Post by Richo »

Well I guess another point for the end game would be what is the maximum power output of each cell.
Obviously without destroying it.
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Thundersky Impedance values

Post by Richo »

coulomb wrote:The cell and the cable are in separate freezers tonight. Image


Are you using a 4-wire measurement?
Or do you take it out then do the test?
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Thundersky Impedance values

Post by Richo »

coulomb wrote:As a first approximation, the other sizes might be inversely proportional to capacity.
So 200 Ah might be 40/200 * 3-4 = 0.6 - 0.8 mR.


I wonder if anyone can convince Rob to take a cell out of his Mustang to do a quick test on a 200Ah cell Image
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Thundersky Impedance values

Post by antiscab »

Richo wrote:
I wonder if anyone can convince Rob to take a cell out of his Mustang to do a quick test on a 200Ah cell


that might not be necessary, hes got a few floating around anyway :P

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Thundersky Impedance values

Post by coulomb »

Richo wrote:Are you using a 4-wire measurement?
Err, we have a multimeter right on the cell, if that's what you mean. It's not quite the same as measuring a resistor.
Or do you take it out then do the test?

The idea is to wear welding gloves and put the circuit together on the bench, with the wires still cooled down from the freezer. I hope they don't snap...
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Post by coulomb »

After being in the freezer overnight, the thermometer reading -4°C, we got these figures for our TS-LFP40AHA:

3.26 V dropping to 2.11 V with 98 A load, for 12 mR internal resistance. That's a big change!

Edit: We also had some older data not published. After being in the fridge overnight, near 100% charge, thermometer reading 9°C, the internal resistance was 7 mR.

With ice water and some time later, after a complete discharge and ~50% charge, the internal resistance ranged from 5.4 mR (8°C indicated) to 4.9 mR (18°C indicated), increasing slowly over time as the temperature increased.
Last edited by coulomb on Thu, 25 Jun 2009, 08:01, edited 1 time in total.
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Post by Johny »

Good work guys. That fits with Peter C's empirical observations in Canberra.
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Post by coulomb »

To summarise various internal resistance measurements, most at about 50% SOC:

Image

The "curve" is basically 1/x, with appropriate scaling and offsets, as shown at the top of the image.

Edit: here down is new.
I should point out that the "temperature of a cell" is very hard to measure. Minutes after pulling the cell out of a freezer (-18°C?), the temperature measured some -4°C; that was with the wires cool too (also in the freezer overnight, but spent an hour or so wrapped in newspaper). So treat the temperature as very approximate.

You can see that the cold from the cell was being conducted out here:

Image

Note how the cold of the cell is causing the water in the air to condense around the wire, but only up to a certain distance from the cell.
Last edited by coulomb on Thu, 25 Jun 2009, 14:55, edited 1 time in total.
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Thundersky Impedance values

Post by Richo »

Bet you wish you had our envrio chamber
Liquid nitrogen cooled to the desired temp.
And nitrogen wont leave the condensation.

Nice results - good work.

Are you expecting your cells to be exposed to such a cold climate?
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Post by weber »

Richo wrote: Bet you wish you had our envrio chamber
Liquid nitrogen cooled to the desired temp.
And nitrogen wont leave the condensation.
Image (Drooling emoticon) :-B
Nice results - good work.

Are you expecting your cells to be exposed to such a cold climate?
Thanks. No we just did it for the folks down south. And to get a better idea of the general trend. But we really need to have a thermometer inside the cell, or as you say, do the tests under stable equilibrium conditions. So please, take the temperatures at which we give those internal resistances with a very big grain of salt.
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Post by coulomb »

Using the Weber and Coulomb Standard Method for voltage sag (internal resistance normalised for battery capacity), we did some measurements on some Sky Energy SE40AHA cells today.

Our method is as follows: First wait for the voltage to settle down (after charging) or settle up (after discharge). I like to see no 2mV rise or fall in about a minute. For example, on a 3.75 digit multimeter (displays 3.XXX volts), the last digit can change from say 3 to 4 to 3, but not 3 to 4 to 5. Or of course it can stay on a 3, or just go 3 to 4 in one minute.

Note the voltage. Apply an approximately 3C load, and start a timer. At the 5 second point, note the voltage and current, then cell temperature if possible. We try to measure near the middle of the largest surface, with polystyrene foam surrounding the temperature probe. (It also helps to hold the meter in place against the cell.)

Calculate as follows: sag/C = (initial voltage - voltage at 5 seconds) / (current at 5 seconds) * capacity (in Ah).

So for example: our first real test. 40 Ah cell at 100% SOC (ambient temp = 15.9°C) measures 3.345V intially. At 5 seconds, we see 3.012, the current is 136.6A, and the temperature is 17.2°C. We calculate (working in millivolts):
(3345 - 3012) / 134.6 * 40 = 98 mV/C @ 17.2°C.

This compares with Thunder Sky measurements of 176 mV/C @ 18°C, and 135 mV/C @ 25°C (the 90Ah mearurement).

This can be converted to milliohms by dividing by the capacity:
For 40 Ah: 98 / 40 = 2.45 mR
For 90 Ah: 98 / 90 = 1.09 mR   (all at 17.2°C)
For 200 Ah: 98 / 200 = 0.49 mR

At 50% SOC, we measured 3.295 and 3.070V, 130.2 A, 33.1°C, for a sag of 69 mV/C @ 33.1°C. This compares with a Thunder Sky measurement of 132 mV/C @ ~30°C (not sure at what SOC, sorry). When this cell cooled down (we used the freezer to help it), we measured 131 mV/C @ 11.9°C and 126 mV/C @ 14.4°C.

The above is one of our "runt" cells; according to the provided documentation, it had the second worst internal resistance of all (given as 0.79 mR, using their crazy measurement at 1000 Hz). By contrast, one of our best cells (at "5.3 mR @ 1000 Hz) measured 106 mV/C @ 16.2°C as delivered (approx 50% SOC). Also 101 mV/C @ 16.5°C.
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Post by weber »

coulomb wrote: Using the Weber and Coulomb Standard Method for voltage sag (internal resistance normalised for battery capacity)...
I know you were being facetious. But it might be better to call it the "5 second 3C discharge method". Image

Here's some possible clarification for others re "sag per C". If the discharge were exactly 3C then the "sag per C" would be obtained just by taking the difference between the "before" voltage and the "5 second" voltage and dividing by 3. But since it's not exactly 3C we need to (in effect) work out exactly how many C it is before dividing, i.e. multiply by "C" and divide by the actual current, as you said.

One "C" here is of course the current required to fully discharge the cell in one hour, also called the "one hour rate" of current. This is obtained by simply dropping the "h" for "hour" from the cell capacity in amp-hours. But should this be the actual capacity of the particular cell under test, or its nominal capacity as given by the manufacturer?

I feel strongly that it should be the nominal capacity because otherwise you would be "rewarding", with a smaller sag-per-C figure, any cell that happened to be lower than the nominal figure. For example, Coulomb ordered a 40 Ah high-rate cell from China HiPower for testing, and only after we measured it to be only about 32 Ah and published the result, did a HiPower representative tell us that it was to be expected that the high-rate cell would have lower capacity.

This was quite odd, because we initially asked for a 30Ah cell as it would have had better dimensions for the MX-5, but they encouraged us to order a 40 Ah cell for testing so it would be a direct comparison with the Thunder Sky which they had already seen our test results for. So 40 Ah was ordered and paid for, and a direct comparison it shall be.

Oh, Coulomb, there's no need for anyone to note the temperature at 5 seconds, only the "before" temperature. It will not have changed. When I wrote down a second temperature on any of the tests yesterday it was sometimes many minutes after the 5 second burst. It was just the highest temp I noted after a burst. It takes that long for the heat to propagate out from the interior. These readings weren't very systematic so please ignore them and use only the "before" temperatures. I'm sorry I didn't explain that sooner.
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