EVE 280Ah acceptable Internal Resistance?

[fafrd]

To all you EVE 280Ah cell owners out there:

-How did you measure internal resistance?

-Was there a specific SOC you targeted to measure internal resistance (90-100%, 40-50%, 20%)?

-What do you consider to be an ‘acceptable’ Internal Resistance (versus EVE’s spec of <=0.25 mOhm for AC Impedance Resistance)?

-How much variation did you have between cells in terms of Internal Resistance?

Thanks in advance.

 

[AussieSim]

I have the same YR1035+ as the battery vendors use in their videos.

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It probably took 2 months to ship from when I ordered, and another 2 months or so to arrive, by which time I had largely assembled my 8s and 16s packs based on cell shipment arrival/matching undulation of the long sides, and the resting voltages at the end of the top balance.

When the YR1035+ finally arrived it was a complete waste of time. All of the cells were 0.17 mΩ +/- 0.015 and did not predict which ones were seeming "weaker" cells on BMS behaviour.

I did find that cell 1+8 and cells 4+5 in a 8s pack behave slightly differently than the others in a "square pack" so reorganizing cells within the pack seems much more useful than IR.

 

[fafrd]

I have the same YR1035+ as the battery vendors use in their videos.

26.37US $ |Upgrade Yr1035 Digital Original Four-line Tr1035 Lithium Battery Internal Resistance Test Electrical 18650 Dry Battery Tester - Tool Parts - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

It probably took 2 months to ship from when I ordered, and another 2 months or so to arrive, by which time I had largely assembled my 8s and 16s packs based on cell shipment arrival/matching undulation of the long sides, and the resting voltages at the end of the top balance.

When the YR1035+ finally arrived it was a complete waste of time. All of the cells were 0.17 mΩ +/- 0.015 and did not predict which ones were seeming "weaker" cells on BMS behaviour.

I did find that cell 1+8 and cells 4+5 in a 8s pack behave slightly differently than the others in a "square pack" so reorganizing cells within the pack seems much more useful than IR.

Thanks, didn’t know those were available.

by any chance did you measure IR when your cells were at high, mid, and low SOC?

The other way I read about to measure IR is to take a voltage reading at 1 current level (such as 20A) and take a second voltage reading at a higher current level (such as 20A).

IR = (V2 - V1) / (I2 - I1)

When I try this method on half-charged cells, I’m getting much higher IR values than 0.25 mOhm, but I wasn’t terribly careful about the wires I used and I suspect that may be why.

Before doing a more careful job with that technique, I’m interested to know whether anyone has successfully used that approach.

And from your feedback, the takeaways for me are that 1/ your cells all met spec (<0.25mohm) and 2/ differences of up to +/-10% are acceptable as far as not having a noticeable impact on cell strength/capacity.

 

[Luthj]

Variance between cells depends on your application. 0.2C can tolerate a lot more than 1C. As far as new cells go, I think plus/minus 5% would be fine for anything by EV service typically. Though I am curious what the more accomplished builders have to say.

The most important measurement is the delta-v between cells at high rate charge/discharge.

As far as SOC for measurement, Pick either full or empty for consistency. My theory is that 100% SOC may be better, as the expansion of the laminate is at maximum, and thus any inconsistencies in manufacture will show up. Though taking them down a bit to 99% may yield more consistent results.

Lots of folks use the AC handheld meters. They are fast, which is a plus. I prefer DC IR measurements, which take a significant amount of discharge current to measure accurately. This is fairly easy to do if you are performing a capacity test. Just note that you will also have the contact resistance included, which means these DIY measurements are not readily compared to the AC/OEM ones. However for building paralleled packs which see maximum current, they are useful for picking matched cells.

 

[fafrd]

Variance between cells depends on your application. 0.2C can tolerate a lot more than 1C. As far as new cells go, I think plus/minus 5% would be fine for anything by EV service typically. Though I am curious what the more accomplished builders have to say.

The most important measurement is the delta-v between cells at high rate charge/discharge.

As far as SOC for measurement, Pick either full or empty for consistency. My theory is that 100% SOC may be better, as the expansion of the laminate is at maximum, and thus any inconsistencies in manufacture will show up. Though taking them down a bit to 99% may yield more consistent results.

From the limited number of delta V versus delta I measurements I’ve made, I’m coming to the conclusion that being in the middle of the plateau and away from either knee may be better.

I’m just charging a newly-empty cell now and at 8-9% SOC just under 3.2V (near the lower knee) I’m getting 1.09 mOhms (23mV deltaV for 21.09A delta I).

I’m going to repeat once the cell is in the middle of the plateau around 50% SOC to see if things change.

In addition, my cell is at 21C so I will warm it up to the specified 23C (25+/-2C). Will report back if the next dV vs dI is significantly different.

Lots of folks use the AC handheld meters. They are fast, which is a plus. I prefer DC IR measurements, which take a significant amount of discharge current to measure accurately. This is fairly easy to do if you are performing a capacity test. Just note that you will also have the contact resistance included, which means these DIY measurements are not readily compared to the AC/OEM ones. However for building paralleled packs which see maximum current, they are useful for picking matched cells.

What currrents do you use, if I can ask?

My approach has been to use 2.11A (Max current of my fine knob) and 23.2A (Max current from both Course and Fine knobs)

I chose these 2 values because it means not having to fiddle around adjusting current levels (fast adjustment).

Is there a problem with this and should I be using different currents from my tester?

Also, is the dV/dA approach only used with discharge current or can the same two measurements be made using delta charge current?

 

[Luthj]

What currrents do you use, if I can ask?

Whatever is convenient for the gear I am working with. A factor of 10-15 between the two current levels works. So 5 and 50A is going to give a good measurement with a meter that's plus/minus 1mv.

I saw good results with the low current level for about a minute (just outside the knee typically at that point). Then switch to the high current and record the voltage immediately. This is really important with lead and its crazy steep curve, but LFP seems more forgiving with its gradual taper.

 

[fafrd]

Whatever is convenient for the gear I am working with. A factor of 10-15 between the two current levels works. So 5 and 50A is going to give a good measurement with a meter that's plus/minus 1mv.

I saw good results with the low current level for about a minute (just outside the knee typically at that point). Then switch to the high current and record the voltage immediately. This is really important with lead and its crazy steep curve, but LFP seems more forgiving with its gradual taper.

Thanks. 22A is the highest I can go with my little 150A tester - do you think that is sufficient or is that current level too low to be meaningful?

I’m eventually going to be setting up a high-current test setup using my inverter powering AC loads, so I’ll be able to switch on and off devices like heaters and toaster overs to get however large of a delta Amps want.

I can measure the resulting voltage drop on individual cells within the 8S battery but the only issue will be that, with the difference between AC amps and DC amps and having the inverter in the pathway, delta DC Amps will be an estimate, at best.

 

[Luthj]

Thanks. 22A is the highest I can go with my little 150A tester - do you think that is sufficient or is that current level too low to be meaningful?

Definitely useful if you have a good volt meter.

 

[fafrd]

Definitely useful if you have a good volt meter.

My DMM is pretty good and has 1mV precision, so I believe I’m OK there.

Currently (no pun intended), I’m reading a 23mV delta V, so measurement precision of 2X +/-2.2% or ~9% total.

That same current range with an 0.25 mOhm internal resistance should translate to a 5.3mV deltaV, so I guess that will approach the limit of what my DMM can deliver using 2.1A at the lower-end... (~2mA of noise on a 5.3mA signal).

The main reason I want to perform the high-current test through my inverter is to identify any cells whose voltage drops more dramatically than the others at high current - this is another ‘internal resistance’ test which is less quantitative but very meaningful in terms of identifying problem cells under actual real-world load...

I was just curious what others are doing as far as internal resistance measurements and what (quantitative) ‘range’ is typical (compared to specification of 0.25 mOhms).

 

[Luthj]

For the last pack I built, I didn't even measure the cells. Just assembled them into a pack and load tested at 0.5C. Less than 15mv difference at that level, and at least half of that was probably just contact variance between the bus bars and cell terminals. Well within the tolerance level for that application.

 

[fafrd]

For the last pack I built, I didn't even measure the cells. Just assembled them into a pack and load tested at 0.5C. Less than 15mv difference at that level, and at least half of that was probably just contact variance between the bus bars and cell terminals. Well within the tolerance level for that application.

Good to hear. That’s kind of where I was headed with my inverter / high-current test.

When you say ‘less than 15mV’ delta V ‘tested at 0.5C’ does that mean you used 0C as the lower-bound? (Just voltage drop at 0.5C minus voltage at 0A)?

(by the way, I’ve discovered the ‘edit’ function and added more info at the beginning of my last post).

 

[Luthj]

When you say ‘less than 15mV’ delta V ‘tested at 0.5C’ does that mean you used 0C as the lower-bound? (Just voltage drop at 0.5C minus voltage at 0A)?

In that case I didn't measure IR (well the BMS reports a value). The Delta-V was between cell groups. I was mostly concerned that the cells were close in IR. Whether they matched the spec sheet was not of important to me, as they delivered advertised capacity and it was a low C application.

On the pack in my van, the BMS shows a 15% difference between the highest and lowest cell group IR throughout most of the SOC range. Most of that is contact resistance on my homemade experimental bus bars though.

 

[Dzl]

-How did you measure internal resistance?
-Was there a specific SOC you targeted to measure internal resistance (90-100%, 40-50%, 20%)?
-What do you consider to be an ‘acceptable’ Internal Resistance (versus EVE’s spec of <=0.25 mOhm for AC Impedance Resistance)?
-How much variation did you have between cells in terms of Internal Resistance?

As a reference point, here are EVE's minimum acceptable parameters for their "energy storage group" LF280N cells sold through their official store:

30 ~ 40% SOC

Capacity ≥ 280Ah
intra-group range ≤ 5.6Ah,

Voltage 3.290~3.305V,
intra-group range ≤ 5mV,

Resistance ≤ 0.25mΩ,
intra-group range ≤ 0.05mΩ

And from the LF280 (no N):

30% SOC condition，Measure the AC 1 KHz AC impedance
≤0.25mΩ

 

[fafrd]

In that case I didn't measure IR (well the BMS reports a value). The Delta-V was between cell groups. I was mostly concerned that the cells were close in IR. Whether they matched the spec sheet was not of important to me, as they delivered advertised capacity and it was a low C application.

On the pack in my van, the BMS shows a 15% difference between the highest and lowest cell group IR throughout most of the SOC range. Most of that is contact resistance on my homemade experimental bus bars though.

When you say cell ‘group’ do you mean you have more than one cell in parallel (in your 4S or 8S pack, ie: a 4S2P pack or an 8S2P pack?

If you are just measuring the difference in voltage between different cells supplying the same current, that is not a measurement of IR. But if all cells started off at the same voltage (whether at 0 Amps or any first lower Amp value), then the delta V you are measuring under load is pretty much the same as a delta IR measurement.

 

[fafrd]

As a reference point, here are EVE's minimum acceptable parameters for their "energy storage group" LF280N cells sold through their official store:





And from the LF280 (no N):

Thanks. I thought I had the latest EVE datasheet, but maybe not - is there a link you can share?

 

[AussieSim]

I measured IR on the naked battery terminals after top balance.

The only thing I found the IR meter useful far was to measure IR of individual assembled cells through the busbars. Because each cell is 5.5kg, my 18P top balance was 100kg, causing my trolly cart to bend. I found some individual parallel cells had a much higher IR, which was corrected by further tightening the busbars.

I should (but haven't yet) do the same for my assembled packs.

 

[fafrd]

I measured IR on the naked battery terminals after top balance.

The only thing I found the IR meter useful far was to measure IR of individual assembled cells through the busbars. Because each cell is 5.5kg, my 18P top balance was 100kg, causing my trolly cart to bend. I found some individual parallel cells had a much higher IR, which was corrected by further tightening the busbars.

I should (but haven't yet) do the same for assembled packs.

Interesting - tracking down / debugging contact resistance issues alone may justify spending the modest amount for one of those IR testers. Are you happy with yours? (Including the supplier). Do you have a link you can share?

 

[fafrd]

I have the same YR1035+ as the battery vendors use in their videos.

26.37US $ |Upgrade Yr1035 Digital Original Four-line Tr1035 Lithium Battery Internal Resistance Test Electrical 18650 Dry Battery Tester - Tool Parts - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

It probably took 2 months to ship from when I ordered, and another 2 months or so to arrive, by which time I had largely assembled my 8s and 16s packs based on cell shipment arrival/matching undulation of the long sides, and the resting voltages at the end of the top balance.

When the YR1035+ finally arrived it was a complete waste of time. All of the cells were 0.17 mΩ +/- 0.015 and did not predict which ones were seeming "weaker" cells on BMS behaviour.

I did find that cell 1+8 and cells 4+5 in a 8s pack behave slightly differently than the others in a "square pack" so reorganizing cells within the pack seems much more useful than IR.

There are several options - which did you select? We’re you happy with the vendor?

 

[Luthj]

When you say cell ‘group’ do you mean you have more than one cell in parallel

Yes, this was a 2P4S pack.

If you are just measuring the difference in voltage between different cells supplying the same current, that is not a measurement of IR.

Its not an absolute measurement, but its a comparison of the IR difference between the cells. Which for most packs is the more important than absolute IR. Of course this takes into account busbar/contact resistance, which is probably 5x more likely to cause issues than random IR variances.

 

[fafrd]

Yes, this was a 2P4S pack.


Its not an absolute measurement, but its a comparison of the IR difference between the cells. Which for most packs is the more important than absolute IR. Of course this takes into account busbar/contact resistance, which is probably 5x more likely to cause issues than random IR variances.

Totally agree. Is there any easy way to measure contact resistance other than to use on of those AC IR testers?