C rating for low power 18650s

[transposon]

I am lucky enough to get as many ncr18650b and ncr18650a cells as I want at almost free of cost. I plan on building 2 large packs. I have a XL frame hardtail, so I have a pretty big triangle.

I was thinking of 14s ~10p packs. I plan on 20mph continuous speed with a few occasional seconds at 25-30mph for fun or if I need the extra speed in an emergency. I will be using one of the nucular controllers, so I believe I can cap the battery current.

I was plaining on about a 1C peak and <= 1/2C continuous draw on both cells. Is this a reasonable expectation, or should I design for a lower draw per cell (maybe 11 or 12p)?

 

[electric_nz]

Ran some new ncr18650a a while back 14s 5p pulling 1500W with occasional 50A peaks - it sagged a few volts but only ever got slightly warm even at 3C. Measured the capacity and internal resistance after 100 odd cycles and only a few percent capacity loss and no change in IR. Really good cells in my opinion but have high internal resistance compared to state of the art today

 

[transposon]

Thanks electric_nz,

That gives me hope. 10p would probably be more than enough then.

 

[e-beach]

The specs rate those cells at a out 2C. So it really depends on your controller. If it is a 20 amp controller, then your 2C max will be with a ??S 10P pack.

Better to be safe then have a battery fire.

 

[transposon]

The specs rate those cells at a out 2C. So it really depends on your controller. If it is a 20 amp controller, then your 2C max will be with a ??S 10P pack.

Better to be safe then have a battery fire.

These cells are 3AH each, so I believe 10p * 3AH = 30A at 1C or 60A at 2C. I figured I should keep it at 1/2 of the manufacturer's limit to be safe. So that would be 30A.

BTW, I will be using a Trek 4500 like one of your builds, but with a 1500W leaf rear hub motor.

 

[electric_nz]

Spec sheet has a 3C curve rating published so presumably its tested for that rate.

 

[e-beach]

...These cells are 3AH each, so I believe 10p * 3AH = 30A at 1C or 60A at 2C. I figured I should keep it at 1/2 of the manufacturer's limit to be safe. So that would be 30A.
BTW, I will be using a Trek 4500 like one of your builds, but with a 1500W leaf rear hub motor.

My bad. Thanks for the correction.

 

[electric_nz]

Out of curiosity where did you get the cells from? Mine were from new old stock toughbook packs.

 

[Hillhater]

Odd discharge curve shown there ?
I do not recall ever seeing a cell that discharges more Ah capacity at high currents, than at low current ??

Also, whilst you have the frame space and cells available to build a large 10p, pack, remember that will also be a HEAVY pack (<7 kg ?).. whilst yo could get your required 30a from a 5p, 3.5kg, pack !
You WILL fell those extra kgs on the bike.
Really it is all down to what ride range you want. !

 

[e-beach]

Odd discharge curve shown there ?
......

https://www.imrbatteries.com/content/panasonic_ncr18650a.pdf

 

[transposon]

Also, whilst you have the frame space and cells available to build a large 10p, pack, remember that will also be a HEAVY pack (<7 kg ?).. whilst yo could get your required 30a from a 5p, 3.5kg, pack !
You WILL fell those extra kgs on the bike.
Really it is all down to what ride range you want. !

I'm worried that 2C peak on these cells will lead to a very poor cycle life. I currently have a ~26A controller and the acceleration leaves a bit to be desired. I'm not sure I want to dip any lower on current draw. Am I being overly protective of my cells?

 

[transposon]

Out of curiosity where did you get the cells from? Mine were from new old stock toughbook packs.

I don't know what the packs were used for, but they were for an industrial application and only had a handful of cycles on them.

 

[Hillhater]

Odd discharge curve shown there ?
......

https://www.imrbatteries.com/content/panasonic_ncr18650a.pdf

Sure,.... but i say it again...i have not seen any other discharge curve where the high current rate indicates more Ah capacity than a low current rate ?
Can you find one ?
( Note: Obviously there is less Wh capacity at high currents because the voltage sag is greater.)

 

[e-beach]

Which graph are you speaking of. The one where the low discharge is -10c?

 

[Hillhater]

No, the standard 25C constant current discharge chart, where the 5900 mA discharge trace shows more mAh capacity than the 550mAh discharge trace ?

 

[electric_nz]

They like being warm in my experience. Performance was greatly improved once the cells were 30-40C which is probably why they have higher measured capacity.

Cycle life of the A version is dependent on 2 main variables from my experience- slow charge (1A per cell charge rate or less) and a rest between discharge and charge cycles. I ran my pack between 90%-20% for the years at up to 3C discharge with no change in IR and only a small drop in measured capacity.

 

[BatteryMooch]

Sure,.... but i say it again...i have not seen any other discharge curve where the high current rate indicates more Ah capacity than a low current rate ?
Can you find one ?

Higher discharge current levels raise the cell temperature more and that results in a lower internal resistance which reduces the voltage sag and allows the cell to run for longer before dropping to the cutoff voltage. There are limits to how much this can help a cell but a warm Li-ion cell could perform better than one at room temperature.

I’ve seen this effect in a lot of cells that I have tested.

 

[docware]

I can confirm the probable temperature effect on the capacity, see real measurement on Panasonic 18650B. However, the energy at 3A discharge is lower thanks to the higher voltage drop.

 

[Hillhater]

Higher discharge current levels raise the cell temperature more and that results in a lower internal resistance which reduces the voltage sag and allows the cell to run for longer before dropping to the cutoff voltage. There are limits to how much this can help a cell but a warm Li-ion cell could perform better than one at room temperature.

I’ve seen this effect in a lot of cells that I have tested.

OK, i understand that high discharge currents cause increased cell temperatures....
...but where do you think that energy comes from to heat the cell ?
...obviously from the cell capacity itself, so there must be less capacity discharged as external energy. !
But note:.. that those Panasonic charts state clearly that the test is at 25C temp, so i would expect that all the different discharge plots were at 25C....not just one uncontrolled temp for the high amp test.
Outside of those “official” Panasonic charts, i have not seen the “increase in Ah” at high currents on any other independent cell test chart.
Can anyone show one. ?
Docware....your 18650B chart shows NO extra Ah at the higher discharge rate, and if you had a 0.2C (600mA) discharge plot it would likely show an even higher Ah capacity.

 

[BatteryMooch]

OK, i understand that high discharge currents cause increased cell temperatures....
...but where do you think that energy comes from to heat the cell ?
...obviously from the cell capacity itself, so there must be less capacity discharged as external energy. !
But note:.. that those Panasonic charts state clearly that the test is at 25C temp, so i would expect that all the different discharge plots were at 25C....not just one uncontrolled temp for the high amp test.
Outside of those “official” Panasonic charts, i have not seen the “increase in Ah” at high currents on any other independent cell test chart.
Can anyone show one. ?

Yes, the energy used to heat the cell causes voltage sag and reduced Wh delivered by the cell before it drops to its low voltage cutoff. But the delivered capacity of the cell can increase at the same time due to its reduced internal resistance. This can increase run time for constant-resistance and constant-current loads where the lower cell voltage would just result in less power to the load. A LED, for example, could run for longer but not as bright due to the extra voltage sag.

The total energy delivered to the LED would be reduced but its run time can still be longer at a higher current level.

Here are some the graphs for a few cells I have tested that show increased capacity (but reduce Wh energy, of course) at certain higher current levels: https://imgur.com/a/ZLH2cpt

Please note that my cutoff for these was 2.8V. These are performance graphs and not capacity tests. The extra capacity for some current levels will be even more apparent if discharged down to 2.5V.

If you only discharge down to, let’s say, 3.0V then you won’t see this increased capacity delivered by the cell. It only becomes apparent at low voltages since the lower internal resistance holds the cell voltage up some but not by huge amounts.

 

[Hillhater]

Here are some the graphs for a few cells I have tested that show increased capacity (but reduce Wh energy, of course) at certain higher current levels: https://imgur.com/a/ZLH2cpt

Please note that my cutoff for these was 2.8V. These are performance graphs and not capacity tests. The extra capacity for some current levels will be even more apparent if discharged down to 2.5V.

Your charts are low resolution and hard to read the ledgends. ( dont have imgur)
But none of them seem to have a 0.2C (industry standard ?) capacity trace, or discharge below 2.8v, so its impossible to make comparisons .

 

[BatteryMooch]

Your charts are low resolution and hard to read the ledgends. ( dont have imgur)
But none of them seem to have a 0.2C (industry standard ?) capacity trace, or discharge below 2.8v, so its impossible to make comparisons .

Imgur is hosting them at full res. I do not know why you are seeing them at a lower resolution. You might have to download them? I am happy to email them to you if you prefer. PM me your email address.

We are not talking about 0.2C capacity testing and results. We are discussing the effect of higher discharge current levels on the runtime and delivered capacity and Wh for cells. This cannot be shown using standard capacity checks at 0.2C. These effects are only apparent at higher discharge current levels and we need to show the difference between discharges at different current levels.

It is easy to make comparisons using a discharge to 2.8V. It is not required to discharge down to 2.5V to see how reduced IR at higher discharge current levels can affect run times. Since all my graphs are down to 2.8V you just have to look at each and compare them. But, no comparisons are necessary. You asked if anyone could show the effect of lowering internal resistance affecting capacity at higher discharge rates and my graphs provide that.

Attached are three of the graphs. You can see the effect clearly on these even with a 2.8V cutoff. The effect is a lot more pronounced at 2.5V.

 

[Hillhater]

.....We are not talking about 0.2C capacity testing and results. We are discussing the effect of higher discharge current levels on the runtime and delivered capacity and Wh for cells. This cannot be shown using standard capacity checks at 0.2C.

Sorry Camlight,.... i can see what you are saying about the marginal effect on your charts between the 10A and 20A plots, (why not others ?).......but we ARE discussing how that original Panasonic graph (above) clearly shows noticably more Ah capacity at 2.0C , than at 0.2C
Here are plots for panasonic PF down to 2.5v, and also Sony VTC3, and VTC5A, down to 2.8v. All at 0.5A and also at 20 amps.
I would suggest that there is no indication of the extra Ah at high currents shown in the original Panasonic charr

 

[BatteryMooch]

Sorry Camlight,.... i can see what you are saying about the marginal effect on your charts between the 10A and 20A plots, (why not others ?).......but we ARE discussing how that original Panasonic graph (above) clearly shows noticably more Ah capacity at 2.0C , than at 0.2C
Here are plots for panasonic PF down to 2.5v, and also Sony VTC3, and VTC5A, down to 2.8v. All at 0.5A and also at 20 amps.
I would suggest that there is no indication of the extra Ah at high currents shown in the original Panasonic charr

In my experience, the effect is highly cell- and current-dependent, which makes sense. Different construction and cell chemistry will affect how the IR changes based on temperature. Many cells I have tested show almost no change as the cell heats up, with the discharge plot lines running parallel to each other for all discharge rates. Other cells have shown the effects we see above. Other cells start off with the voltage plummeting at higher discharge current levels but then the recovers a lot as the cell warms up.

The discharge current level matters though. At some lower levels the cell doesn’t get warm enough to change its IR much. At higher levels the voltage drop during discharge swamps out the visible effect of decreasing IR as the cell heats up. The voltage would sag even further without the decreasing IR though.

Other things that could affect the voltage sag might be current dependent too. The gradient in ion density across the cell changes depending on the discharge current level. This could be SOC as well as temperature dependent and affects the voltage sag we see.

If we use HKJ’s 10A and 20A plot of the Samsung 30T and compare it to mine we can see that his tests do show a “crossover” to greater capacity at about 2.8V, as my test shows. My test shows that occurring at about 2.95V though. The differences can be due to cell grades/condition and ambient temps and airflow. I keep all air flow away from my cells during a test.

The effect seen in my charts would be much greater if the discharge was down to 2.5V, as pronounced or even more so compared to what is seen in the NCR18650A graph above. IMO, the effect seen in the NCR18650A graph is real and common.

 

[Hillhater]

The effect you are seeing is marginal at best, and does not really show on the Lygt test ! (30T )
But none of this explains the Panasonic chart with 6A crossing the 0.5A plot significantly.
I will say again......I have not seen such a discharge effect before.
If you believe it is a “common” effect, please show another chart, from any cell, to verify this. !
The 0.2C tests have always produced the highest Ah capacity results