Manufacturer's Dogmatic "Magical" 18650-cell's Amp-rating

[Matador]

After reading this "Use The Right Size Pack For Your Ebike Build : Ignore ‘Peak Power’ BMS Ratings" blog by Karl Gesslein (https://electricbike-blog.com/2017/01/07/use-the-right-size-pack-for-your-ebike-build-ignore-peak-power-bms-ratings/), I posted (still awaiting moderator approval/modifications) a comment on his blog because I enjoyed reading it, and I made me think more outside the box :

I was just thinking... We take the 18650-cell's manufacturer discharge current amp-rating for granted.
In my view, that rating is not a magical threshold/cutoff value bellow which a cell magically will stay cool and have the longest possible life. It sort of a continuum. A "dose-dependant relationship" if I may say by analogy.
For exemple that 30Q's 15-amp rating threshold is kind of abritrary and relies on what Samsumg believe is an acceptable drain for an okay temperature rise (81°C !!! once 100% discharged) and max cycle life.
Yet that HG2 is rated for a 20-amps by LG chem, but I've read somewhere that they perform slightly worst than the 30Q under similar condition, even though they have a higher amp rating.

I think 18650 lithium cell manufacturer's max discharge current rating are to take with a grain of salt. Knowing that temp is lithium cell's worst enemy, I think the best way to know what cells can take is to look for temperature rise curves at different discharge amp values on graphs. Like in this example : http://www.powerstream.com/18650-high-discharge-rate.htm
Also good to look for internal resistance charts at different SOC and temps. The DC-IR are obviously related to temperature curves at different discharge rates with ohm's law.

Just wanted to point that out : take those discharge current(amp)-rating values provided by manufacturer with a grain of salt.

For some reason, my post on Karl blog is still awaiting moderation and doesn't seem to pass that step. So here is my opinion on the subject (my original answer to his post). Here it is :
EDIT : My post has been aproved witout any moderator modifications on Karls blog. Here it is anyways :
______________________________________________________________________________________________________________________________________
I think the “amp-rating of a lithium cell” is not a “dogmatic exact cutoff/threshold value” that will garantee the cell will last if you stay exactly at, or just below that value… It’s more of a “dose-dependant relashionship”. In my opinion, there is a direct relationship between the chronic stress-load (in amps) imposed/drawn on a pack, the magnitude of it’s chronic temperature rise and ultimately, it’s cycle life. Heat (my personal cutoff is 50°C) is lithium cell’s worst enemy when it comes to battery cycle life. Sure you can completely drain a PF-cell (10amp-rated) or a 30Q-cell (15amp-rated) at exactly that 10 or 15 amp rating per cell respectively, but at what cost (maybe fine in acute emergency power usage needs, but bad in a chronic usage setting) !!! Temperature of these cells at these load values will rise at or above 75° !!!! See for yourself with scientific graphs (the scientific method is better than the dogmatic method IMHO) : https://endless-sphere.com/forums/viewtopic.php?f=14&t=68556&start=200#p1251357. Temp wil presumably rise even more than that when the cells are packed together in an enclosed, non-ventilated battery box/heatshrink. The less the temperature rises, the better it is for the cells cycle life. Temp will not just start to dangerously rise once you magically cross that 15A cell’s stated amp-rating value… Temp will proportionally rise according to the amp draw rate imposed on it. The more you pull the more it cooks. Look at this graphical exemple… If you completely drain a cell at a higher amp-draw curve, in then end the temperature wil be much higher, it’s not rocket science. This website shows that in very graphical manner… look at temperature curves : http://www.powerstream.com/18650-high-discharge-rate.htm

So sure a Mighty Mini Cube (14S 2P) made with good 15amp-rated 30Q-cells will be rated for max 30 amp-draw (2P x 15amps = 30 amps), and it will work with the 30A-BBSHD (surely not with a Ludacriss BBSHD@50A on throttle only mode though)… But if you pull 30amps ALL THE TIME out of that 30amp-rated 2P Mighty Mini Cube pack (the better one, with 30Q cells) until you COMPLETELY drain the pack @30Amps all the time, then the TEMPERATURE OF CELLS WILL RISE ABOVE 75°C! THAT IS NOT GOOD for a long term cell life IMHO. Somebody made the experience in the past and it’s 2P battery pack quickely failed on his BBSHD : https://endless-sphere.com/forums/viewtopic.php?f=6&t=83238&hilit=Sony+VTC4&start=25#p1250578

I tend to stay conservator with battery ratings in order to enjoy a long battery life (high number of charge cycles). What it think is good IMHO is posted on this forum link (at least double-spec your pack for amps. Triple it or more if you can afford) : https://endless-sphere.com/forums/viewtopic.php?f=3&t=85629#p1252830

I enjoyed this article. Thanks for taking the time to explain beginners about battery amps rating. The battery is the most important and most expensive part of an ebike. We might as well make it last for more than a year !
______________________________________________________________________________________________________________________________________

I propose that this thread serves to post discharge curve of cells with their concordant superposed temperature rises curves... So we can get a sense of internal resistance of cells, and know what to expect from cells. I mean, do Sony even recommend that the VTC4 be allowed at 97°C to test at the rated "Magical dogmatic" 30A-rated threashold they claim ???? (see graph for VTC4 below). I really wonder if cells submitted to the max rating claimed have comparable life in cycles... Certain cell tolerate high temps without degrading ? Or is LiCoMnO2 just behaves the same in diffent manufacturer brands ? Otherwise are amps rating just are a manufacturing gimmick... Le me start the graph sharing (NB : remember with superimposed temp curves please)

Panasonic NCR18650 GA (Rated : 10A, 3500 mAh) rises to 62°C @ 100% DoD with 10A-load (as per max rating) :


Panasonic NCR18650 PF (Rated : 10A, 2900 mAh) rises to 65°C @ 100% DoD with 10A-load (as per max rating) :
Somebody even pushed the PF to 15A at 100% DoD... Temp went up to 92°C ! (REF for graph : https://www.e-cigarette-forum.com/forum/threads/panasonic-ncr18650pf-10a-2680mah-18650-bench-test-results-a-great-battery-beats-mh1-and-mj1.692438/ )

Sony US18650 VTC6 (Rated : 20A, 3000 mAh) rises to 88°C @ 100% DoD with 30A-load (as per max rating) :


Samsung INR18650-30Q (Rated : 15A, 3000 mAh) rises to 81°C @ 100% DoD with 15A-load (as per max rating) :


Samsung INR18650-25R (Rated : 20A, 2500 mAh) rises to 95°C @ 100% DoD with 20A-load (as per max rating) :


Sony US18650 VTC5 (Rated : 20A, 2600 mAh) rises to 77°C @ 100% DoD with 20A-load (as per max rating) :


Sony US18650 VTC4 (Rated : 30A, 2100 mAh) rises to 97°C @ 100% DoD with 30A-load (as per max rating) :


Sony US18650 VTC3 (Rated : 30A, 1600 mAh) rises to 83°C @ 100% DoD with 30A-load (as per max rating) : (sorry, I could not find better data for temp curves ; these are old technology cells).
View attachment 2

 

[Matador]

It would also be awesome if somebody could make comparative graphs :
Temperatures curves of different 18650 brands/type at a given amp draw.... We could compare temperature rise under the same stress load between brands. Which ones runs the coolest ???

Exemple of what I mean :

Graph 1 : Temps of different 18650 cell types/model/brand at 5A (y-axis) versus mAh discharged (x-axis)
Graph 2 : Temps of different 18650 cell types/model/brand at 10A (y-axis) versus mAh discharged (x-axis)
Graph 3 : Temps of different 18650 cell types/model/brand at 15A (y-axis) versus mAh discharged (x-axis)
Graph 4 : Temps of different 18650 cell types/model/brand at 20A (y-axis) versus mAh discharged (x-axis)
Graph 5 : Temps of different 18650 cell types/model/brand at 25A (y-axis) versus mAh discharged (x-axis)
Graph 6 : Temps of different 18650 cell types/model/brand at 30A (y-axis) versus mAh discharged (x-axis)

 

[liveforphysics]

It's a complex set of relationships that composite together to create cell decay.

For modern EV cells (which already do multi-thousand cycles fine), it's merely time sitting at temperature that plays a more significant decay roll than if the cell were ever even cycled or not.

In cycling related decay, charge rate generally plays a bigger factor than discharge rates (again though things get very complex towards the far ends of either).

I should add, I wouldn't call any of the above shown cells 'modern EV cells', most are designed for consumer electronics which often has an order of magnitude shorter cycle-life in exchange for energy density while keeping impedance low.

Used modern Tesla cells of cars that lived in cool climates, or modern LEAF or GM bolt/volt cells are a pretty attractive option if you can source and package them in your application.

 

[spinningmagnets]

I don't think any lithium 18650 cell should ever be allowed to go over 140F / 60C at any time, for even a few moments.

I feel 120F / 48C is a much better peak temp to determine what the maximum discharge of the cell should be. I have no scientific basis for this, but if you can't hold it in your hand for more than 10 seconds, its just too hot. I'm open to hearing why a certain particular temp is ideal, but I'd slowly raise the continuous drain on a cell in steps, and when the stable temperature of the cell reached 120F / 48C, I'd say that amount of current is it's safe peak amps.

100F / 38C is warm to the touch.

 

[Matador]

It's a complex set of relationships that composite together to create cell decay.

For modern EV cells (which already do multi-thousand cycles fine), it's merely time sitting at temperature that plays a more significant decay roll than if the cell were ever even cycled or not.

In cycling related decay, charge rate generally plays a bigger factor than discharge rates (again though things get very complex towards the far ends of either).

I should add, I wouldn't call any of the above shown cells 'modern EV cells', most are designed for consumer electronics which often has an order of magnitude shorter cycle-life in exchange for energy density while keeping impedance low.

Used modern Tesla cells of cars that lived in cool climates, or modern LEAF or GM bolt/volt cells are a pretty attractive option if you can source and package them in your application.

Interesting LFP!

I always charge slowly (temp never goes 10-15°C above ambiant... and I live in Canada, so should be good). Anyways, I just ride a moderate power hotrodded 30amp BBSHD, but I want to have an overbuild 10-years long lasting battery that will sustain 30A continous all the time without boiling up. For charging, I will probably never go above 1C with my 58.8V 5amp charger, as I have no intention for going lower than 4P for my next build as a very strict minimum. I actually target around an 14S-8P if I opted for 2500 mAh-range cells (range around 20Ah)... So that would be charging at 0.25C. No abuse in charging at those rates I think... Although I don't know about the ideal C rate of charging for maximium longevity. Also I mostly charge to a max of 4.10V per cells (SoC of around 86%). Also I do "shallow" discharge cycles, stoping at 3.77V (resting voltage/no load) or around 40% SoC. I think that shallow cycling also doesnt allow temperature rise (as seen on curves) as much as when pushing to 100% DoD. Hence, why it's also better to go with a higher capacity pack one can afford... Finally, I only charge when pack is at or close to ambiant temp.

I just wonder what makes Tesla cells any different than other modern 18650 ?
Is it the chemistry ? I think Tesla are LiNiCoAlO2 cells right ? Aren't there any cell of this chemistry available at NKON or other cell sellers.
I really though LiNiMnCoO2 were considered "modern EV cells", but I'm no expert.... I would like to be part of that "Game" here, to gain acess to cutting edge lithium-cell technologies... I guess I didn't look at the best cells to begin with... But where to learn about them... Telsa seems to protect the knowled with patents of all kind.

I really crave for more info on lithium cells... There must be some electrochemist out there that wrote a thesis on the subject somewere ? Or maybe Telsa is keeping a patent and doesn't want to share the knowledge ?

 

[Matador]

I don't think any lithium 18650 cell should ever be allowed to go over 140F / 60C at any time, for even a few moments.

I feel 120F / 48C is a much better peak temp to determine what the maximum discharge of the cell should be. I have no scientific basis for this, but if you can't hold it in your hand for more than 10 seconds, its just too hot. I'm open to hearing why a certain particular temp is ideal, but I'd slowly raise the continuous drain on a cell in steps, and when the stable temperature of the cell reached 120F / 48C, I'd say that amount of current is it's safe peak amps.

100F / 38C is warm to the touch.

I hear you. I get the same feeling... I once went up to 75°C with laptop cells on BBSHD at 60% SoD (14S 9P 20.3Ah overall). The pack reached 75°C.. I thought it was going to go «nuclear chain-reaction» on me ! (Crappy laptop cells I used - my pack testing specs are here ) : View attachment Li-Ion.xlsx

I wonder if there are any difference in the way high-amp cell' metal sheets (lithium, copper, isolation layer) are "rolled" compared to low-amp but higher mAh-rated cells ? Do thicker rolled lithium sheets with less turns or the opposite (thiner lithium sheets with more turns) to fit in the 18650 metal can have an effect of amp rating of cells ? What is it that make a cell better suited for high drains....

Do certain cell tolerate higher temps which makes them better suited to high drain and high temps ?

EDIT : On my crappy experimental laptop lithium-pack, some cells lost around 10-15% capacity in 10 cycles at 60% SoD, while draining them between 15 to 30A continous (9P pack of averaging 2300mAh/cell, discharge tested at 0.5A down to 2.8V)... Is it just because there were too old ? I mean, I charged them slowly and stored them at 25°C. It's just when I discharged them that temp rose and capacity was lost... But, I also made the mistake to store them at 100%SoC- 4.20V during the months that I capacity-tested the cells.

 

[Matador]

I'm open to hearing why a certain particular temp is ideal.

I get the feeling it's for the same reason you get a better more homogenous, albeight slower process, in electoplating metal at lower temperatures. But I'm not sure, I'm not an electrochemist.
My rational is : less heat, less kinetic energy... Slower rates, but more homogenous electrodeposition of element... More uniform electroplated layer... That could influence cycle life, I mean if that could help preserving battery electrodes.

 

[Matador]

https://chargedevs.com/features/tesla-tweaks-its-battery-chemistry-a-closer-look-at-silicon-anode-development/

Changing from graphite-anode toward's silcon-anode is one of the parameter telsa is after...
I guess they also have crazy sophisticated BMSs and have so many cells in parallel that cycle life is incredible.

LTO (LiTi2O4) might seem amazing for cycle life... but nominal voltage and capacity seem to suck for weight, volume and cost...
How about NCA (LiNiCoAlO2) ? Is it one of tesla's ingredient ?

 

[dogman dan]

Well,,, I've been saying "cut all manufacturers C rate spec in half, and Hobby lipo spec in half twice" since like 2009.

Spec means it can do it once. They don't really say it can do it 2000 times. Those cycle tests are at 1c, .5c, etc. Of course you get lots of cycles at .5c. ( I'm talking about the average cell here, not the car grade cells.)

Build to last,, aim to discharge at 1c,, 1.5c burst. That in general won't get hot, or sag excessively, if the spec is 3c.

And always,, bear in mind,, that cells internal resistance is not going to get lower without it's shorted. It just gets higher as it ages, so your working c rate just goes down and down. So for most bike size batteries under 15 ah,, you are maxing its new lower c rate by the end of the second or third year. This is why they seem fine, seem fine,, seem fine, then wham it's a brick. That last six months, you were really flogging it.

When the pack gets warmer than it used to,, you are on the way out with it.

 

[Matador]

Build to last,, aim to discharge at 1c,, 1.5c burst. That in general won't get hot, or sag excessively, if the spec is 3c.

And always,, bear in mind,, that cells internal resistance is not going to get lower without it's shorted. It just gets higher as it ages, so your working c rate just goes down and down. So for most bike size batteries under 15 ah,, you are maxing its new lower c rate by the end of the second or third year. This is why they seem fine, seem fine,, seem fine, then wham it's a brick. That last six months, you were really flogging it.

When the pack gets warmer than it used to,, you are on the way out with it.

I have a 30A BBSHD with hotrodded controller parameters (still 30A max). I use thumb throttle without restrain and I'm not pedalling a lot, if not at all (except when there's a steep hill to climb where I pedal to not push my luck on the motor kit and battery) !

I am considering bying used Makita battery Packs (with less than 100 cycles)... Dunno the year batchs.
The BL1830 Makita pack contains 10 Sony US18650V (rated 7.5A;15A burst, 1600 mAh)
The BL1840 Makita pack contains 10 Sony US18650 VTC4 (rated 30 amps, 2100 mAh)
I will balance everything, check DCIR, residual mAh, and pair groupes for 2 parameter : equal capacity for parallel groups and comparable DC-IR.

I have three questions :

1) I will make a 14S battery (52V).... What would be a good P number (for those VTC4) ? I mean regardless of range I'm needing, what is the safe minimum P number I should choose so the battery will stay cool and have the longest life ? Money is not such a limiting factor as I could afford up to 300 VTC4 cells (14S20P!), but that's heavy! What a conservator P number for a reasonable pack weight.??? i'm trying to find the sweet spot here.

2) The VTC4 are high-drain quality cells.... Will they also "Wham, it's a brick" after 3-4 years regardless of how I baby them in high P-numbers arrays ???

3) Do you think my project with Makita Pack is a good idea or would you just buy new cells instead at higher cost ?

 

[999zip999]

Makita used battery packs are defective and returned. I have had 45 packs 450 cells. I have seen fire flash out the top of cells. Just by taking the cover off. Yes more than once. More. Some are water damage some with other problems.

 

[Matador]

Makita used battery packs are defective and returned. I have had 45 packs 450 cells. I have seen fire flash out the top of cells. Just by taking the cover off. Yes more than once. More. Some are water damage some with other problems.

Yes, but by testing them, i think I should be able to discard the bad cells (the ones that bricked walled the BMS) and retain the good ones. I'd be happy to get 8 good ones out of a 10 cell pack. But under 6 good cells per 10 cells I would be a bit disapointed... ? out of 10, do you think that's a yield to be expected from your previous experiences 999zip999 ? I can get these cells in my location, hence my asking more experienced builders their impression on these cells.

Fire flashes ? As in something metalic touching and shorting cells inside de packs or did de cell just went "spontaneous combustion" ?

 

[liveforphysics]

NCA is more reactive and delicate than NCM (in general, as particle shapes, binder, electrolyte all play a huge factor). Silicon in anodes are very much more delicate than graphite anodes (again in general, mechanical structures plays a huge factor).

However, cathode material is only a single and relatively tiny/insignificant aspect of what makes a cell cycle well.

It's possible to make a cobalt oxide cell which has tremendous safety and cycle endurance, just as it's possible to make an LiFePO4 or NCM/NCA cell that has terrible safety and rapidly fails cycling.

The metrics folks always talk about in cells (like the composition of cathode material) is less of a safety and endurance concern than the anode structure design, how the SEI was formed and what material purity and electrolyte base and stabilizers, binders, and electrolyte were used in the cell.

Tesla has success using consumer-electronics-esque cycle life cells due to having packs large enough that they rarely get cycled deeply, so in practice they survive and function fine. This method can be applied to ebikes as well if you size the battery generously for the applications needs.

 

[dogman dan]

Once you are into used cells, the spec really does not matter any more.

Much like a 2-3 year old pack,, the cells spec has changed by now. You will have to of course test each cell and toss those that are obviously higher internal resistance than the norm for that batch of used cells.

Then, how many in P? As many as it takes. Keep adding cells until sag under the load is tolerable. For 30 amps,, likely you will need at least 20ah worth,, maybe more. I don't know. Maybe 15 ah is plenty. Only testing the cells in your hand will tell.

If that thing gets all hot when you ride,, build another pack and parallel the two.

 

[Esparza]

I'm testing some of the cells like the Samsung 30Q and it seems to run much cooler than on those graphs with a room temperature of 24ºC.

The problem is that the average drops to 3.236 V, so you only get 8 Wh with 2.8 cut off voltage.

We are planing to make a racebike with this or the VTC-6 cells, they are lighter than LiPo but the voltage sag is really big, what do you think? There will be peaks of 17.5A and average current of 10-12A.

This was our last prototype

 

[liveforphysics]

If it's a dedicated race vehicle with high power needs, not a bad candidate for RC pouch cells, or something like GM Volt hybrid cells.

 

[dogman dan]

In any case, you won't win till you parallel enough of them to use them hard with minimal sag. So a lighter bike with less saggy cells may be the best approach.

The length of the race will matter,, you need to have good voltage at the end of it, so you may have to carry more than you think. Carrying that much,, the weaker cell may not sag as much anymore.

 

[dustNbone]

Another thing I'd consider when using any cells that have been in Makita packs is how hot these packs get in "normal" tool usage. I've used these packs for about as long as Makita has been making cordless 18V tools, and I can say that after driving a dozen 3" wood screws into lumber the pack is very, very warm, even if it's a freshly charged pack.

So even a pack that has under 100 cycles has likely been heated well beyond what most on here consider "safe" temperatures several times. They also tend to get killed very dead before being recharged, as people don't like to stop what they're working on until absolutely necessary. I was personally guilty of doing this until I started doing ebike stuff and learned a bunch more about batteries, now I run them much more gently, and they live much longer.

Of all the common applications for 18650 cells, I can't imagine one that hammers harder on them than tools in the hands of a busy professional, we expect alot from our tools, and we're generally willing to sacrifice their longevity in exchange for productivity. Makita tools in particular are the ones you'll most often find in these "pro" environments, at least where I live.

 

[Esparza]

Its a dedicated racebike but the organizers give the same motor to all teams and is not very powerful (13kW continuous and 35 kW peak), they pretend it to be like a Moto3. Voltage is limited to 110 volts at full charge.
We race 5 laps at Motorland Circuit (5km per lap with a straight of 1.7 km) and last year we reached only 170kmh (106mph) and lap average speed of 110kmh (70mph). The bike was 129kg and we used RC LiPos that performed perfectly, low voltage sag and low temperature.

The point is that we have near 50Ah and out max motor requirement is 350A, so 7C as maximum pulse discharge rate, not that high. With lipos we had 4.5kWh in 31kg, with 18650 we can have around 5kWh in 24 kg but huge voltage sag, from test i've done our Avg voltage will be 0.3 volts lower so near 8 volts lower at the whole pack, especially at low SOC. Temperature is not a problem as cycle life is not really important. Also cylindrical pack is near 30% more economical and we are students
The lower voltage will be translated into lower topspeed but same torque no?
We were planing to make a light and efficient motorbike, but maybe is better to just search for more and more power as LFP says.

Regards

 

[Matador]

I just updated first postny adding the Sony VTC6 profiles (mAh AND temperature) curves...
As you can see, if an ebike has an LVC of 3.0V per cell....
Pulling 10 amps per cell on 2900 mAh Panasonic NCR18650PF cells will only yield 2250 mAh... and temps will rise to 58°C
While pulling 10 amps per cell on 3000 mAh Sony VTC6 cells will yield 2725 mAh , while temps will rise to 54°C

So manufacturer capacities are to take with a grain of salt.... especially considering voltage sag, LVC and temp rises that will shorten cycle life.

 

[Punx0r]

That's an interesting problem you have there, Esparza. If the motor spec and upper pack voltage are both fixed then it feels like the team having the least pack voltage sag is going to end up with a performance advantage at some point on the track - Not just battery capacity wasted as ohmic heating in the pack, but will be able to deliver greater motor torque at higher speeds.

On the other hand, as you say, the 18650 pack would be lighter for the same capacity compared to less saggy RC lipo pouches, which infers its own performance advantages.

I wonder where the optimal trade-off will occur?