High drain vs low drain battery chemistry? What's the difference?

[Farnsworth]

Hello,

I'm curious as I've seen high drain vs low drain being thrown around a lot. Most articles or explanations discuss what the basic differences are between the two types of batteries, but I can't find anything that actually explains why these types of batteries are different functionally or chemically.

The question is, let's say we are comparing two 18650 cells to each other.
An LG HG2 rated at 20A continuous discharge (6.67C rating) and an LG MJ1 rated at 10A continuous discharge (2.86C rating).

When I look at the specs of these two batteries, to me it almost seems as if the two cells are using the same chemistry, but they have been given different ratings by the manufacturer for marketing reasons.
Either this is the case, or the chemistry really is different on the inside.

So how does a manufacturer make a cell high drain and a cell low drain? Or is it simply marketing on their part to create a wider application use? Is an HG2 secretly an MJ1 but with the heat trip limit turned up to 6.67C? Could a "low drain" MJ1 for example be used in the same way that the HG2 is rated? If it can or cannot, why?

I believe that these cells most likely are different somehow, it's just interesting to see what ends up making them so different.
If you have any info supporting this it would be really helpful to share.

Btw great forum, I've found tons of useful info here.
Thanks!

 

[john61ct]

The ability to put out a very high C-rate is a new development, and only demanded by certain niche use cases.

Propulsion being one of them, and the rise of EVs the main driver of R&D.

The actual LI chemistry, e.g. NMC vs NCA is a relatively obscure topic area, most consumers do not know nor care.

But the rising power capacity - lower resistance - is from relatively subtle tweaking, not just of the chemistry formulas, but physical design and high precision QA processes.

From a practical POV, just follow the real life testing results published here and elsewhere.

The mfg ratings are all over the place IMO best ignored.

And note that there are compromises in other areas of "value", high power density is just one.

Energy density, price, longevity are just as or more , important for many, there is no one measure of "the best", all depends on the use case and buyer priorities.

 

[Farnsworth]

Hello John,

Thanks for your reply.

The actual LI chemistry, e.g. NMC vs NCA is a relatively obscure topic area, most consumers do not know nor care.
But the rising power capacity - lower resistance - is from relatively subtle tweaking, not just of the chemistry formulas, but physical design and high precision QA processes.

Yes this is what I'm interested in knowing more about.

From a practical POV, just follow the real life testing results published here and elsewhere.
The mfg ratings are all over the place IMO best ignored.

But this is what I'm curious about as well. Why should they be ignored? Why are they "all over the place"? How could I best explain why one battery is rated 3C and one is rated 6C to a client other than saying "just ignore those"?

Cheers

 

[Addy]

The ability to put out a very high C-rate is a new development, and only demanded by certain niche use cases.

High C-rate cells have been around for a long time, look at older power tool batteries and RC lipos.

Why are they "all over the place"? How could I best explain why one battery is rated 3C and one is rated 6C to a client other than saying "just ignore those"?

There's more to it than the lithium chemistry. I would assume that the anode/cathode materials will be made differently. High drain cells might require thicker anode/cathode layers or different amounts of surface area.

 

[john61ct]

Yes this is what I'm interested in knowing more about.

Maybe find out what bars the battery designer / engineers from Samsung, Sanyo-Panasonic, LG and Sony hang out.

They might even speak decent English.

They certainly won't think you're a spy, since English speaking contries have no dog in that hunt anymore.

But of course you probably need years of training and study to comprehend their answers, even if you speak Korean and Japanese fluently.

> But this is what I'm curious about as well. Why should they be ignored? Why are they "all over the place"?

Because there are no objective criteria for arriving at the number, no standards body to enable the tests to be comparable, as with solar panels.

And in some companies, the marketing department's influence is greater than at others.

IOW the mfg ratings have no intrinsic meaning, cannot be used to differentiate except between the output from that mfg.

Again, forum members' standardized tests will reveal the actual C-rate that can be discharged, for X minutes befor the temp climbs to Y, or voltage sags to Z.

Comparing two cells in the same price range, **if** you actually need that kind of power density, makes the choice easier.

There are also longevity tests to search for, but those tend to be at less abusive C-rates, maybe can be extrapolated maybe not.

 

[john61ct]

The ability to put out a very high C-rate is a new development, and only demanded by certain niche use cases.

High C-rate cells have been around for a long time, look at older power tool batteries and RC lipos.

As in "recent decades".

Actually sustaining 25C was not a thing thirty years ago, incremental improvements over the years has led to radically higher power densities.

At the expense of energy density, and sometime longevity, and certainly pricing.

So again, if the use case does not actually **require** high C-rate discharge, then better value is found avoiding the extreme examples.

 

[Farnsworth]

Yes this is what I'm interested in knowing more about.

Maybe find out what bars the battery designer / engineers from Samsung, Sanyo-Panasonic, LG and Sony hang out.

They might even speak decent English.

They certainly won't think you're a spy, since English speaking contries have no dog in that hunt anymore.

But of course you probably need years of training and study to comprehend their answers, even if you speak Korean and Japanese fluently.

I get the joke but that's not at all helpful.

I found a lot of interesting presentations that cover battery specifications on youtube that might be helpful to others in the future. It only required an afternoon to get the jist of it too, not years.
https://www.youtube.com/watch?v=9qi03QawZEk

Thanks!

 

[Hillhater]

..
Maybe find out what bars the battery designer / engineers from Samsung, Sanyo-Panasonic, LG and Sony hang out.

They might even speak decent English.

They certainly won't think you're a spy, since English speaking contries have no dog in that hunt anymore.

You may not have to go far , or worry about the language,..
..try the bars around Freemont where Tesla has its secret battery development team.
However with the current restrictions, you will probably have more luck hacking into one of their Video Conference meetings ! :wink:
But yes, technical interpretation may still be an issue !

Why are they "all over the place"? How could I best explain why one battery is rated 3C and one is rated 6C to a client....?...

One reason is the lack of any uniform “standards” for cell performance, and a lot of compromises are possible.
So a cell manufacturer can decide what parameters he wants to target.
There are many performance parameters for a manufacturer to consider,..
Capacity, weight, resistance, thermal stability, cost, cycle life, safety, ease of manufacture, Quality, etc etc, and many of them are “mutually exclusive” ..IE there are limits to how much capacity you can get from a set weight of cell materials.
So many compromises have to be made in cell design
As a crude example , a cell can be sold as having a higher C rate than other similar cells, but it will likely have a lower cycle life as a consequence,

 

[spinningmagnets]

I'm always on the lookout for the details inside the cells. I'm sure there is a lot more to the cells that are well-known as "high drain", but I only know of one aspect. If you take two 18650 cells from the same manufacturer and the same chemistry, one might be "high capacity (3400-mAh MJ1, 35E, GA, etc), and others might be high C-rate...

Most high C-rate 18650's at the very least rely on multiple tabs being connected from the jelly-roll to the two electrodes. The internal construction is a long and thin ribbon of materials that make a sandwich of anode/cathode, separator/electrolyte/etc. Then, it is rolled up into the "jelly roll" which is then inserted into the metal 18650 can. If there is only one tab for the cathode, and only one tab for the anode, then the active material is maximized within the set volume.

I'm sure there is more to high C-rate cells than what I'm about to say, but...if you cut them open, high C-rate cells have multiple tabs along the ribbon, which all converge together to provide multiple tabs between the jelly roll and the electrode.

Not only would a smaller area of just two conductive tabs get hot with high current, but the active material itself will get hotter if the current has to travel farther to get to a tab.

Tabs are thin, but they do take up some internal volume, so high drain cells with multiple tabs will never have as much capacity as a high energy cell of the same chemistry.

 

[Farnsworth]

I'm always on the lookout for the details inside the cells. I'm sure there is a lot to the cells that are well-known as "high drain", but I only know of one aspect. If you take two 18650 cells from the same manufacturer and the same chemistry, one might be "high capacity (3400-mAh MJ1, 35E, GA, etc), and others might be high C-rate...

Most high C-rate 18650's at the very least rely on multiple tabs being connected to the two electrodes. The internal construction is a long and thin ribbon of materials that make a sandwich of anode/cathode, separator/electrolyte/etc. Then, it is rolled up into the "jelly roll" which is then inserted into the metal 18650 can. If there is only one tab for the cathode, and only one tab for the anode, then the active material is maximized within the set volume.

I'm sure there is more to high C-rate cells than what I'm about to say, but...if you cut them open, they have multiple tabs along the ribbon, which all converge together to provide multiple tabs between the jelly roll and the electrode.

Not only would a smaller area of conductive tab get hot with high current, but the active material itself will get hotter if the current has to travel farther to get to a tab.

Tabs are thin, but they do take up some internal volume, so high drain cells with multiple tabs will never have as much capacity as a high energy cell of the same chemistry.

Hi Ron,

This is exactly the type of info I was looking for to help explain some differences. How many more tabs would I expect to find in a high C-rate cell? It's interesting, I will do some more research on this.

Also, thank you for all of your articles, I've already read several of them and they've been extremely helpful!

Cheers

 

[john61ct]

I was not "joking" just making the point that detailed information on specific cell models is highly guarded proprietary info and difficult to grok even if you were given access.

 

[Farnsworth]

I was not "joking" just making the point that detailed information on specific cell models is highly guarded proprietary info and difficult to grok even if you were given access.

Oh my bad, I thought it was a joke.

Thank you for the serious and helpful information, I've started my courses on Japanese and Korean languages. I've also applied to MIT, I'm in it for the long run.

 

[john61ct]

Or, the practical approach, learn how to find and understand the comparative test results published by objective members

and realize that the actual performance statistics that they reveal, under conditions related to your use case, are a better guide for selecting the appropriate cells

than any theoretical abstract understanding as to how the sausage is made.

I am not just arguing with you for the fun of it, but spending time to help you.

 

[john61ct]

https://endless-sphere.com/forums/viewtopic.php?t=106001

 

[docware]

Quotations from the article bellow :

"Commercial lithium ion cells are now optimised for either high energy density or high power density. There is a trade off in cell design between the power and energy requirements.

The cells designed for high power applications used smaller particles of the active material in both the anodes and the cathodes. The cathodes for high power cells had higher porosities, but a similar trend was not observed for the anodes. In terms of cell design, the coat weights and areal capacities were lower for high power cells. The tag arrangements were the same in eight out of nine cells, with tags at each end of the anode, and one tag on the cathode. The thicknesses of the current collectors and separators were based on the best (thinnest) materials available when the cells were designed, rather than materials optimised for power or energy. To obtain high power, the resistance of each component is reduced as low as possible, and the lithium ion diffusion path lengths are minimised.

Electrodes. Higher power density is achieved by using lower coat weights, lower areal capacities, smaller active particles and higher cathode porosities

Tag Configuration. It was expected that high power cells would use more tags, but eight out of nine cells used the same three tag configuration.

Current Collectors. The thicknesses did not follow any design trend, but were also related to the availability of components at the point of design. However, the high power Sony VTC5A cell had thicker current collectors and tags than the high energy Sony VTC6 cell, despite being designed in the same year.

High power density requires the minimisation of every component of the overall cell resistance, based on lower electrode coat weights, thinner separators with lower tortuosity and thicker tags and current collectors. The electrode resistances and lithium ion diffusion path lengths can be reduced by using smaller particle sizes for the active materials, and higher loadings of conductive additives.

For all these cells, the maximum charge rate in the cell specification sheet was much lower than the maximum rate of discharge."

Hopefully this can help a little bit.

View attachment Design Strategies for High Power vs. High Energy Lithium Ion Cells.pdf

 

[john61ct]

great stuff thanks!

 

[DogDipstick]

Any relation to Philo?


If so, I thank thee immensely. We all love Philo. He did quite a bit for us, as a nation, lol, of couch potatoes.

 

[Farnsworth]

Quotations from the article bellow :

Hopefully this can help a little bit.

Design Strategies for High Power vs. High Energy Lithium Ion Cells.pdf

This is fantastic, thank you!

Any relation to Philo?

If so, I thank thee immensely. We all love Philo. He did quite a bit for us, as a nation, lol, of couch potatoes.

Hahah no relation actually, just a common ancestor from like 1675 or something.

 

[spinningmagnets]

New Tesla patent does away with tabs. There is an added internal cap for the positive collector, but all the negative tabs have been removed. Since the entire bottom and sides of the can are the negative electrode, the negative collector (ribbon) is just a hair longer on the negative edge, so once it's "squished" into the can, the entire bottom of the jelly roll is the connector to the bottom of the can.

https://www.youtube.com/watch?v=L6rZZBH4A_A&feature=youtu.be

This makes more contact area than the previous "multiple tab" model of a high current cylindrical cell construction. This was a concern for Tesla because they are finding that customers are using the fast-charger more often than they thought they would (even though in doing so, the customer voluntarily gives up a small portion of the battery warranty due to "fast charge" heat). Large connector area means less heat during fast charging.

Several European EV's and chargers are embracing an 800V system, which was done to speed up charging. Tesla and Nissan are closer to 400V, which means their BMS's are much simpler, and there are half as many series connections. The upscale Euro cars at 800V are piggy-backing on the 800V public bus system that is developing there. The bus system went to 800V so they can achieve their power goals with fewer amps.

 

[john61ct]

New Tesla patent does away with tabs. There is an added internal cap for the positive collector, but all the negative tabs have been removed. Since the entire bottom and sides of the can are the negative electrode, the negative collector (ribbon) is just a hair longer on the negative edge, so once it's "squished" into the can, the entire bottom of the jelly roll is the connector to the bottom of the can

CONDUCTIVE ADHESIVE

no-weld, solderless for the win!

 

[amberwolf]

Do you realize how high the resistance of such adhesive is?

And what it's capability is(n't) to withstand environmental factors?

Have you read the previous threads discussing such adhesives?


EDIT: just to see what's out there, I poked around and found a paper testing various particle types, sizes, densities, epoxies
https://res.mdpi.com/d_attachment/polymers/polymers-03-00427/article_deploy/polymers-03-00427-v2.pdf
It doesn't seem to explicitly say what thin-layer applications would be (it discusses centimeter-thick testing, which is odd given that conductive adhesives aren't going to be used like that). But if I understood what it showed correctly, one might expect milliohm-per-millimeter-scale resistances from the best types experimented with. The paper was a decade ago, so technology has probably changed, but that's pretty high when you're talking about currents on the scale of EVs.

I am not sure what the *area* tested was, though, which changes everything if it was a large-contact-area vs a point-source area. So let's assume a 1cm x 1cm contact area (if it's much less, then performance is actually much better than estimated).

If you were to use a thin layer, say, a tenth of a millimeter thick, and started with a milliohm per millimeter, then you'd have a resistance at each connection of 0.0001ohms. That would be doubled, if adhesive used at each end of a cell. If you pass a 10A current thru that, you get 0.002v drop across the adhesive. It's not much, but it means you also get 0.002v x 10A = 0.02w power loss generated as heat inside the adhesive. Doesn't seem like much.

Let's say you have a 20s20p battery for an ebike. That's 400 cells, so 400 * 0.02w = 8w of heat generated just in the adhesive.


Now, compared to the actual cell losses, that's not much, but it's still an additional loss.

If you had actual solid metal making those interconnects, it should have less resistance, and less heat loss, depending on interconnect method and size, though I haven't looked up the numbers yet.



When I go look up actual available adhesives, I find that there *are* adhesives available that are in the above range:
https://www.masterbond.com/properties/electrically-conductive-adhesive-systems

What I don't know (can't tell from available data) is how durable such a connection is under the vibration and temperature-change-cycling environments seen on bikes and vehicles (vs a stationary application). If the entire pack, cells and interconnects (busbars, etc) that are glued together, are all sufficiently immobilized relative to each other, it might be ok, but I'm not sure how the connections would be affected over time by the different (possibly wildly different) thermal expansion rates of adhesive and metals.

 

[john61ct]

That's why the article tweaked my interest

Not as if Tesla won't have moved material science along at least a little.

 

[john61ct]

For my design/doodling

a copper "coin" gas-tight crimped around tinned fine-stranded copper wiring

or maybe the "splayed" strands themselves

are held in direct contact with the endcap terminal, with a spec'd pressure through foam/rubber.

The conductive adhesive is not between the two, just holding it securely in place above and around the copper edges.

The foam rubber also adhered around its edges to the cell end.

Really, adhesive not being necessary, more belt & suspenders, could try paste if that improves conductivity.

The pack itself wrapped or potted in shock-absorbing foam/rubber, no strain on wiring,

also all well sealed against moisture.

Each cell and its pair of wires being freely "moving" independent of the others wrt any shock / vibration issues.

Can't really imagine the temperature expansion being an issue.