LIFEPO4 C

[minde28383]

Why two batteries (for ex. ping 48 15Ah battery – 2C(30A); cell a123 26650 – 70A continuous) with the same chemistry LIFEPO4 have extremely different C ratings (discharge rating). Initially I was about to think that battery's chemistry would determine C rating...

Both LIFEPO4 but C ratings differs:
ping 48V 15Ah battery – 2C(30A);
cell a123 26650 – 70A continuous

ps. I own "ping 48V 15Ah" and wish it was a123 pack instead.

 

[Jeremy Harris]

The continuous/maximum discharge current rating (the 'C' rating) depends on a number of factors, but primarily the internal resistance of the cell. This varies depending on the manufacturing technique, quality of materials used, chemistry, the cell physical construction and the cell termination method, so irrespective of chemistry there are a large number of other variables at work.

In general terms, the majority of pouch cell LiFePO4 cells (with the exception of the A123 cells and maybe one or two others) have a fairly low maximum C rate - Ping cells are typical. On the other hand, the majority of pouch cell LiPos are capable of very high C rates.

Sorry this isn't a clear answer, the question isn't that simple, unfortunately.

Jeremy

 

[minde28383]

Thanks for answer. Now I'll now that magic abbreviation/word LIFEPO4 does not put equal on battery's parameters of different manufactures.

 

[sangesf]

Thanks for answer. Now I'll now that magic abbreviation/word LIFEPO4 does not put equal on battery's parameters of different manufactures.

Look at it this way..

All things being equal..

Each specific "cell" has it's own C rating.
Take a small cylindrical cell that's 1.1ah
Let's say that cell has a normal 1c rate. (3.2v 1.1amps)
If you put 15 of them in parallel, you still have a 1c rate, but that is now 15amps (3.2v 15a)
If you have a 15ah prismatic "cell" with a 1c rate, again that's 15amps.
If you have a high c rate cell that's 3.2v and 1.1 ah and has a 5c rating, that's 5amps you can pull. If you put 15 of them in parallel, you still have 5c rate, but it's not 5amps it's 75a.

Get it?

 

[dnmun]

i don't think that is what he was asking.

the reason that the A123 cells can have such a high discharge and charge rate compared to the ordinary pouch type lifepo4 cells is due to the nature of the surface structure of the anode.

the anode of the A123 cells has been prepared by a secret patented process that somehow increases the effective surface for the deposition and formation of lithium metal as the cell is discharged and the lithium ions rush through the electrolyte to the anode to carry the current out of the cell to the load.

the process was discovered and patented by dr goodenough of the university of texas and they have the patent. except one of his graduate students has joined with some venture capital to make a competing process, but i think they are under restraint now by court order.

from what i have read, there is some special sintering process that they subject the anode material to which causes the surface to form a network of short dendrites growing all over which have low resistance to the anode, and where the lithium can grow into solid quickly without the precipitation outside the anode surface similar to supercooling when the lithium actually precipitates in formations separated from the anode, leading to higher resistance and surface damage to the anode which shortens the anode life time.

there is also a lot of new stuff about the lifepo4 cathodic crystals too, and the material is priced and sold based on the amount that shows up in the different screen lots after sifting the powder. the smaller the particles, the higher the value.

also there was a paper last fall from some chinese post docs at MIT who showed that non stoichiometric mixtures of the lifepo4 material would form special types of crystals when sintered at a high temperature, allowing the Li ions to be drawn directly to the spot on the surface of the crystal where the ions could be sucked in along the preferred 110 axis, but that is still new science. but the rates are on the order of 150-200C. comparisons to capacitors were made in the paper.

 

[minde28383]

Thanks for answer. Now I'll now that magic abbreviation/word LIFEPO4 does not put equal on battery's parameters of different manufactures.

Look at it this way..

All things being equal..

Each specific "cell" has it's own C rating.
Take a small cylindrical cell that's 1.1ah
Let's say that cell has a normal 1c rate. (3.2v 1.1amps)
If you put 15 of them in parallel, you still have a 1c rate, but that is now 15amps (3.2v 15a)
If you have a 15ah prismatic "cell" with a 1c rate, again that's 15amps.
If you have a high c rate cell that's 3.2v and 1.1 ah and has a 5c rating, that's 5amps you can pull. If you put 15 of them in parallel, you still have 5c rate, but it's not 5amps it's 75a.

Get it?

I already had understanding relating C rating, but it always good to see such examples. It strengthens my understanding relating C ratings. Thanks.

 

[PedalingBiped]

My own limited understanding is you get what you pay for.
A123 holds the patent and sets the bar.

Cheap Chinese manufactures make knock offs. But since they can't make them quite the same way, they lower quality.

With the lower C rate, you need a bigger battery to make sure you don't exceed the C rate.
At 1 C you need a 30 ah battery to draw 30 amps
At 2 C you need a 15 ah.

At 30 c you only need a 1 ah, but you would drain it to fast and therefore have to increase the ah to increase you range

Ok, end of my knowledge.

 

[minde28383]

i don't think that is what he was asking.

the reason that the A123 cells can have such a high discharge and charge rate compared to the ordinary pouch type lifepo4 cells is due to the nature of the surface structure of the anode.

the anode of the A123 cells has been prepared by a secret patented process that somehow increases the effective surface for the deposition and formation of lithium metal as the cell is discharged and the lithium ions rush through the electrolyte to the anode to carry the current out of the cell to the load.

the process was discovered and patented by dr goodenough of the university of texas and they have the patent. except one of his graduate students has joined with some venture capital to make a competing process, but i think they are under restraint now by court order.

from what i have read, there is some special sintering process that they subject the anode material to which causes the surface to form a network of short dendrites growing all over which have low resistance to the anode, and where the lithium can grow into solid quickly without the precipitation outside the anode surface similar to supercooling when the lithium actually precipitates in formations separated from the anode, leading to higher resistance and surface damage to the anode which shortens the anode life time.

there is also a lot of new stuff about the lifepo4 cathodic crystals too, and the material is priced and sold based on the amount that shows up in the different screen lots after sifting the powder. the smaller the particles, the higher the value.

also there was a paper last fall from some chinese post docs at MIT who showed that non stoichiometric mixtures of the lifepo4 material would form special types of crystals when sintered at a high temperature, allowing the Li ions to be drawn directly to the spot on the surface of the crystal where the ions could be sucked in along the preferred 110 axis, but that is still new science. but the rates are on the order of 150-200C. comparisons to capacitors were made in the paper.

dnmun, Thanks for such info. It will take time till I'll grasp all of it What is apparent to me is that “anode preparation of the A123 cell” is patented and kept in secret, and it may be the reason why other manufacturers are not able or may I say: knowledgeable to manufacture cells which would have such big C rating as a123 has.
“restraint by court order” does not allow, competing process using the same surface structure of anode as a123, - to go alive. That’s why we basically see only one A123 manufacturer, and quit high prices of a123 cells?

 

[liveforphysics]

Ehh, It's only in LiFePO4 that A123's perform decent/well.

HobbyCity LiPo cells perform about 4x better than A123.


Also, something worth mention. Some folks say nobody needs more than 5c cells or whatever. Well, if you want the energy out of your pack then you sure as hell do want more than 5c cells. lol

For example, if you have 2 packs both with identical voltage and capacity, say (48v 10Ah), you could call both packs 480watt-hours.
However, if one pack runs some low C cells that drop from 3.3v to 2.6v under your operating discharge current, you've actually just lost 22% of the energy your battery is storeing to just waste heat in the cells, meaning your 480watt-hours pack is actually only giving you 378watt-hours that get used towards moving your bike. Higher C-rate cells may only drop a percent or two in voltage under the same operating discharge current, so while both packs on paper seem like identical amounts of energy stored, in practice it can be quite different. This is where high-C cells are helping you out even if you only ever draw perhaps 1/5th of what they are rated for.

 

[minde28383]

Ehh, It's only in LiFePO4 that A123's perform decent/well.

HobbyCity LiPo cells perform about 4x better than A123.


Also, something worth mention. Some folks say nobody needs more than 5c cells or whatever. Well, if you want the energy out of your pack then you sure as hell do want more than 5c cells. lol

For example, if you have 2 packs both with identical voltage and capacity, say (48v 10Ah), you could call both packs 480watt-hours.
However, if one pack runs some low C cells that drop from 3.3v to 2.6v under your operating discharge current, you've actually just lost 22% of the energy your battery is storeing to just waste heat in the cells, meaning your 480watt-hours pack is actually only giving you 378watt-hours that get used towards moving your bike. Higher C-rate cells may only drop a percent or two in voltage under the same operating discharge current, so while both packs on paper seem like identical amounts of energy stored, in practice it can be quite different. This is where high-C cells are helping you out even if you only ever draw perhaps 1/5th of what they are rated for.

I see you point: higher C rating minimizes Voltage drop possibility, but is there much difference between 40C and 30C. Is it worth to spend few bucks more for pack with 40C when 30C is more then enough (what do you think?). Taking into consideration that such pack will be used with controller of 25Amps.
I already started some thread "Considering battery pack": http://endless-sphere.com/forums/viewtopic.php?f=14&t=22160

Why you say that "HobbyCity LiPo cells perform about 4x better than A123." when a123 website claims their cell 26650 have constant 70A discharge capability.

 

[liveforphysics]

Because HobbyCity offers 5Ah LiPo cells with 0.8mOhm internal resistance.

A123 M1 26650 cells are 7mOhm for a 2.3Ah cell.

This is a specific Ri difference with respect to capacity of 4mOhm/Ah for LiPo vs 16.1mOhm/Ah for A123.

The LiPo also takes up quite a lot less space and weight for a given capacity as well.

Dominates A123 performance in a big way in all aspects except cycle life.

 

[minde28383]

Because HobbyCity offers 5Ah LiPo cells with 0.8mOhm internal resistance.

A123 M1 26650 cells are 7mOhm for a 2.3Ah cell.

This is a specific Ri difference with respect to capacity of 4mOhm/Ah for LiPo vs 16.1mOhm/Ah for A123.

The LiPo also takes up quite a lot less space and weight for a given capacity as well.

Dominates A123 performance in a big way in all aspects except cycle life.

Please correct if I’m wrong:
Even A123 has extremely high C rating (continued 70A discharge), its high 7mOhm resistance creates drawback – voltage drop on high discharge. But does this 7mOhm resistance is constant or it increases with discharged amps? On what amperage this resistance is measured?


A123 claims that cell can be charged with 10A which is amazingly high amperage. I assume due cell’s 7mOhm resistance quite lots of energy must be wasted during charge process vs low LiPo 0.8mOhm resistance.

LiPo cells low resistance 0.8mOhm also must mean that it has to accept very high charge (charging process) amperage, but I don’t see claims that LiPo could be charged with 10amps as a123 claims. Why we can’t charge LiPo packes from “hobyking” with high amperage if it has only 0.8mOhm resistance?


Yes, LiPo does take little bit less weight and space for given capacity.

My believe that a123 is best cells on the earth is about to vanishe  of course a123 is safest cell.



Ps.Could you share how you get these numbers?: “4mOhm/Ah for LiPo vs 16.1mOhm/Ah for A123”?


Not all I said is based on numbers and facts but rather on assumptions, but currently I’m capable only with it

 

[dogman dan]

Yup. Lipo rocks except for cycle life over lifepo4. Many of us are into it enough not to care if we wear out some batteries, we can't wait to replace the 30c's with 100c's or whatever it is next year.

But I'd agree that 5c cells are quite sufficient for anybody only drawing about 2-3 c anyway. The internal resistance gets to be less of a problem if your amp rate is low in the first place. But once you cross the 1000 watts line, lipo makes more and more sense. Lifepo4 can be easier to charge with a simple low amp bms helping out.

Is the higher c rate lipo worth the money? Hell yes if you are going to use them like that. For my racing setup I paid quite a bit for the 30-40 c turnigy packs. It may be crucial to me having some speed left in the last lap.

But I just bought some 15 c that I intend to use just in the practice laps or the heat race. Both those applications won't require as much range, and I saved about $160.

 

[liveforphysics]

The 5ah hobbcity cells can be 10c charged (nanotech 50/90c cells of course). Meaning 50A charge rates for each cell in parallel.

The math to find specific Ri is very simple. Multiply the cell Ri by its capacity. This gives you a common ground to compare cells.

 

[minde28383]

I do use "ping 48V". Ping pack is rated for "continues 1C", and "2C max continues". My stock Infeneon controller draws no more then 22amps. Pack hot off the charger is 53V. When I hit full throttle for ~10 seconds (on flat) it drops to 48V. When I cruise ~25km/h then pack keeps 50V-51V.
So it looks right, when it's up to 1000W or little bit over then this low C rating is ok, but I plat to run this NC 6*10 with 72V, so C rating will be more influential. Basically all this C rating is determined by internal pack resistance, less better.

 

[liveforphysics]

Basically all this C rating is determined by internal pack resistance, less better.

Yep.

 

[minde28383]

The 5ah hobbcity cells can be 10c charged (nanotech 50/90c cells of course). Meaning 50A charge rates for each cell in parallel.

The math to find specific Ri is very simple. Multiply the cell Ri by its capacity. This gives you a common ground to compare cells.

Now I see how you got these numbers:

R = 7mOhm – a123
R = 0.8mOhm - Polyquest 45C-90C, Turnigy Nano-tech 45C-90C

For a123: 7mOhm * 2.300Ah = 1.61Ohm / cell (not per Ah as you wrote)
For LiPo: 0.8mOhm * 5.000Ah = 4mOhm / cell (not per Ah as you wrote)
Correct if wrong.

I’m looking into “Turnigy 5000mAh 5S 30C Lipo Pack” which suppose to have higher Ri due lower C rating. Ri is not specified in Spec. in hobbycity site. Also charge C rate or amps are not specified either.

 

[liveforphysics]

The 5ah hobbcity cells can be 10c charged (nanotech 50/90c cells of course). Meaning 50A charge rates for each cell in parallel.

The math to find specific Ri is very simple. Multiply the cell Ri by its capacity. This gives you a common ground to compare cells.

Now I see how you got these numbers:

R = 7mOhm – a123
R = 0.8mOhm - Polyquest 45C-90C, Turnigy Nano-tech 45C-90C

For a123: 7mOhm * 2.300Ah = 1.61Ohm / cell (not per Ah as you wrote) WRONG
For LiPo: 0.8mOhm * 5.000Ah = 4mOhm / cell (not per Ah as you wrote) WRONG

Correct if wrong.

It's per Ah, and you don't get to divide on the A123, and multiply on the LiPo... Also, aside from getting the number wrong, you forgot the "m" in mOhm.

The point of specific Ri is to have a number you can compare between cells. Doesn't matter if it's a 10Ah cell or a 0.5Ah cell, you can immeadiately have a clean discharge performance index of how a pack of the same capacity would perform if it were made from X cells or Y cells.

I've given enough time to you.

Best Wishes,
-Luke

 

[minde28383]

The 5ah hobbcity cells can be 10c charged (nanotech 50/90c cells of course). Meaning 50A charge rates for each cell in parallel.

The math to find specific Ri is very simple. Multiply the cell Ri by its capacity. This gives you a common ground to compare cells.

Now I see how you got these numbers:

R = 7mOhm – a123
R = 0.8mOhm - Polyquest 45C-90C, Turnigy Nano-tech 45C-90C

For a123: 7mOhm * 2.300Ah = 1.61Ohm / cell (not per Ah as you wrote) WRONG
For LiPo: 0.8mOhm * 5.000Ah = 4mOhm / cell (not per Ah as you wrote) WRONG

Correct if wrong.

It's per Ah, and you don't get to divide on the A123, and multiply on the LiPo... Also, aside from getting the number wrong, you forgot the "m" in mOhm.

The point of specific Ri is to have a number you can compare between cells. Doesn't matter if it's a 10Ah cell or a 0.5Ah cell, you can immeadiately have a clean discharge performance index of how a pack of the same capacity would perform if it were made from X cells or Y cells.

I've given enough time to you.

Best Wishes,
-Luke

not sure what you meant by "... and you don't get to divide on the A123, and multiply on the LiPo..." but I believe I got basic idea.

Quote: “ Multiply the cell Ri by its capacity. This gives you a common ground to compare cells.”
7mOmh * 2.3mAh = 16.1mOhm/Ah (per Ah, not pack, right)

Thanks for sharing wisdom

 

[7circle]

Often Manufactures Quote Impedance of cells at Frequency say 1kHz.
This is different to the resistance measured from the Sag at steady DC.
You might see 2.6V from a 3.4V No load 2.3Ah Cell, when drawing 40A.

That's V/I = R = (3.4-2.6)/(40) = 20 mOhm

But they show this:
eg http://www.a123systems.com/cms/product/pdf/1/_ANR26650M1A.pdf

Nominal capacity and voltage 2.3 Ah, 3.3 V
Internal impedance (1kHz AC) 8 m1 typical
Internal resistance (10A, 1s DC) 10 m1 typical

Does n't really add up.

 

[dogman dan]

All that math zings right over my head, But take it from me, a guy who loves his pingbatteries, Go for the lipo, and a lyens controller to do 72v.

20s lipo rocks. 20c zippy's are cheap and in stock.

 

[sangesf]

The bet way is just to figure out how many amps max, your going to pull..
Currently (pun intended) I'm running 12-100ah Thundersky cells..
I know for a fact, that I NEVER pull more than 50a (highest I ever pulled was 48.x, so at less than a 1c rate, my pack should last quite a while.

Realize however, I'm only running 36v 750w motor/BMS/controller (set for max 50a)

Prior to that I was running a 36v 25ah pack and even tho I was still pulling 50a, it was a 2c rate, so it wasn't that bad.
(FYI, I still charge at a c/3 rate on every battery (lithium) I've ever owned.. I always try to stay below the 1c rate discharge and at about the c/3 charge rate.
But I'm super careful.)

For those who run higher voltage and higher wattage motors, they have other concerns and need/want as much power as they can get.

Also I try to get as much range as possible while still keeping voltage/wattage/speed/cost down.
In Florida where I live, max 20mph for a bicycle without needing a license (which I don't currently have)

 

[Spacey]

So me running my 12ah Headway cells at 40amps is going to be ok as it says that they are 5C continuos 10C peak?

The voltage drop is about 4volts I think off the top of my head.

 

[dogman dan]

Should be ok. But it could still be a problem depending on the type of ride. Racing, climbing huge mountians, etc. But it's really unlikely that you pull 40 amps continuous for all that long. Chances are, you pull 25 amps more of the time.

Depth of discharge can be another issue, so keeping the discharge less than 80% really helps the cells stay balanced and happy.

 

[liveforphysics]

So me running my 12ah Headway cells at 40amps is going to be ok as it says that they are 5C continuos 10C peak?

The voltage drop is about 4volts I think off the top of my head.

People often wrongly think C-rate is a big factor in cell life. It's actually one of the very small things.
Never charging a cold battery is a MUCH bigger factor.
Charging only 90% full and discharging to ~15-20% capacity is another very big factor in cycle life.

The C-rate the battery is used, as long as it's not allowed to over-heat, is a very minor factor in cycle life.