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Discussion Starter · #1 ·
Hi all,

I have been looking through the interweb on the definition of C charge rates and how they are calculated by manufacturers for each battery.

Can someone tell me how manufacturers come up with their 'C' rating for a specific type of cell?

Is it the internal resistance of the battery the factor?

For example the rating for the Calb CA series cells is 3C but is this to 100% or can it achieve higher C rating before hitting the CC/CV level?

Thanks!

Leigh
 

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OEM C rates tend to be a combination of several battery effects all summed up... to a number that makes the engineer's / user's job easier.

The goal of the C rate limit recommendations is to prevent or greatly reduce the amount of permanent cell damage.... without making it so complex that everyone involved has to have a degree about electrochemical reactions.

- - - - - - - -

So yes internal resistance plays a part ... so does cell initial temperature ... ambient temperature ... the SoC ... the diffusion rate of that particular battery ... etc.

And there is also a difference between the continuous C rate and the Pule C rate 10s or 1s ... because as the usage context changes , you get different results.

- - - - - - - -

Some people push the limits of the cells ... and the cells might take it for a while ... but it can come at the cost of shortened operational life time ... others baby the cells but getting longer operational life times.
 

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So , why then do manufacturers quote a different max charge rate vs the max discharge rate ??
( which was the OP question)

Its called CYA, its a fact that those CA cells can handle 3c charge rate or even more, whether or not it shortens its life time, hard to tell, all the tests I've seen show no appreciable heating or loss of capacity at 3C, which is generally a good indicator.

But its actually irrelevant, cause where you going to get 3C from, unless its a real small cell.

Roy
 

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without making it so complex that everyone involved has to have a degree about electrochemical reactions.
But to be fair ... if you want to get more into the details of how / why / etc ... Perhaps one good place is the attached 150page pdf... beware, it is a little bit dry and technical.
 

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Discussion Starter · #11 ·
Its called CYA, its a fact that those CA cells can handle 3c charge rate or even more, whether or not it shortens its life time, hard to tell, all the tests I've seen show no appreciable heating or loss of capacity at 3C, which is generally a good indicator.

But its actually irrelevant, cause where you going to get 3C from, unless its a real small cell.

Roy

Hi Roy,

So why would a Calb CA cell for example have a 3C charge rate and has been rated for 12C discharge, if your theory is correct wouldn't these numbers be interchangeable?

I am working on a project that has a 200KW DC source so I do have enough power.

Leigh
 

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I was talking about charge rates, there is a distinct difference between the the charge and discharge rates in most cells. I just wanted to know why they are so different.
That pdf is a good place to start with the chemical reasons why there is a difference ... but it would also be worth noting the marketing effects on the OEM claims ... Higher Discharge Rates are more marketable than higher Charge rates are ... both are nice ... but 10C vs 5C discharge will sell better than 10C vs 5C Charge rates.

Part of that is infrastructure ... how common is a 100kw line available to charge a 10kwh battery.

Part is usage patterns ... most people's operational conditions require the battery to be discharged much faster than charged ... they often have more time to allow it to charge... ~8 hours over night to charge a BEV battery is not as much of a inconvenience as ~8 Hours to do the commute trip.
 

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I have this testing done on my cells before we quote a C rate. The C-rate for discharging gives different power levels. You can check these out at my facebook page "Lonestar EV Racing"

 

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Discussion Starter · #15 ·
I have this testing done on my cells before we quote a C rate. The C-rate for discharging gives different power levels. You can check these out at my facebook page "Lonestar EV Racing"

Hi John,

These seem to all be from LiPo cells, I am assuming that the charge cycle of a LiPo cell would be significantly different compared to a LiFePo4 cell.

Can anyone else shed some light on this?

I have been reading into the PDF and understand that the charging regime for the first couple of cycles is important because the SEI and cell properties have not been fully formed but after that what stop the cell from being charged as fast as it can be discharged?

Thanks

Leigh
 

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Hi Roy,

So why would a Calb CA cell for example have a 3C charge rate and has been rated for 12C discharge, if your theory is correct wouldn't these numbers be interchangeable?

I am working on a project that has a 200KW DC source so I do have enough power.

Leigh
The question is why would you want to do that in the first place unless you were racing a vehicle, and at that point ask some who have done so, there are a few in here. I know for a fact you can put a lot more in those CA cells than stated by CALB. But you need to understand that they will always be on the conservative side, for reason I stated before.

Just because you have such a source available doesn't mean you have to use it, why stress the cell, unless you have a good reason to do so.

If you have a need to know more, go to EVTV and look at some of the tests done there. Don't take my word or anyone else's here, watch the tests and judge for yourself.


Roy
 

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Hi !

In the data sheet for the A123 pouch you can see the relasionship between charge and discharge rates pretty well.

http://www.raceyard.de/tl_files/Newsletter/Dateien/A123-AMP20-M1HD-A-1-Data-Sheet.pdf

it shows best discharge power rate in 10s pulse to 45C @ 1,6v
and best charge power rate in 10s pulse to 14,5C @ 3,8v

regards
/Per
Hi Per,

Your C-rate calculations are best case for discharge and worst case for charge. If you use the nominal power of 65 Watts (@ 1 hour) for your base, then the maximum discharge E-rate = 23 and charge E-rate = 17. They also list a nominal discharge power of 1200W. From the 10s pulse graph, that would be at 50% SOC. The charge power for 10s at 50% SOC would be 950W. From both of these comparisons, it looks like charge power can be 70 to 80% of discharge power.

Different ways of looking at it ;)

major
 

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The question is why would you want to do that in the first place unless you were racing a vehicle......,
With respect Roy, .... it doesnt matter why he might want to... he was just asking why the difference.
...and he didnt say he was going to do it.!

....Just because you have such a source available doesn't mean you have to use it, why stress the cell,....
..and there you seem to be giving a reason , but one that still needs a technical explanation[/QUOTE]
 
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