# C Rating



## evmetro (Apr 9, 2012)

I am shopping around for a pack for a new build, and wanted to clarify the meaning of C rating. I looked pretty hard at the wiki, but could not find much there. Does anybody know of a link or to to point me to? Or maybe just a quick definition?


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## Ziggythewiz (May 16, 2010)

That's how much a battery (or cell) can handle specified in terms of it's capacity. It's used to indicate nominal and max charge and discharge rates.

A 50AH cell putting out 100A is being discharged at 2C (100/50).


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## EVfun (Mar 14, 2010)

Every time I get on the throttle it is more like 5C, pulling 300 amps from a 60 amp hour cell. I charge at 0.2C, 12 amps into my 60 amp hour cells.


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## evmetro (Apr 9, 2012)

Thanks. If a 600 amp load where applied to a 200 ah pack rated for 3c, would it make any difference in performance compared to a 200ah pack that was rated for 10c? Is there a downside to running a pack right up to its max c rating?


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## MN Driver (Sep 29, 2009)

C ratings are chosen by cell manufacturers and some have been exagerated a bit. Basically you don't want the cells to overheat, for high short term (10-15 second) acceleration discharges you need to be sure that internal interconnects aren't overheating and that there aren't going to be any hot spots internally in the cell that would cause those overheated spots to cause a problem that might reduce the capacity or raise the internal resistance. Worst case, you melt the separater and short the cell.

For a continuous rating it's basically a function of the cell heating over the discharge to a point, where if you draw too much current for a long period of time you can damage the cycle life of the cell. ..or extreme enough, destroy it quickly.

It turns out that some cells can do very well at decent C rates such as CALB CA cells. We used to have people on this forum who used older Thundersky cells in 2008 that tried to do a continuous discharge tests on their cells and they couldn't maintain a voltage above 2.5 volts over a long discharge at 3C and even an acceleration discharge had them sagging pretty low. The CALB 180Ah SE cells sagged to 2.5V at 1000 amps and that was good for an acceleration discharge. Smaller cells can have higher C rate discharges because their voltage sags less. Headway cells have a reputation for a higher C rate, yet they get pretty hot if you discharge them continuously at their specced C rate which leads me to believe they are overrated a bit. I don't think I'd ever build a car with them myself, they are decent cells though but many people like the A123 cells as they sag less and don't produce much heat when discharged heavily. Each form factor has it's different difficulties with installation.

Basically the more voltage sag, the more the cell heats up which limits your discharge C rate. It's a function of resistance.


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## Elithion (Oct 6, 2009)

evmetro said:


> wanted to clarify the meaning of C rating


Don 't we all!

C-rating is a marketing tool, not a physical parameter.

The very same cell could be marketed as having a rating of 1 C, and a lifetime of 2000 cycles, or having a rating of 30 C, and no lifetime mentioned.

What IS a useful physical parameter is the internal_ DC resistance_, but only a couple of cell manufacturers specify it. A whole bunch specify the _AC impedance_ at 1 kHz, which is a great tool for the cell manufacturer and easy to measure, but has _no bearing _on the DC resistance.

To that effect, I am promoting the concept of "Short Discharge Time", a characteristic of cell technology that is mathematically derived from measurable, physical parameters. It is a useful tool to compare cells and determine their suitability to high power use: lower Short Discharge Time = more efficient. (And it is free of marketing influences.)


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## Caps18 (Jun 8, 2008)

It is probably not very good for the battery, and the 10C rate is only for 10 seconds in most cases. 3C is the continuous rate.

Now, I don't get the difference between the motor amps, controller amps, and battery amps. I would like to think that 900A limit on the motor, 900A limit on the controller (1000A max), and a 9C rate with a 100Ah battery producing 900A would be correct. Is this right? You might have some voltage sag and not be able to create as many watts at the motor if the battery isn't full.


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## EVfun (Mar 14, 2010)

Now it gets tricky (not really.) A controller works kinda like a transformer, only for DC. The term "controller amps" probably shouldn't be used as it would need defining each time. Battery amps is the current flowing to the controller from the battery pack. Motor amps is the current flowing through the motor and cables between the controller and motor. 

Motor amps can, and usually is, higher than battery amps. This is not magic, energy is conserved. Motor voltage is usually lower than battery pack voltage. The watts (volts times amps) being supplied by the battery pack equal the watts being fed to the motor. The motor voltage can never be higher than the pack voltage and so the motor current can never be lower than the battery current. This is a limit of the common buck circuit used in almost every large DC motor controller.


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## evmetro (Apr 9, 2012)

Davide, this is the kind of information that I was hoping for when I started the thread, thanks. Marketing for products is often misleading. If the dc internal resistance data can be found, what is a good reference starting point for a "respectable" number to use for short term discharge ability, and continuous discharge ability in lieu of using the c rating?


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## Elithion (Oct 6, 2009)

evmetro said:


> If the dc internal resistance data can be found, what is a good reference starting point for a "respectable" number to use for short term discharge ability, and continuous discharge ability in lieu of using the c rating?


See, here lies the problem: the answer depends on the capacity of the cell. 
1 mOhm may be a great value for a 5 Ah cell, but a terrible one for a 200 Ah cell. 

That is why I am trying to steer people towards using Short Discharge Time instead, whose value is independent of capacity and voltage.










What's good? Depends on your application and your budget.


A Short Discharge Time of 150 s is plenty good for standard EVs (not race EVs), and is met by Thundersky-like cells.
A Short Discharge Time around 50 s is very good, appropriate for HEVs and race vehicles, and is met by many cells.
A Short Discharge Time below 20 s is excellent, but it may be overkill for your application.
Short Discharge Time is only one parameter. You also have to consider cost, availability and energy density.


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## Sunking (Aug 10, 2009)

evmetro said:


> Thanks. If a 600 amp load where applied to a 200 ah pack rated for 3c, would it make any difference in performance compared to a 200ah pack that was rated for 10c? Is there a downside to running a pack right up to its max c rating?


Yes it makes a huge difference. First thing you need to read up on is Peukert Law which states: As the rate increases, the battery's available capacity decreases. It mostly pertains to lead acid batteries but does affect Lithium to a lessor degree. For LA batteries rated at say 100 AH at a C/20 discharge rate can change it to 20 AH at a 1 hour rate.

For an EV with say lithium batteries C rate is an expression of either the charge/discharge rate with respect to the AH rating of the cell. So you are correct if you have a 200 AH pack and either charge or discharged at 600 amps = 3C or C3.

What is most important and a very elusive number is the battery internal resistance, and how it effects voltage sag. For example a battery is practically useless if it can deliver 5C and its voltage drops 50%. Let's say the battery is rated 50 AH @ 3.6 volts. Apply a load that draws 250 amps but the voltage falls or sags to 1.8 volts. That means the cell internal resistance is 1.8 volts / 250 amps = .0072 Ohms. That is actually pretty low internal resistance for a battery, but way too high to be useful in a EV application. 

The problem is compounded as the battery discharges because as the SOC goes lower the internal resistance goes up.


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## Elithion (Oct 6, 2009)

Sunking said:


> Let's say the [cell] is rated ... 3.6 volts. Apply a load ... the voltage falls or sags to 1.8 volts. ... That is actually pretty low internal resistance for a battery, but way too high to be useful in a EV application.


Well, no, on the contrary. What you describe is the peak power point, which is far from useless: that's when you can get the most power out of a cell. Which is good. In fact, that's where race EVs like to operate.

However, the efficiency at the peak power point is 50 %: half the power does work, the other half heats the cell. Which is bad: it is wasteful and the heat causes damage to the cell. That's why a standard EV does not want to operate there.


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