What is the C-rate?

In the late 1700s, Charles-Augustin de Coulomb ruled that a battery that receives a charge current of one ampere (1A) passes one coulomb (1C) of charge every second. In 10 seconds, 10 coulombs pass into the battery, and so on. On discharge, the process reverses. Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.

Most portable batteries are rated at 1C, meaning that a 1,000mAh battery that is discharged at 1C rate should under ideal conditions provide a current of 1,000mA for one hour. The same battery discharging at 0.5C would provide 500mA for two hours, and at 2C, the 1,000mAh battery would deliver 2,000mA for 30 minutes. 1C is also known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is a half-hour discharge.

The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. The analyzer discharges the battery at a calibrated current while measuring the time it takes to reach the end-of-discharge voltage. An instrument displaying the results in percentage of the nominal rating would show 100 percent if a 1,000mAh test battery could provide 1,000mA for one hour. If the discharge lasts for 30 minutes before reaching the end-of-discharge cut-off voltage, then the battery has a capacity of 50 percent. A new battery is sometimes overrated and can produce more than 100 percent capacity; others are underrated and never reach 100 percent even after priming.

When discharging a battery with a battery analyzer capable of applying different C‑rates, a higher C‑rate will produce a lower capacity reading and vice versa. By discharging the 1,000mAh battery at the faster 2C, or 2,000mA, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same as with a slower discharge since the identical amount of energy is being dispensed, only over a shorter time. In reality, internal resistance turns some of the energy into heat and lowers the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over two hours, will likely increase the capacity to above 100 percent.

To obtain a reasonably good capacity reading, manufacturers commonly rate lead acid at 0.05C, or a 20-hour discharge. Even at this slow discharge rate, the battery seldom attains a 100 percent capacity. Manufacturers provide capacity offsets to adjust for the discrepancies in capacity if discharged at a higher C‑rate than specified. Figure 1 illustrates the discharge times of a lead acid battery at various loads as expressed in C-rate.

Typical discharge curves of lead acid as a function of C-rate

Figure 1: Typical discharge curves of lead acid as a function of C-rate

Smaller batteries are rated at a 1C discharge rate. Due to sluggish behavior, lead acid is rated at 0.2C (5h) and 0.05C (20h).

While lead- and nickel-based batteries can be discharged at a high rate, a safety circuit prevents Li-ion with cobalt cathodes from discharging above 1C. Manganese and phosphate can tolerate discharge rates of up to 10C and the current threshold is set higher accordingly. 

Comments

On April 29, 2011 at 9:22am
Luca wrote:

So the C-rates are just another way to express a current? 1C=1A (Ampere), 0.5C=0.5A= 500mA, 2C=2A=2,000mA ?

On August 22, 2011 at 11:49am
Matt wrote:

C-rates are the rate of discharge (or charge) as compared to the capacity of the battery.  A 1C discharge will be the current that would seemingly go through the rated ampere-hours of the battery in an hour.  A 2C rate is twice that current.  A 0.5C rate is half that current.  In the specific case of a 1 Ah battery given above, the numbers come out equivalent to amps, but in the general case, the C rate is different.  For example, a 5 Ah battery would (by rating alone) have a 1C current of 5 amps.  A 2C current would be 10 amps.  The 0.5C current would be 2.5 amps.  The real capacity of the battery changes with discharge rate though, so actual performance differes from the ideal capacity.

On September 14, 2011 at 11:02pm
amutha wrote:

how much charging time is required for 1 day back up of the rechargeable battery?

On October 21, 2011 at 8:31am
Crazy Casta wrote:

Your graph is confusing. Shouldn’t 1h=1C, 3h=0.333C and the 1C line be coincident to the 1h line?

On November 22, 2011 at 12:04pm
Srdjan Dragojlovic wrote:

I have system with 110pcs of 450Ah/2V in serial connection. I have problem with battery system. How I can find problematic battery?

On January 19, 2012 at 2:34pm
Howard Silverwater wrote:

With using a 38 Ah deep cycle lead acid battery and at discharge rate of 20 hours (ham radio 75% duty cycle; receive at 1.7A and transmit at 5.5A) with a solar panel supplying 0.8 A, why does a 12.25 V reading during transmit (5.5A) give a lower SoC (state of charge) than if I was NOT to consider the solar panel contribution and used HIGHER discharge rate (of ~14 hours)? According to typical lead acid graphs showing SoC vs. voltage while under discharge, a reading of the same 12.25 V shows a higher SoC for a faster discharge rate. I would expect an influx of 0.8 V from solar energy that contributes to slow down the discharge rate to show a higher SoC. What’s wrong with line of thought?

On February 12, 2012 at 6:00am
Johny wrote:

So, to achieve the desired C-rate, the batteries must be connected in parallel circuit…

If I ‘ve got 4 x 0.5C batteries with 12V, I can set a combination to 2C and 12V. Is it correct understanding?

On March 8, 2012 at 12:23pm
Jon wrote:

Johny—
I believe that in your example you’d still have a 1C battery (pack).  Assuming the following batteries:
  (4) 12V, 10AHr batteries (1C: thereby capable of delivering 10A over 1Hr)
Hooking them up in parallel, you’d end up with a configuration that is:
  (1) 12V, 40AHr battery pack (1C: thereby capable of delivering 40A over 1Hr).  Therefore, you’d still have 4x the current capability of a single battery (40A, instead of 10A), but it would not be possible to drain the pack in about 1/2 hour (as with a 2C battery), or about 15 minutes (as with a 4C).

On March 15, 2012 at 10:32am
gianni pup wrote:

On Li-ion battery cell technical specification I found the following charateristic :
Standard charge = ” Standard charge ” means charging the cell with charge current 1075 mA and costant voltage 4.2 V. at 25°C., 0.02 cutoff.
Could someone explain me the meaning of cutoff ?
Many thanks in advance !

On April 14, 2012 at 9:58pm
gurivireddy wrote:

Hi,
can some if explain me what happens to the C-rating if I connect two identical batteries in series, and in parallel.
do they change or they remain same?

thanks in advance

On May 4, 2012 at 3:06pm
styvens wrote:

@ Jon
your quick example is much more explanative than the whole paragraph who seems to circle around…

On May 16, 2012 at 2:19pm
Steve Spence wrote:

The term everyone seems to be dancing around is peukert. If you have a 100ah battery at the c20 rate, you can discharge at 5 amps, for 20 hours. If you pull faster, you’ll have less capacity, if you pull slower, you’ll have more. You can calculate this at http://green-trust.org/peukert/

On May 28, 2012 at 1:48pm
MySchizoBuddy wrote:

What about min and maximum charging current for a particular battery capacity.

On September 6, 2012 at 3:13am
hifasath wrote:

sir ,,
        i had on no of 12v 150ah c20 & 2 nos of 80 ah
which will make better performance

On March 26, 2013 at 12:01am
thinira wrote:

can anyone explane what is charge capacity, discharge capacity and coulombic efficiency?

On May 22, 2013 at 11:40pm
gunwoo wrote:

1C is a 1000mAh battery in one hour to supply 1000mA when you when 0.5C, 2C change when going to both the current time and why? When the total current 1000mA La 0.5C to 2C if ilttaen 2 hours 30 minutes 1000mA 1000mA supply is wondering whether or not haneunge?

On July 10, 2013 at 3:21pm
Olddawg wrote:

I came her to understand what a 1C is compared to a C1 rate.
What I understand is that it’s only an invert stat that means the same.

A C20 battery, which I work with, is a battery rated at a 20 hour draw.
Then could be stated as 20C

A 1C battery is the Ah at 1 hour draw
Then could be stated C1

It all comes down to the amount of time the battery can be drained.

Drain your battery, time it.
That’s your real C rate of use.
Want to improve the result because it’s under-performing??

De-sulfate the battery.

On July 15, 2013 at 8:33pm
Jason wrote:

On March 15, 2012 at 10:32am
gianni pup wrote:
On Li-ion battery cell technical specification I found the following charateristic :
Standard charge = ” Standard charge ” means charging the cell with charge current 1075 mA and costant voltage 4.2 V. at 25°C., 0.02 cutoff.
Could someone explain me the meaning of cutoff ?
Many thanks in advance !
===============================
In the case of your Lithium-ion charge, it interprets as the charger will begin from Constant Current(CC)mode, at the point when the voltage reaches 4.2V, the charger switches to CV(constant voltage) mode; the charge process won’t end until the current drops to the threshold of 0.02(20mA)

On November 18, 2013 at 12:27am
naresh wrote:

can any one explain, how do we choose the C rates, when we are developing the new battery. I Mean we do not know how much capacity it can store, in this case how we can choose the C rates.

On November 18, 2013 at 1:54pm
Olddawg wrote:

You don’t choose C-Rate. The response of dis-charge determines the C-Rate.

Since you’re developing a ‘new’ battery (of which I’d love to char further), I would start dis-charging small and work your way up.

You only know what you anticipate from the battery.
I used half that to draw and time it.
You may find you are correct and it draws 30-40+hrs before done.
Then you have a C30-C40 battery.

Up the draw till you hit the 20hr time length.
Now you have the draw matched to the battery.

‘Done’, becomes the next question..
I’ll await..

On November 18, 2013 at 11:45pm
naresh wrote:

Thank you for your replay mr. Olddawg.

You mean to say that we have to select the charging current such that it can full charge (100% capacity) the battery in 20 hr. Once we do this we will find out the full capacity of battery and according to this full capacity we have to select the C-rates. Initially I thought that we need to select the C-rates according to the theoretical capacity of anode and cathode (combined capacity) do we select C-rates according to theory?.

On November 19, 2013 at 2:22pm
Olddawg wrote:

You’re in new territory for me with a newly designed battery!
You’re thought of using the theoretic capacity is good for charging.
I still would go lighter and time it.
You may again find it will take 30hrs, yet better that going full and charging in 10hrs and slamming the battery with too much.

To me (MHO), the real test of capacity is going to come with dis-charge.

Without knowing what you truly have as a concept, and the possibilities being abundant, it’s difficult to say “Do this”...

Initial charge of something I just built, personally, I’d have a hard time going over 50% of what I believe it will do. Especially for the first run..
I’ve burned out too many good ideas (circuits) because I ms-judged the capability. 

I wish you great success in your endeavor!
Please keep me posted as to your results.

 

On November 19, 2013 at 10:15pm
naresh wrote:

Thank you,

Actually I am new to this field. That is why I am getting these silly questions and I am learning slowly and after that only I will start making the battery.

  One last question is that while calculating the theoretical capacity of a new battery do we take only cathode capacity or only anode capacity or combined capacity.

On November 20, 2013 at 3:32pm
Olddawg wrote:

Oh so many more questions…

What is the seperation?
What is the solution involved?
How is it being circulated?
Area of plate (if plates are involved)?
Quality of all?
Many more could come..

Oh my!
It’s not just as simple as capacity of either.. or both combined..

If no one else here ‘chimes in’ and you wish to continue..
Please send me a personsal email.
I will hook you up with some very good minds I work with!

Let’s see if this site can get you going first.
If not, write me!

 

 

 

On January 17, 2014 at 3:09am
Anne wrote:

I am also new on developing new batteries and have a similar question.

We have two devices: the old we used for CV and impedance and with the new (better) one we can also measure e.g. constant current measurement.

So when we want to start a test with the new device we have to add a c-rate. Since we do not know the capacity of the test cell, do we first have to measure CV on our old device to know the highest achievable capacity, so that we can calculate e.g. 0,1C?

And:
On our old device every cycle of one test cell needed the same time. In the first tests with the new one we noticed that the time for one cycle depends on the capacity of the cell (so a short cycle shows less capacity in the capacity-cycle-diagram). Does this mean that the new device notices when all active material has reacted and automatically starts the charging process, or what else can be the reason?

On January 20, 2014 at 6:04am
Jaan wrote:

The C-rating has nothing to do with the physical unit “coulomb”, so the first sentence of the article is misleading. 1 C is indeed the standard abbreviation for the SI unit 1 coulomb, but in this article, the notation 1C is used to mean a totally different thing! “Coulomb” is a unit of charge; it is just another word for ampere-second and could be used for measung battery capacity instead of milliampere-hours: 1 coulomb = 1 A * 1 s = 1 A * 1/3600 h = 1/3600 Ah = 0.278 mAh. What is denoted by 1C in this article, has actually the units of frequency, i. e. 1C (in this article) = 1/(1 h) = 1/3600 Hz = 0.278 mHz (millihertz).

On January 27, 2014 at 5:43pm
Steve wrote:

It sounds like to me you need to know the theoretical capacity of your battery before charging/discharging to apply this system? Is it possible to test your battery’s capacity using different C values if you are testing new anode/cathode technologies?

On January 28, 2014 at 1:22am
Anne wrote:

@Steve:
If you just run cyclic voltammetry (potentiostat) you do not need any capacity value, you get it from this experiment. If you use a battery cycler you need a C-rate (so: yes, using different C values is possible, most people does). If you need the theoretical capacity of your system (e.g. for Li-S, in the literature) or the capacity for your exact testcell I don’t know (that’s one of my questions).

On January 29, 2014 at 2:37pm
Steve wrote:

@Anne:
Thanks for the response!
I am at the same place you are then smile
When you run cyclically (potentiostat), I assume you then get the capacity related to the current over time?
Do you know how this compares to a “typical” charging profile where you hold the current (then the voltage) constant? http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

Bump for the question on a method to calculate the theoretical capacity!

On January 30, 2014 at 12:28am
Anne wrote:

(Do you use skype? If yes we can also discuss there. -  It would be awsome if a discussion of e.g. batteries could be done between different scientist via skype or something else, so that everyone get soon help…)

Hm, I just tell about our devices, maybe this helps you. If we run cyclic voltammetry in our old device we just have to add the parameters voltage range, scan number, Esteps and scanrate. Than you get a diagramm in which the voltage is the x-axis and current is the y-axis. Than you just have to calculate the area under the redox-peaks and with this (and the active mass) calculate the specific capacity.

If we use our new device we get a diagram in which the time is the x-axis, one y-axis is capacity and the other one is the voltage. Before the measurement here you have to add a c-rate. I guess it is like this: If you take the theoretical capacity of your system (Ah/g), transform this to your actual active mass you get the number you need for e.g. 0,1C.

Right now I can not tell you more, I do not really know what this fig.1 in the link is (but I work not Li-ion batteries).
Does this help somehow?

On February 27, 2014 at 4:02am
harry potter wrote:

do you use skype? If yes we can also discuss there. -  It would be awsome if a discussion of e.g. batteries could be done between different scientist via skype or something else, so that everyone get soon help…
i just want to ask something about that battery and explain me how to make it in russia\
benelmokadem
harry potter movies in order

On April 14, 2014 at 8:20pm
Jagat Modi wrote:

Can any body share the the probability of lead acid automotive battery being exploded dusting cracking of an engine ? And also the same battery being used for DG starting with permanent float cum boost charger connected ?
And what are the SOP for lead acid batteries used for such applications ?

On April 14, 2014 at 8:23pm
Jagat Modi wrote:

Can any body share the probability of lead acid automotive battery being exploded durting cracking of an engine ? And also the same battery being used for DG starting with permanent float cum boost charger connected ?
And what are the SOP for lead acid batteries used for such applications ?

On April 16, 2014 at 4:42pm
Steve wrote:

@Anne:

I have been busy working over the past few months, if you still check this site i would be happy to try and get a discussion going on. I am somewhat hesitant to put my skype name or email up on a permanent online post, but I think there could be a way to get in contact at least through an email.

Let me know if you are still using this with a post! I plan on checking this page for a week or two.

On April 16, 2014 at 4:51pm
Steve wrote:

@anne:

In follow up I have some questions about the voltammetry; I plan on using the technique myself XD but in my initial learning I didnt see how it could be applied to a theoretical capacity. Wont the measurement only tell you the potentials at which reactions inside the cell are occurring? i.e. lithium insertion/removal? I think that you would need some sort of time scale to know the amount of charge you are putting into the batter? Or perhaps you are cycling until you no longer see a reaction taking place?

On April 17, 2014 at 5:00am
Anne wrote:

@Steve
Hi, I also was (better said am) busy, but I want to still write here.

First of all: what device do you use (can you give me a name or so)? As I wrote we have two different ones, so it depends a bit if its more of a battery cycler or a potentiostat (at least in this way we name or two ones).
... It would be best if we could write mails, please write me on this adress (just a note is enough, that I have your address): vi-friends.v.af(at)web.de
Since is address I normally don’t use, its just for such cases (I will write you back from another, so don’t wonder; I do not want to write my real one here, because I already get enough spam mail ; )

On April 17, 2014 at 5:04am
Anne wrote:

@Steve
(Ignore the mistakes, I wrote too fast…)
When you send me a mail I will reply with some images, this makes discussion easier (and I can not add pictures here, so mail is the best).

On April 17, 2014 at 10:01am
Steve wrote:

@Anne

I have sent an email to that address, let me know through that address if you have received it :D

I can answer your questions from this forum through that address if it works!

On July 29, 2014 at 5:24am
Yogesh Yadav wrote:

Dear All

we have battery charger of 270 V, 10 Amp,  i want to know that what no of batteries i can charge on this of 12V 7.5 AH.