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.
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
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.
how much charging time is required for 1 day back up of the rechargeable battery?
Your graph is confusing. Shouldn’t 1h=1C, 3h=0.333C and the 1C line be coincident to the 1h line?
I have system with 110pcs of 450Ah/2V in serial connection. I have problem with battery system. How I can find problematic battery?
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?
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?
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 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 !
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
@ Jon
your quick example is much more explanative than the whole paragraph who seems to circle around…
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/
So the C-rates are just another way to express a current? 1C=1A (Ampere), 0.5C=0.5A= 500mA, 2C=2A=2,000mA ?