What Exactly Is a Battery C Rating? Here’s What You Need to Know!

The charge and discharge rate of a battery is controlled by the battery C rate. The battery C rating is a measurement of the current when the battery is charged and discharged. Battery capacity is usually rated and marked with a 1C rating (1C current), which means that a fully charged battery with a capacity of 10Ah should be able to provide a current of 10 amps for one hour. The same 10Ah battery discharged at a C rating of 0.5C will provide a current of 5 amps in two hours, and if discharged at a 2C rating, it will provide a current of 20 amps for 30 minutes. It is important to understand the C rating of a battery because for most batteries, the available stored energy depends on the speed of the charge and discharge current.

Battery C rate table

The figure below shows different battery C rates and how long they can be used. It is important to know that although discharging a battery at different C rates should use the same calculations as the same energy, there may actually be some internal energy losses. At higher C rates, some of the energy may be lost and converted into heat, which can cause a capacity reduction of 5% or more.

To get reasonable capacity readings, manufacturers often rate alkaline and lead-acid batteries at a very low discharge rate of 0.05C, or 20 hours of discharge. Even at this slow discharge rate, lead-acid batteries rarely reach 100% capacity because the batteries are overrated. If the discharge rate is higher than specified, manufacturers provide a capacity offset to adjust for the difference.

How to Calculate a Battery's C Rating

The C rating of a battery is defined by the rate of time it takes to charge or discharge. The C rating can be increased or decreased, so this will affect the time it takes to charge or discharge the battery. The charge or discharge time for the C rating will vary depending on the rating. 1C equals 60 minutes, 0.5C equals 120 minutes, and the 2C rating equals 30 minutes, using the following formula:

t = time

Cr = C Rate

t = 1 / Cr (view in hours)

t = 60 minutes / Cr (view in minutes)

0.5C Rate Example

2300mAh Battery

2300mAh/1000=2.3Ah

0.5C x 2.3Ah = 1.15 Amps Available

1 / 0.5C = 2 hours

60 / 0.5C = 120 minutes

2C Rate Example

2300mAh Battery

2300mAh/1000=2.3Ah

2C x 2.3Ah = 4.6 Amps Available

1 / 2C = 0.5 hours

60 / 2C = 30 minutes

30C Rate Example

2300mAh Battery

2300mAh/1000=2.3Ah

30C x 2.3Ah = 69 Amps Available

60 / 30C = 2 Minutes

The output current, power, and energy of a battery can be calculated from its C rating using the following formula.

Er = Rated Energy (Ah)

Cr = C Rate

I = Charge or Discharge Current (Amps)

I = Cr * Er

Cr = I / Er

How to Calculate a Battery's C Rating

Smaller batteries are often rated at 1C, also called a one-hour charge rate. For example, if a battery is labeled 3000mAh at a one-hour rate, the 1C rating is 3000mAh. You will usually find the C rate of a battery on the battery label and on the battery datasheet. Different battery chemistries sometimes show different C rates; for example, lead-acid batteries are often rated for a very low discharge rate, usually 0.05C or 20 hours. The battery's chemistry and design will determine the maximum C rate of the battery. For example, lithium batteries can withstand much higher discharge C-rates than other chemistries such as alkaline. If you cannot find the battery C-rating on the label or datasheet, we recommend contacting the battery manufacturer directly.

Applications Requiring High C-Rates

There are an increasing number of applications and devices on the market that require high C-rate batteries. These include industrial and consumer applications such as remote control models, drones, robots, and car jump starters. All of these applications require powerful bursts of energy in a short period of time.

Most jump starters may require discharge rates up to 80C, and in the RC industry, high discharge rate batteries are available at rates up to 50C! There are some batteries on the market that claim to have higher C-rates based on maximum pulse discharge rates, which requires the battery to be fully discharged in a few seconds. However, most applications do not require such high C-rates.