What is C Rating in a battery?

What does the C rating of a battery mean?

Guide to Battery Specifications: What is C rating of a battery?

Discharge current in a lead acid battery is expressed as a C Rate in order to normalise battery capacity. Batteries are usually subjected to constant current discharge & the capacity is estimated as Ampere Hours at a particular rate & a standard temperature. The variation in capacity with discharge is given by the Penkert equation 

Int = C where n & c are constants & I is the discharge current & t is the duration of discharge

A C1 rating means that the rated discharge current will discharge the entire battery capacity in One hour. For a battery with a capacity of 150 Ah, this translates to a discharge current of 150 amps for 1 hour. A C5 rate for this battery would be 150 Amps at a higher rate of discharge, and a C2 rate would be 150 Amps at an even higher rate of discharge current as shown in the example below.

The difference is only in the rate of discharge. A 150 AH battery at C₂₀, will last for 20 hours on a load of 7.5 A. 

A 150 AH battery at C₁₀ will last for 10 hours on a load of 15 A. A 150 AH battery at Cwill last for 5 hours at a load of 30 A.

C10 rated battery is capable of discharging at higher current, whereas a C₂₀ rating can discharge at lower current. C₁₀ rated batteries are always recommended for solar and industrial purposes with the best charging and discharging rates. As the high load uses battery power, it is capable of delivering more energy in a short time, C₁₀ 

Ampere Hour or C₂₀ is an indicator of how much energy is stored in a battery. It is the energy a battery can deliver continuously for 20 hours at 25°C without falling below 10.5 volts. A Battery rated at C₁₀ by the manufacturer will give a capacity higher than 100 % when discharged at C₂₀ rate or lower rate. But it requires adequate volume of acid to yield the higher AH capacity.

Rate of
Discharge
Discharge current
for 100Ah Battery
rated at C₁₀ __
Amperes
Discharge
Duration
in Hours
Capacity
as % of C₁₀
yield
Remarks
C20 5.5 A 20 hr 110** ** Adequate acid volume required for higher Ah
C10 10 A 10 hr 100 Standard for UPS, inverter battery, IS 13369
C9 10.87 A 9 hr 97.9
C8 11.7 A 8 hr 93.6
C7 13.1 A 7 91.7
C6 14.65 A 6 87.9
C5 16.66 A 5 83.3
C4 19.55 A 4 78.2
C3 23.9 A 3 71.7
C2 31.65 A 2 63.3
C1 50 A 1 50

C Rating of Automotive batteries

Automotive batteries are traditionally rated in C20. This has no particular significance. The requirement for an auto battery is to start the engine and help the vehicle to reach a service station in 2 hrs. This is the old requirement. Presently the SLI battery has other functions like supporting the working of Air-Conditioner and other many other power-hungry gadgets when the vehicle stops at traffic signals or during slow driving in crowded roads. Therefore the auto battery   for a small car has to meet the following minimum

Power requirement of a small car at traffic Junctions and slow run  in India

Function Current Amps Duration
Starting 150-250 Amps Seconds CCA for cold climates
Air Conditioner 8-10 Amps 3-5 Minutes Longer duration in slow traffic
Other gadgets Radio GPS etc 5 Amps 5-10 minutes

The above drain on batteries exist during driving at all traffic signal stops and while driving in heavy traffic bumper to bumper.

Therefore, a modern battery is designed to withstand all  road conditions.  A recent introduction of a battery called ENHANCED FLOODED BATTERY (EFB)  currently used in India, Europe, Japan etc are meant for STOP – START applications where the car automatically stops at traffic junctions and restarts as required. Such batteries need a special design & hence called EFB batteries

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