How does a battery C rating describe its characteristics?

Engineers have given batteries a standardized C rating to describe discharge characteristics and safe operating ranges.  This rating compares the amount of current drawn from a pack to the overall pack Amp-hour (Ah) specification. To figure the rate, multiply the C rating by total pack Amp-hours.  This product determines the maximum current that can be drawn from each cell without fear of cell damage or explosion.

LiFeBATT cells have a peak 20C rating and, within a 10Ah pack, LiFeBATT cells can discharge impulses of 200 Amps for 5 seconds. LiFeBATT cells also have a 14C pulse rating, which translates to 140 Amps discharged for 18 seconds from a 10Ah pack. LiFeBATT cells also have a 12C continuous rating, which translates to 120 Amps discharged continuously from a 10Ah pack. 

At 5C and less, LiFeBATT cells have such a solid voltage that for as long as the pack is in shallow discharge cycle (70% of total capacity or less) the battery pack voltage will remain unaffected by the current draw. Basically, you can draw 50A continuously for every 10Ah in the pack and the pack voltage will be sustained until current capacity is 70% depleted.

The comparative disadvantage with PbA chemistries is the dynamic internal resistance of each cell that increases as more current is discharged while the electrolyte, anode and cathode components undergo a chemical transformation that has to be reformed during the recharging period.  The results of this non-linear chemical change is called the “Peukert” effect of lead-acid batteries, where cell voltage drops off exponentially as more current is drawn and internal resistance continues to increase with more rapid chemical transformation.  A more rapid exponential voltage drop reduces the overall capability of PbA chemistry to supply the rated power to a load consistently. For instance, a battery pack with a 260Ah rating may provide current for 20 hours if the battery discharges current to a light load with steady current draw.  However, a heavier load like an electric motor that draws current from the battery within 5 hrs, accompanied by sharp increases or decreases in current demand, may cause the pack voltage to drop off more quickly as internal resistance rises.  The battery pack may only have an effective rating of 215Ah per charge cycle for this application. The more constant internal chemistry and voltage characteristics of LiFePO4 battery packs provide a more constant supply of total power to a system, as a function of the balance between voltage and current where Power = Voltage * Current. Since the voltage does not drop out as quickly in LiFePO4 chemistries as it does in PbA chemistries, the current does not need to increase proportionally in order to maintain power. This means the power delivered to the motor will be very consistent until the very end, and less power will be lost when accelerating and cruising in an electric motor vehicle.

Because the PbA cell internal resistance increases and causes the pack voltage to drop, more current must be drawn from the battery to produce the same power.   Current draw continues to increase over time as more internal resistance is formed with further chemical transformation of the battery components, resulting in an accelerating discharge of the total power available within the PbA battery pack.