Battery: C-Rates
The C-rate of a battery is a measure that describes the rate at which a battery is charged or discharged relative to its maximum capacity (the capacity recorded on the battery or manufacturers datasheet). It is used to express the current flowing in or out of the battery in terms of a fraction or multiple of its total capacity.
For example (a 10 Ah battery):
A 1C rate means the battery charges or discharges its entire capacity in one hour, implying a 10 A discharge/charge rate.
A 0.5C rate means it charges or discharges half its capacity in one hour, which would fully charge or discharge in two hours - this implies a 5A discharge/charge rate.
A 2C rate means the battery can charge or discharge twice its capacity in one hour, indicating a faster charge or discharge process - this implies a 20 A discharge/charge.
C-rates: Lithium notation (left) and lead notation (right)
The C-rates indicated on the left are written in the notation typically used to express the charging/discharging of lithium-ion battery chemistry, whereas the notation on the right is typically used in the lead-acid industry. Lithium-ion uses a fraction-based system, where 0.2C is calculated from 1/5, where 5 indicates the duration of the discharge/charge. This is much clearer in the notation for lead-acid batteries, where C5 means charge/discharge over 5 hours.
The C-rate is important because it helps determine how quickly a battery can be used and recharged, affecting applications like electric vehicles and portable electronics.
Different C-rates resulting can be attributed to several factors primarily related to the battery's internal chemistry and physical limitations. Here's why:
Internal Resistance and Heat Production: Higher C-rates involve drawing more current from the battery, which increases the internal resistance and heat production. Heat can negatively affect the chemical reactions inside the battery, potentially reducing the efficiency of energy conversion and thus the usable capacity.
Peukert's Law: Particularly evident in lead-acid batteries, Peukert's Law states that the higher the discharge rate, the lower the available capacity. This is because internal losses due to resistance have a greater impact at higher currents, effectively reducing the amount of actual energy that can be extracted from the battery.
Electrochemical Limits: At higher discharge rates, the speed at which the ions and electrons can move through the electrolyte and between the electrodes may not keep up with the demand, leading to incomplete reactions. This limits the amount of energy that can be effectively used from the battery.
Voltage Drop: Increasing the current (higher C-rates) often leads to a greater voltage drop across the internal resistance of the battery. A lower voltage under load reduces the total energy (power x time) that can be delivered before the voltage drops below a usable level.
Understanding C-rates is important when configuring a battery management system and selecting the battery capacity required for a particular application. The fundamentals are generally overlooked but provide a crucial insight into the safe and effective use of batteries.