C-Rate determines how quickly a storage system can be loaded and unloaded. 1C means the storage system can be fully charged or discharged within an hour.
A storage system rated 0.5C takes two hours to charge or discharge, while a system rated 2C takes just half an hour.
Fast charging and discharging of batteries are essential for high system performance. If the C-Rate of a system is low, this system must have greater storage capacity to supply the necessary power, which will have an impact on the final price of the system.
In other words, a system with a higher C-Rate is capable of making all of its stored energy available in less time and recovering it more quickly. The illustration below represents a brief comparison of the output power of the same system, using different C-rates:
C-Rate and E-Rate
When specifying batteries, discharge current is usually expressed as a rate C to normalize battery capacity, which tends to differ between battery manufacturers.
A battery with a capacity of 100 Ah has a discharge current of 100 A if working at 1C. A 5C rate for this battery would give a current of 500 A and a C/2 rate would be 50 A.
Simply put, if a system requires 100 kW and its battery is capable of working at a maximum of 0.5C, a bank of 200 kWh will be needed, while a system that can work at 1C can meet the same demand with a capacity of 100 kWh without compromising its useful life. Likewise, an E rate describes the discharge power.
A rate of 1E means the battery can be completely discharged in 1 hour. Energy or nominal energy (Wh, for a specific C rate) is the “energy capacity” of the battery, that is, the total watt-hours available when the battery is discharged at a certain discharge current (specified as C rate). from 100% state of charge to cut-off voltage.
Energy is calculated by multiplying the discharge power (in watts) by the discharge time (in hours). Just like battery capacity, power decreases with increasing C rate.
Precautions
One of the worst possible scenarios when considering battery life is operation above the C-rate indicated by the manufacturer. When operating at a charge or discharge rate above what the battery is designed to withstand under normal conditions, there is an increase in the flow of ions in the battery electrolyte, which causes an increase in the micropores of the separator, causing irreversible damage.
This operation can also crack the battery's anode and cathode, in addition to oxidizing them. The following image, found in the magazine Journal of Power Sources 307 (2016) from Elsevier, highlights several harms arising from the inappropriate use of C-rate above that indicated by the manufacturer, such as:
- Island formation;
- Mechanical fractures of electrodes;
- Solvent inter-cancellation;
- Graphite exfoliation;
- Gas formation;
- Acid dissolution;
- Drying out of the electrolyte;
- Salt precipitation;
- Collector corrosion;
- Reduction of electrolyte porosity;
- Decomposition of junctions;
- Lithium galvanization;
- Growth of dendrites;
- Structural disorder and connective oxidation, among other things.
It is not the focus of this article to scientifically address the issues mentioned in the previous paragraph, however this list gives us an idea of the number of problems that arise in batteries when the maximum C-Rate is not respected.
All these factors drastically reduce battery life due to various phenomena. Such effects cannot be predicted in relation to the number of cycles reduced in the system's useful life.
Conclusion
Systems with a higher C-Rate have greater charging and discharging potential, which may mean that an installation has a smaller battery bank to meet its demand. An installation that needs 500 kW in one hour, with rate C = 1, may have a 500 kWh bank.
However, a prior analysis that identifies the needs of each case is extremely important. A fast charge/discharge system, for example, requires a high output power and can be optimized with a high C-Rate, however it will not always be necessary to use a high rate for charge and discharge.
