Energy Storage Batteries: Why Is It Always 0.5C?
The most typical characteristic of an energy storage system is that it contains an electricity storage medium – batteries. An important performance indicator of batteries is the charging and discharging speed or charging and discharging capacity. In tender technical requirements or battery technical parameters, we often see a parameter like “***C”, such as “0.2C”, “0.3C”, “1C”, or “2C”. In industrial and commercial energy storage systems, “0.5C” is the most common. So, why is 0.5C the most prevalent?
1. What Is "C"?
2. How Is "C" Calculated or Derived?
The charging and discharging rate (C) is a logical concept rather than a fixed concept like current (A) or voltage (V).
For example, if a circuit has a current of 1A passing through it, regardless of the measuring equipment used, the value of this 1A current remains the same.However, for the charging and discharging capacity of 1C, it is also related to the specific capacity of the battery.
For a battery with a capacity of 1Ah, its 1C charging and discharging current is 1A; for a battery with a capacity of 2Ah, its 1C charging and discharging current is 2A, and so on.
Therefore, the charging and discharging rate (C) of a battery = the charging and discharging current of the battery ÷ the rated capacity of the battery.
For example, for a 1000mAh battery, 0.2C represents 200mA (0.2 times the capacity of 1000mAh), and 1C represents 1200mA (1 times the capacity of 1200mAh).
The concept of using “C” can conveniently compare the charging and discharging capacities of two batteries with the same total battery capacity under the same conditions.
For example, if both batteries have a capacity of 1Ah, but Battery No. 1 can achieve 3C, that is, it can be charged and discharged with a current of 3A, while Battery No. 2 can only be charged and discharged with a current of 0.5C, that is, 0.5A. Then it can be intuitively shown that the instantaneous charging and discharging capacity (charging and discharging burst ability) of Battery No. 1 is significantly better.
3. Why Is 0.5C the Most Common?
In the specification parameter table of lithium batteries, “S” represents series connection (String), and “P” represents parallel connection (Parallel). Batteries (cells) increase the voltage through series connection and increase the discharging current through parallel connection.
Then, according to the above, how large should the charging and discharging rate of the battery be appropriate? It is necessary to clearly understand what impacts the value of the charging and discharging rate C has on the battery. In fact, although we need the battery to exhibit a strong charging and discharging capacity instantaneously, if the C value is too high, it will have a relatively large impact on the service life of the battery. Specifically, there are mainly the following three points: ① It leads to an increase in polarization and internal resistance: The larger the charging and discharging rate, the faster the polarization and internal resistance inside the battery increase, resulting in a decline in the battery’s storage capacity. ② It leads to the loss of active substances and Li+: The larger the charging and discharging rate, the faster the loss speed of active substances and Li+, resulting in the attenuation of battery capacity. ③ It leads to the consumption of electrolyte: The larger the charging and discharging rate, the greater the consumption of the electrolyte, further affecting the battery life.
Composition of Lithium Metal Batteries
An overly large charging and discharging rate has an impact on the battery life, so it should not be set too large. Of course, if C is too small, such as 0.1C, 0.2C, 0.3C, which are common rates for lead-acid batteries, the charging current is small and the speed is slow. Although it provides better protection for the battery, in industrial and commercial energy storage projects of the State Grid that aim to obtain benefits from peak-valley price differences during peak, valley, and flat periods, it will obviously reduce the number of kWh of charging and discharging in the same period, thereby reducing the daily income and prolonging the payback period. So it is also not appropriate.
Summary
Overall, choosing a charging and discharging rate of 0.5C takes into account both the charging and discharging capacity of the battery and the protection of the battery’s service life. At the same time, it also considers compatibility with peak and valley periods. For example, for a single-cabinet system with a capacity of 209 kWh or 215 kWh, when paired with a 100 kW PCS, it can be fully charged or discharged in 2 hours, which is quite in line with the length of peak and valley periods defined by various power grid companies. It can just perform charging and discharging within the corresponding periods, without wasting power or too much time, and can also obtain the expected income. So it is reasonable.