Why the energy storage battery -- residential and commercial -- C rate is 0.5C ?

  Energy storage systems and off-grid systems contain batteries, and an important performance indicator of batteries is the speed of charge and discharge or the charge and discharge capacity. You can often see a "xxC" parameter in the technical requirements or battery technical parameters, such as "0.2C", "0.3C", "1C", or "2C". In energy storage systems, the most common is "0.5C", so why is 0.5C the most common?

1. What is "C"?

  C is the first letter of the unit of charge, Coulomb. This concept was first proposed by French physicist Coulomb and defines the amount of electricity passing through the cross-sectional area of ​​a wire in 1 second. In energy storage batteries, C is used to indicate the charge and discharge rate of the battery. Generally, the size of the charge and discharge current is expressed by this charge and discharge rate. A charge and discharge rate of 1C means that the energy storage battery can discharge all its electricity within 1 hour; 2C means that the energy storage battery can discharge all its electricity within 0.5 hours.

2. How is "C" calculated or obtained?

  C (charge and discharge rate) is a logical concept rather than a fixed concept like current (A) and voltage (V). For example, a circuit passes a current of 1A. No matter what device is used to measure it, the current value of 1A is the same. As for the charge and discharge capacity of 1C, it is also related to the specific capacity of the battery. For a battery with a capacity of 1Ah, its 1C charge and discharge current is 1A; for a battery with a capacity of 2Ah, its 1C charge and discharge current is 2A. And so on.

In energy storage battery systems, the common design choice of 0.5C charge and discharge rate (i.e., the battery capacity is fully charged or discharged within 2 hours) is mainly based on the following core reasons:

1. Extending battery life

- The cost of high-rate charge and discharge:

The higher the battery charge and discharge rate (C-rate), the faster the lithium ions are inserted/extracted from the electrode material, resulting in:

- Intensified chemical side reactions (such as SEI film thickening, electrolyte decomposition);

- Increased material structural stress (electrode expansion/contraction, particle rupture);

- Increased internal heat generation (accelerated aging).

These factors will significantly shorten the battery cycle life (e.g., 1C discharge may reduce the life by 30%-50% compared to 0.5C).

- Life requirements of energy storage scenarios:

Energy storage systems (such as home energy storage, grid-level energy storage) usually require a life of more than 10 years (6000+ cycles).

The use of a mild charge and discharge strategy of 0.5C can reduce the battery attenuation rate and meet the long life requirements.

2. Reduce the difficulty of thermal management

-The relationship between heat and rate:

The heat generated by the internal resistance of the battery is proportional to the square of the current (\(P = I^2 \cdot R\)).

- 0.5C current: Assume that the battery capacity is 100Ah and the current is 50A;

- 1C current: The current is 100A → the heat is 4 times that of the former.

- Heat dissipation cost and risk:

Energy storage systems usually use large-scale battery packs, and high-rate operation requires more complex heat dissipation systems (such as liquid cooling), which are costly and increase the risk of failure.

The 0.5C design simplifies thermal management (natural convection or air cooling can meet the requirements), reduces costs and improves safety.

3. Matching the requirements of energy storage application scenarios

- Energy type vs. power type application:

- Energy storage system: mostly energy type requirements (such as peak shaving and valley filling, photovoltaic storage), which requires long-term stable energy output and low instantaneous power requirements;

- Power battery (such as electric vehicles): requires power type design (1C~3C) to meet high power requirements such as acceleration and fast charging.

- Applicability of 0.5C:

Taking typical household energy storage as an example:

- Battery capacity is 10kWh, and 0.5C discharge power is 5kW, which can cover most household loads (air conditioning, lighting, etc.);

- If higher power is required (such as short-term impact load), it can be solved through system design (such as increasing inverter capacity) without increasing battery rate.

4. Exceptions in actual applications

- Short-term high power scenarios:

Some special energy storage scenarios (such as grid frequency regulation, UPS backup power) require fast response, and may use higher rate batteries (such as 1C~2C), but at the expense of life and cost.

- Battery technology progress:

With the maturity of solid-state batteries, silicon-based negative electrodes and other technologies, energy storage batteries may support higher rates (such as 1C) while maintaining long life in the future, but 0.5C is still the mainstream choice at present.

  Too large a charge and discharge rate will affect the battery life, so it should not be set too high;

of course, C is not too small either. For example, 0.1C, 0.2C, and 0.3C are common rates on lead-acid batteries. The charging current is small and the speed is slow. Although it protects the battery better, in the industrial and commercial energy storage projects where the State Grid has peak-valley-flat periods and the main purpose is to obtain peak-valley price difference benefits, it will obviously reduce the number of KWh charged and discharged in the same time period, thereby reducing the daily income and lengthening the payback period, so it is not suitable.

  On the whole, choosing a charge and discharge rate of 0.5C takes into account both the charge and discharge capabilities of the battery and the protection of the battery's service life, while also taking into account compatibility with peak and valley periods.

  For example, a single-cabinet system of 209KWh or 215KWh, with a 100KW PCS, can be fully charged or discharged in 2 hours, which is consistent with the length of the peak and valley periods designated by local power grid companies. Charging and discharging can be performed within the corresponding period, so that power and time are not wasted, and the expected benefits can be obtained, which is reasonable.

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