The amount of electricity that the battery can give under certain discharge conditions is called the capacity of the battery, which is commonly expressed as C. However, when the battery is used as a power source, since its terminal voltage is a variable value, it is more accurate to select the ampere-hour (Ah) to represent the power supply characteristics of the battery. The definition of battery capacity is that theoretically, t can tend to infinity, but in practice, when the battery discharges below the termination voltage, it continues to discharge, which may damage the battery, so there is a limit to the t value. The so-called termination voltage means that when the battery is lower than this specified voltage, the battery cannot work normally. In other words, if the battery continues to discharge and use below the termination voltage, it may cause permanent damage to the battery.

In the battery industry, the continuous discharge time of the battery is expressed in hours or minutes. Commonly there are nominal capacity values such as C24, C20, C10, C8, C3, and C1. Battery capacity can be divided into theoretical capacity, actual capacity, and rated capacity. The definitions are as follows:

(1) The theoretical capacity is the highest theoretical value obtained by calculating the mass of the active material according to Faraday’s law.

(2) The actual capacity refers to the amount of electricity that the battery can output under certain conditions. It is equal to the product of discharge current and discharge time, and its value is less than the theoretical capacity.

(3) Rated capacity, also known as nominal capacity and guaranteed capacity, is the minimum capacity that the battery should release under certain discharge conditions according to the standards issued by the state or relevant departments. The capacity released by the fixed battery generally adopts the 10-hour rate as the rated capacity of the battery, and is used to calibrate the model of the battery. The rated capacity or nominal capacity of the battery is indicated by the letter C. For example, a battery with a rated capacity of 6Ah, C=6Ah; a battery with a rated capacity of 24Ah, C=24Ah. In order to compare different series of batteries, the concept of specific capacity is commonly used, that is, the amount of electricity that a battery can give per unit volume or unit mass, which are called volume specific capacity and weight specific capacity, respectively. The unit is Ah/L (Ampere hour/liter) or Ah/kg (Ampere hour/kg).

Among the indicators to measure the battery, the rated voltage and rated capacity of the battery are the two most commonly used technical indicators. For example, the rated voltage of the Japanese Yuasa NP6-12 battery is 12V, and the rated capacity is 6Ah/20h; the rated voltage of the German Sunshine A406/165 battery is 6V, and the rated capacity is 165Ah/20h.

In the case of constant current discharge, the battery capacity is

Q=I×t

In the formula: Q is the electricity discharged by the battery, Ah; I is the discharge current, A; t is the discharge time, h.

The concept of capacity is essentially a representation of battery energy conversion. For example, considering the fact that the terminal voltage of the battery is V=12V, which remains almost unchanged in actual use, its output energy expression is W(t)=I×t. Therefore, from the perspective of energy effect, 6Ah can be understood as The NP6-12 type battery releases energy while keeping the terminal voltage unchanged. If it is discharged with a current of 6A, it can be discharged for 1h or discharged with a current of 1A for 6h.