## Capacity of electric car power battery

The amount of electricity that the power battery can give under certain discharge conditions is called the capacity of the power battery, which is usually represented by C. As the power storage battery is used as the power source, because its terminal voltage is a variable value, the selection of ampere hour (Ah) indicates that the power supply characteristics of the power storage battery are more accurate. The power battery capacity is defined as

Q=∫^{1}_{0}idt

Theoretically, t can tend to infinity, but in fact, when the power battery is discharged below the termination voltage, it continues to discharge, which may damage the power battery, so there is a limit to the value of t. The so-called termination voltage refers to the voltage at which the power battery cannot work normally when the power battery is lower than this prescribed voltage. In other words, if the power battery continues to be discharged and used when the voltage is lower than the termination voltage, it may cause permanent damage to the power battery.

In the power storage battery industry, the continuous discharge time of the power storage battery is expressed in hours or minutes. Commonly used are C_{24}, C_{20}, C_{10}, C_{8}, C_{3}, C_{1} and other nominal capacity values. Power battery capacity can be divided into theoretical capacity, rated capacity, and actual capacity.

(1) 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 power battery can output under certain conditions, which is equal to the product of the discharge current and the discharge time, and its value is less than the theoretical capacity.

(3) Rated capacity is also called nominal capacity and guaranteed capacity. It is the minimum capacity that the power battery should be discharged under certain discharge conditions according to the standards issued by the country or relevant departments. The fixed power battery generally uses the capacity released at a rate of 10 hours as the rated capacity of the power battery, and is used to calibrate the model of the power battery. The rated capacity or nominal capacity of the power battery is indicated by the letter C. For example, a power battery with a rated capacity of 6Ah, C=6Ah; a power battery with a rated capacity of 24Ah, C=24Ah.

In order to compare different series of power batteries, the concept of specific capacity is commonly used, that is, the amount of electricity that a unit volume or unit mass of a power battery can give, respectively called volume specific capacity and weight specific capacity, and the units are Ah/L (Ah/L ) Or Ah/kg (Ah/kg).

Among the indicators for measuring the power battery, the rated voltage and rated capacity of the power battery are the two most commonly used technical indicators. For example, the Japanese Yuasa NP6-12 power battery has a rated voltage of 12V and a rated capacity of 6A/20h; the German Sunshine A406/165 power battery has a rated voltage of 6V and a rated capacity of 165Ah/20h.

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

Q=I×t

In the formula, Q is the amount of electricity discharged by the power arrow battery, in Ah; I is the discharge current, in A; t is the discharge time, in h.

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