**(1) Battery discharge system**The battery discharge system refers to the discharge rate, discharge mode (constant current, variable current or pulse), termination voltage and temperature. Termination voltage refers to the battery voltage at the end of charge and discharge, which is divided into charge termination voltage and discharge termination voltage. When studying the battery capacity, a uniform discharge current should be specified, which is usually expressed at a rate of n hours. If the current I is used to discharge, the battery discharge in n hours is the rated capacity, this discharge rate is called the n-hour discharge rate.

**(2) Battery capacity**The capacity of a battery refers to the amount of electricity output when a fully charged battery is discharged to the final voltage under specified conditions, and the unit is A·h. Regarding the battery capacity, there are concepts such as theoretical capacity, i-hour rate discharge capacity, rated capacity, actual capacity, and remaining capacity, which will be explained separately below.

①Theoretical capacity is the amount of electricity that can be calculated based on Faraday’s law assuming that all the active materials in the battery participate in the flow reaction. The theoretical capacity is the maximum limit value of the battery capacity. The actual capacity of the battery is only a part of the theoretical capacity.

②i hour rate discharge capacity Under the condition of constant current discharge, the amount of electricity that can be released when a fully charged battery is discharged to the final voltage in just i hours, usually expressed by C

_{i}. Normally, the starting battery is represented by C

_{20}, the traction battery is represented by C

_{5}, and the battery for electric vehicles is represented by C

_{3}.

③Rated capacity is the amount of electricity that the battery should discharge under specified conditions. The rated capacity is the ampere-hour capacity indicated by the manufacturer, as an important technical indicator for the acceptance of battery quality.

④The actual capacity refers to the amount of electricity that a fully charged battery can output under certain conditions, which is equal to the product of the discharge current and the discharge time.

⑤Remaining capacity After the battery has been used, the amount of electricity that can be discharged from the battery under the specified discharge rate and temperature.

**(3) Battery energy**The energy of the battery refers to the electric energy output by the battery under the discharge system stipulated by a certain standard, and the unit is watt-hour (W·h) or kilowatt-hour (kW·h).

**(4) Energy density**The energy density of a battery is divided into mass energy density and volume energy density. Mass energy density refers to the electric energy that a battery can output per unit mass, in watt-hours/kg (W·h/kg). Volumetric energy density refers to the electrical energy that a battery can output per unit volume, in watt-hours/liter (W·h/L).

**(5) Battery power and power density**The power of a battery refers to the energy output by the battery per unit time under a certain discharge system, in watts (W) or kilowatts (kW). The power output per unit mass of the battery is called the mass power density, and the unit is W/kg , The power output per unit volume of the battery is called volume power density, and the unit is W/L.

**(6) The state of charge of the battery**The State of Charge (SOC) of the battery describes the percentage of the remaining capacity of the battery to the rated capacity. The state of charge of the battery during operation is calculated by formula (1).

In the formula, SOC_{0 }is the initial SOC of the battery; I_{bat} is the working current of the battery at time t, which is positive when charging and negative when discharging, A; t is the charging and discharging time, h; C is the rated capacity of the battery, A·h.

**(7) Depth of Discharge (DOD)**DOD is the ratio of the battery’s discharged power to the battery’s rated capacity, and its mathematical expression is

In the formula, I_{bat}, t and C are the same as in formula (1).

**(8) Cycle Life of the battery**The cycle life of a battery refers to the total number of charge and discharge cycles that the battery has experienced before the battery capacity drops to a certain value according to a certain test standard.

**(9) Abuse Tolerance**Refers to the battery’s tolerance to abnormal use such as short circuit, overcharge, over discharge, mechanical vibration, impact, extrusion, and exposure to high temperature and fire.