1) The capacity of the battery
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 use ampere-hour (A·h) to represent the power supply characteristics of the battery. The battery capacity is defined as
In theory, 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 value of 1.
The so-called termination voltage means that when the battery is lower than this specified voltage, the battery cannot provide the normal working voltage for the load. 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 used are: C24, C20, C10, C8, C3, C1 and other nominal capacity values. Battery capacity can be divided into theoretical capacity, rated capacity, and actual capacity.
(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 should be released by the battery under certain discharge conditions according to the standards required by the state or relevant departments to ensure that the battery is stored. 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. For example, a battery with a rated capacity of 6A·h, C=6 A·h; a battery with a rated capacity of 24 A·h, C=24 A·h.
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, and their units are A·h/L (Ah/L). / liter) or A h/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, Japan’s Yuasa NP6-12 battery has a rated voltage of 12 V and a rated capacity of 6A·h/20h; Germany’s Sunshine A406/165 battery has a rated voltage of 6V and a rated capacity of 165A·h/20h.
In the case of constant current discharge, the battery capacity is
In the formula, Q is the power discharged by the battery, the unit is A h; I is the discharge current, the unit is A; t is the discharge time, the unit is h.
The concept of capacity is essentially a representation of battery energy conversion. For example, considering the fact that the terminal voltage E=12 V of the battery remains almost unchanged in actual use, its output energy expression is W(t)=I·V·t=I·E·t, therefore, 6A ·h From the perspective of energy effect, it can be understood that the NP6-12 battery releases energy while keeping the terminal voltage unchanged. If it is discharged with a 6A current, it can be discharged for 1h or with a 1A current for 6h.
2) The voltage of the battery
(1) Open circuit voltage. The terminal voltage of the battery in the open circuit state is called the open circuit voltage. The open circuit voltage of the battery is equal to the difference between the positive electrode potential and the negative electrode potential of the battery when the battery is disconnected from the external circuit (that is, when no current flows through the two poles). The open circuit voltage of the battery is represented by Vk, that is
In the formula, Ez is the positive electrode potential of the battery; Ef is the negative electrode potential of the battery.
(2) Working voltage. Refers to the voltage displayed during the discharge process after the battery is connected to the load, also known as the load (load) voltage or discharge voltage. It is usually expressed as V, that is,
In the formula, I is the discharge current of the battery; R0 is the ohmic resistance of the battery; Rj is the polarization resistance of the battery.
(3) Initial voltage. The working voltage of the battery at the beginning of discharge is called the initial voltage.
(4) Charging voltage. The charging voltage refers to the voltage applied by the external power source to both ends of the battery when the battery is being charged.
(5) Float voltage. The float voltage of the battery is the voltage value set when the charger floats the battery. The battery requires that the charger should have an accurate and stable floating charge voltage value. A high floating charge voltage value means a large amount of stored energy. The floating charge voltage value of a battery with poor quality is generally small. Artificially increasing the floating charge voltage value is harmful to the battery and not beneficial. .
(6) Termination voltage. The battery discharge termination voltage is the lowest working voltage at which the battery discharge voltage drops to the point where it can no longer discharge. Generally, it is stipulated that when the fixed battery is discharged at a rate of 10 hours, the termination voltage of the single battery discharge is 1.8V (relative to the single 2V battery).