## Basic parameters of electric vehicle power battery

1. The voltage of the power battery
(1) Open circuit voltage. The terminal voltage of the power battery in the open circuit state is called the open circuit voltage. The open circuit voltage of the power battery is equal to the difference between the positive electrode potential and the negative electrode potential of the power battery when the power battery is open (that is, when there is no current passing through the two poles). The open circuit voltage of the power battery is expressed by Vk, namely
Vk=Ez-Ef
In the formula, Ez is the positive potential of the power battery; Ef is the negative potential of the power battery.

(2) Working voltage. The voltage displayed during the discharging process after the power battery is connected to the load, also known as the load (load) voltage or the discharge voltage. The discharge voltage is usually represented by V
V=Vk-I(Ro+Rj)
In the formula, I is the discharge current of the power battery; Ro is the ohmic resistance of the power battery; Rj is the polarization resistance of the power battery.

(3) Initial voltage. The working voltage of the power battery at the beginning of discharge is called the initial voltage.
(4) Charging voltage. Charging voltage refers to the voltage applied to both ends of the power battery by an external power source when the power battery is being charged.
(5) Float charging voltage. The floating charge voltage of the power battery is the voltage value set when the charger floats the power battery. The power battery requires the charger to have an accurate and stable float voltage value. The high float voltage value means that the energy storage is large, and the float voltage value of the poor quality power battery is generally small, and artificially increasing the float voltage value is harmful and not beneficial to the power battery.
(6) Termination voltage. The end-of-discharge voltage of the power battery is the lowest working voltage at which the power battery discharges when the voltage drops to the point where it can no longer be discharged. Generally, it is stipulated that when the fixed-type power battery is discharged at a rate of 10 hours, the final discharge voltage of the single power battery is 1.8V (relative to a single power battery). Body 2V power battery, at 25℃).

2. Charging and discharging curve of power battery
The curve of power battery voltage changing with charging time is called charging curve, and the curve of power battery voltage changing with discharging time is called discharging curve.

3. discharge time rate and discharge rate
(1) Discharge time rate. The discharge time rate of the power battery is the length of the discharge time to express the discharge rate of the power battery, that is, the capacity of the power battery at the specified current during the specified discharge time. The discharge time rate can be determined by the following formula
Tk=Ck/Ik
In the formula, Tk (T10, T3, T1) respectively represent the hourly discharge rate of 10, 3, 1, etc.; Ck (C10, C3, C1) respectively represent the hourly rate discharge capacity of 10, 3, and 1, (Amp-hour); Ik (I10, I3, I1) respectively represent 10, 3, 1 hour rate discharge current, (A).
(3) Discharge rate. The discharge rate (X) is a multiple of the discharge current for the rated capacity of the power battery, namely
X=I/C
In the formula, X is the discharge rate; I is the discharge current; C is the rated capacity of the power battery.
In order to compare power batteries with different capacities, the discharge current is not expressed as an absolute value (ampere), but expressed as the ratio of the rated capacity C to the discharge system time, which is called the discharge rate or discharge rate. The discharge rate of the 20h system is C/20=0.05C, and the unit is A. For the NP6-12 type power battery, 0.05C is equal to 0.3A current.

4. Energy and specific energy
(1) Energy. The energy of the power battery refers to the electric energy that the power battery can give under a certain discharge system, usually expressed in W, the unit is Wh, the energy of the power battery is divided into theoretical energy and actual energy, and the theoretical energy can be the product of the theoretical capacity and the electromotive force. , And the actual energy of the power battery is the product of the actual capacity and the average working voltage under certain discharge conditions.
(2) Specific energy. The specific energy of the power battery is the energy given by the power battery per unit volume or unit weight, which is called volume specific energy and weight specific energy, and the units are Wh/L and Wh/kg.

5. Power and specific power
(1) Power. The power of the traction battery refers to the amount of energy given by the traction battery in a unit time under a certain discharge system. It is usually expressed by P and the unit is W. The power of the traction battery is divided into theoretical power and actual power. The theoretical power is in The product of the discharge current and the electromotive force under a certain discharge condition, and the actual power of the power battery is the product of the discharge current and the average working voltage under a certain discharge condition.
(2) Specific power. The specific power of the power battery refers to the power output by the power battery per unit volume or unit mass, respectively called the volume specific power W/L or the mass specific power W/kg. The specific power is an important performance technical index of the power battery. The large specific power of the power battery indicates that it has a strong ability to withstand large current discharge.

6. cycle life
Cycle life, also known as service cycle, refers to the number of charge and discharge times that the power battery experiences before the power battery capacity drops to a certain value under certain discharge conditions.

7. self-discharge
The self-discharge of the power battery refers to the automatic discharge of the power battery when it is left open. The self-discharge of the power battery will directly reduce the power output of the power battery and reduce the capacity of the power battery. The main reason for self-discharge is that the electrodes are in a thermodynamically unstable state in the electrolyte, which is the result of the oxidation-reduction reactions of the two electrodes of the power battery. In the two electrodes, the self-discharge of the negative electrode is the main one. The occurrence of self-discharge causes the active material to be consumed and transformed into unusable heat energy. The magnitude of self-discharge can be expressed by the self-discharge rate, that is, the percentage of the reduction in the capacity of the power battery within a specified time.
Y%=[(C1-C2)/C1×T]×100%

In the formula, Y% is the self-discharge rate; C1 is the capacity of the power battery before shelving; C2 is the capacity of the power battery after shelving; T is the shelving time of the power battery, generally expressed in days, weeks, months or years.

The self-discharge rate of power battery is determined by dynamic factors, mainly depends on the nature of the electrode material, surface state, electrolyte composition and concentration, impurity content, etc., and also depends on the environmental conditions such as temperature and humidity And other factors.

8. internal resistance