What are the performance parameters of electric vehicle batteries?

As we all know, the most important thing for an electric car is the performance of its battery. A good battery is definitely a good thing for an electric car. So what are the performance parameters of electric vehicle batteries? Below small make up for you to introduce it!

Performance parameters of electric vehicle battery: brief introduction

(1) electromotive force

The electromotive force of the battery, also known as the standard or theoretical voltage of the battery, is the potential difference between the positive and negative poles when the battery is cut off. The electromotive force of a battery

It can be calculated from the change in free energy of the thermodynamic function of the battery system.

The rated voltage

Rated voltage (or nominal voltage) refers to the accepted standard voltage when the battery of the electrochemical system is working. For example, zinc-manganese dry cells are 1.5v, nickel-cadmium batteries are 1.2v, lead-acid batteries are 2V and lithium-ion batteries are 3.6v.

Self opening voltage

The open circuit voltage of the battery is the battery voltage without load. The open circuit voltage is not equal to the electromotive force of the battery. It must be pointed out that the electromotive force of the battery is calculated from a thermodynamic function, while the open circuit voltage of the battery is actually measured.

Of operating voltage

Refers to the actual discharge voltage of a battery under a load, usually within a voltage range. For example, the working voltage of lead-acid battery is 2V ~ 1.8v; The operating voltage of nimh battery is 1.5v ~ 1.1v. The working voltage of lithium ion battery is 3.6v ~ 2.75v.

Set termination voltage

Voltage value at discharge termination, depending on load and service requirements. Take lead-acid battery as an example: the electromotive force is 2.1v, the rated voltage is 2V, the open circuit voltage is close to 2.15v, the working voltage is 2V ~ 1.8v, and the termination voltage is 1.8v ~ 1.5v (the termination voltage is different according to the discharge rate).

Performance parameters of electric vehicle battery: brief introduction

? charging voltage

Refers to the voltage applied by the dc voltage of the external circuit to the battery charge. The general charging voltage is greater than the open circuit voltage of the battery, usually within a certain range. For example, the charging voltage of nickel-cadmium batteries ranges from 1.45v to 1.5v. The charging voltage of lithium ion battery is 4.1v ~ 4.2v; Lead-acid batteries charge at 2.25v ~ 2.5v.

Once internal resistance

The internal resistance of the battery includes the resistance of the positive and negative electrode plate, the resistance of the electrolyte, the resistance of the separator and the resistance of the connector.

A. Positive and negative plate resistance

At present, the positive and negative plate of lead-acid battery is pasted, which is composed of lead-antimony alloy or lead-calcium alloy plate frame and active material. Therefore, the plate resistance is also composed of the plate gate resistance and the active material resistance. In the inner layer of the active material, no chemical changes will occur when the grid is charged or discharged, so its resistance is the inherent resistance of the grid. The resistance of the active material varies with the charging and discharging state of the battery.

When the battery is discharged, the active material of the electrode plate is changed into lead sulfate (PbSO4). When the battery is charged, lead sulfate is reduced to lead (Pb). The smaller the lead sulfate content, the smaller the resistance.

B. Electrolyte resistance

The resistance of the electrolyte varies according to its concentration. Once a concentration is selected within the specified concentration range, the electrolyte resistance will vary with the degree of charge and discharge. When the battery is charged, the electrolyte concentration increases while the active material of the electrode is reduced, and its resistance decreases. When the battery is discharged, the electrolyte concentration decreases and its resistance increases at the same time as the sulfur of the active material on the electrode plate.

C. Barrier resistance

The resistance of the baffle plate varies with the porosity of the baffle plate. The resistance of the baffle plate of the new battery tends to be a fixed value, but with the extension of the battery operating time, the resistance increases. Because some of the lead and other sediments in the battery run on the separator, the porosity of the separator decreases and the resistance increases.

D. Connector resistance

The connector includes the inherent resistance of the metal such as the connection strip when the single battery is in series, the connection resistance between the battery plates, and the metal resistance of the connection body composed of positive and negative plates. If the welding and connection contact are good, the connection body resistance can be regarded as a fixed resistance.

The internal resistance of each battery is the sum of the resistance of the above objects. The relation of the internal resistance R of the battery to the electromotive force, terminal voltage and discharge current: Rs=(e-uf) If.

The internal resistance of the battery increases during discharge and decreases during charge. Therefore, in the process of charging and discharging, the terminal voltage will also change due to the change of its internal resistance. Therefore, the terminal voltage is lower than the electromotive force of the battery when discharging, and higher than the electromotive force of the battery when charging.

Being capacity

The unit of capacity of the battery is coulomb (C) or ampere hour (Ah). There are three terms used to describe the capacity characteristics of a battery:

A. theoretical capacity. Refers to the electric quantity calculated according to the electrochemical equivalent number of the active substance participating in the electrochemical reaction. Generally, in theory, 1 electrochemical equivalent substance will give off 1 Faraday charge, namely 96500C or 26.8ah (1 amount of electrochemical equivalent substance, equal to the atomic or molecular weight of the active substance divided by the number of electrons in the reaction).

B. Rated capacity. Refers to the minimum amount of electricity that is required or guaranteed to be released by the battery under specified discharge conditions during the design and production of the battery.

C. Actual capacity. Refers to the amount of electricity that can be released by the battery before the termination of voltage under certain discharge conditions, namely, under certain discharge current and temperature.

The actual capacity of the battery is usually 10% ~ 20% larger than the rated capacity.

The size of the battery capacity is related to the amount and activity of active substances on the positive and negative electrodes, as well as the structure and manufacturing process of the battery and the discharge conditions (current and temperature) of the battery.

The comprehensive index that affects battery capacity is the utilization rate of active material. In other words, the more the active material is used, the higher the capacity given by the battery.

The utilization rate of active substances can be defined as:

Utilization =(actual battery capacity/theoretical battery capacity) 100%.

Or, utilization =(theoretical dosage of active substance/actual dosage of active substance) 100%.

Specific energy and specific power

The output energy of the battery refers to the electric work that the battery can make under certain discharge conditions, which is equal to the product of the discharge capacity of the battery and the average working voltage of the battery, and its unit is commonly expressed as watt-hour (Wh).

There are two specific energies of a battery. One is called specific energy by weight, expressed in watt-hours per kilogram (Wh/kg); The other is called volume specific energy, expressed in watt-hours per liter (Wh/L). The physical meaning of specific energy is the effective electrical energy of a battery per unit weight or volume. It's an important indicator for comparing battery performance.

It must be pointed out that the specific energy of a single battery is different from that of a battery pack. The specific energy of a battery pack is always less than that of a single battery, because the battery pack always has a connection bar, an external container, an internal packaging layer and so on.

The power of the battery refers to the energy that the battery can output in unit time under certain discharge conditions. The unit is watt (W), or kilowatt (kW). The power per unit weight or volume of a battery is called the specific power of the battery in watts per kilogram (W/kg) or watts per liter (W/L). If the specific power of a battery is large, it indicates that more energy is given in unit time, unit weight or unit volume, indicating that the battery can discharge with a larger current. Therefore, the specific power of the battery is also one of the important indexes to evaluate the performance of the battery.

We maintain storage properties and self discharge

After dry storage (without electrolyte) or wet storage (with electrolyte) for a certain period of time, the capacity of the battery will decrease by itself. This phenomenon is called self-discharge. The so-called "storage performance" refers to the battery open circuit, in a certain condition (such as temperature, humidity) after a certain time of storage self-discharge size.

During the storage period, although the battery does not give out electric energy, but there is always self-discharge phenomenon in the battery. Even dry storage, because the seal is not strict, into water, air and carbon dioxide and other substances, so that in the thermodynamic unstable state of part of the positive and negative electrode active materials constitute a micro-battery corrosion mechanism, REDOX reaction and waste consumption. This is especially true if it is stored wet. The active material in the electrolyte is also unstable. Most of the negative active materials are active metals, which will undergo anodic autolysis. In acidic solution, the negative metal is unstable, and it is not very stable in alkaline solution and neutral solution.

The size of self-discharge of battery is generally expressed as the percentage of capacity reduction per unit time, that is:

Self-discharge =(co-ct /Cot) 100%.

Type: Co ─ ─ before the storage battery capacity, Ah;

Ct ─ ─ after storage battery capacity, Ah;

T ─ ─ storage time, day, week, month or year.

The size of self-discharge can also be expressed by the number of days when the battery is stored to a specified capacity, which is called storage life. There are two kinds of storage life, namely dry storage life and wet storage life. For the battery storage life of electrolyte added before use, it is also known as dry storage life. Dry storage life can be very long. For the battery storage life which has been added electrolyte before leaving the factory, it is conventionally called wet storage life (or wet charge life). The self-discharge is serious in wet storage and the life is short. For example, the dry storage life of silver-zinc battery can reach 5 ~ 8 years, but its wet storage life is usually only a few months.

The measures to reduce the self-discharge in the battery are generally using the raw materials with higher purity, or pretreating the raw materials to remove the harmful impurities. In addition, metals with higher hydrogen overpotential, such as Ag and Cd, can also be added to the negative electrode metal grid, and corrosion inhibitors can also be added to the solution. The purpose is to inhibit the precipitation of hydrogen and reduce the occurrence of self-discharge reaction.

⑾ life

The battery life has two concepts: dry storage life and wet storage life. It must be pointed out that these two concepts are only for the self-discharge size of the battery, not the actual life of the battery. The real life of a battery is how long the battery actually lasts.

For primary batteries, the battery life is characterized by the working time given the rated capacity (which is related to the discharge rate).

For secondary batteries, battery life is divided into charge and discharge cycle life and wet use life of two kinds.

Cycle life of charge and discharge is an important parameter to measure the performance of secondary batteries. It undergoes a charge and discharge called a cycle (or cycle). Under a certain charging and discharging system, the number of times the battery can withstand before the battery capacity drops to a certain specified value is called the cycle life of the secondary battery. The longer the cycle life of charge and discharge, the better the battery performance. Among the commonly used secondary batteries, the cycle life of cadmium nickel battery is 500 ~ 800 times, lead acid battery 200 ~ 500 times, lithium ion battery 600 ~ 1000 times, and zinc silver battery is very short, about 100 times.

The cycle life of secondary battery is related to discharge depth, temperature and charging and discharging mode. The "discharge depth" is the percentage of the capacity released by the battery to the rated capacity. The cycle life of the secondary battery can be greatly extended by reducing the discharge depth (" shallow discharge ").

Wet shelving life is also an important parameter to measure secondary battery performance. It refers to the time when the battery starts charging and discharging cycle after adding electrolyte until the cycle life of charging and discharging is terminated (including the time when the battery is in the discharge state of wet shelving in the charging and discharging cycle). The longer the wet shelving life, the better the battery performance. Among the commonly used batteries, the wet shelf-life of cadmium nickel battery is 2 ~ 3 years, lead acid battery 3 ~ 5 years, lithium ion battery 5 ~ 8 years, zinc silver battery is the shortest, only about 1 year.

In addition, the performance of the battery: low temperature performance, overcharge resistance, safety performance.

About the introduction of electric vehicle battery performance parameters, small make up for you to introduce 11 points, do not know through small make up of the introduction of everyone know how much? I hope the introduction of small make up to help you!

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