Although some performance parameters of the power battery are more theoretical, they are the most basic elements of the power battery, just like a key, through which the quality of the power battery can be evaluated.

Let's take a look at the parameters of these power batteries!

1. Energy density (Wh/L&Wh/kg)

The energy released by a unit volume or unit mass battery, if it is a unit volume, that is, the volume energy density (Wh/L), which is directly referred to as the energy density in many places; if it is a unit mass, it is the mass energy density (Wh/kg), in many places Also called specific energy. For example, a lithium battery weighs 300g, has a rated voltage of 3.7V, and a capacity of 10Ah, and its specific energy is 123Wh/kg.

These 9 parameters can be called the "key" of the power battery, do you know it all?

According to the "Energy Saving and New Energy Vehicle Technology" released in 2016, we can roughly have a concept of the development trend of power batteries. As shown in the figure above, by 2020, the specific energy of a pure electric vehicle battery will reach 350Wh/kg.

2. Power density (W/L&W/kg)

Divide energy by time to get power in W or kW. In the same way, power density refers to the power output per unit mass (some places are also directly called specific power) or unit volume of the battery, and the unit is W/kg or W/L. Specific power is an important indicator to evaluate whether the battery meets the acceleration performance of electric vehicles.

What is the difference between specific energy and specific power?

To give a vivid example: a power battery with high specific energy is like the tortoise in the tortoise and the hare. It has good endurance and can work for a long time, ensuring a long mileage of the car.

The power battery with high specific power is like the rabbit in the tortoise and the hare race. It is fast and can provide high instantaneous current to ensure good acceleration performance of the car.

3. Battery discharge rate (C)

The discharge rate refers to the current value required to discharge its rated capacity (Q) within a specified time, which is numerically equal to the multiple of the rated capacity of the battery. That is, charge and discharge current (A)/rated capacity (Ah), and its unit is generally C (short for C-rate), such as 0.5C, 1C, 5C, etc.

For example, for a battery with a capacity of 24Ah:

Discharge with 48A, the discharge rate is 2C, on the other hand, discharge at 2C, the discharge current is 48A, and the discharge is completed in 0.5 hours;

Charging with 12A, the charging rate is 0.5C, conversely, charging at 0.5C, the charging current is 12A, and the charging is completed in 2 hours;

The charge and discharge rate of the battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery.

4. State of charge (%)

SOC, the full name is StateofCharge, the state of charge, also known as the remaining power, represents the ratio of the remaining capacity of the battery after discharge to the capacity of the fully charged state.

Its value ranges from 0 to 1. When SOC=0, it means that the battery is fully discharged. When SOC=1, it means that the battery is fully charged. The battery management system (BMS) mainly ensures the efficient operation of the battery by managing the SOC and making estimates, so it is the core of battery management.

At present, SOC estimation mainly includes open circuit voltage method, ampere-hour measurement method, artificial neural network method, Kalman filter method, etc. We will explain it in detail later.

5. Internal resistance

Internal resistance refers to the resistance to the current flowing through the battery when the battery is working.

Including ohmic internal resistance and polarization internal resistance, wherein: ohmic internal resistance includes electrode material, electrolyte, diaphragm resistance and the resistance of various parts; polarization internal resistance includes electrochemical polarization resistance and concentration polarization resistance.

In terms of data, the figure below shows a battery discharge curve, the X-axis indicates the discharge capacity, and the Y-axis indicates the open-circuit voltage of the battery. The ideal discharge state of the battery is the black curve, and the red curve is the real state when the internal resistance of the battery is considered.

These 9 parameters can be called the "key" of the power battery, do you know it all?

Illustration: Qmax is the maximum chemical capacity of the battery; Quse is the actual capacity of the battery; Rbat is the internal resistance of the battery; EDV is the end-of-discharge voltage; I is the discharge current.

It can be seen from the figure that the actual battery capacity Quse<the theoretical maximum chemical capacity Qmax of the battery.

Due to the existence of resistance, the actual capacity of the battery will be reduced. We can also see that the actual capacity of the battery Quse depends on two factors:

What is the product of the discharge current I and the internal resistance R of the battery, and the end-of-discharge voltage EDV.

It should be pointed out that the battery internal resistance Rbat will gradually increase with the use of the battery.

The unit of internal resistance is generally milliohm (mΩ). A battery with a large internal resistance will consume a lot of power and generate heat during charging and discharging, which will cause accelerated aging and life attenuation of the battery, and will also limit the high-rate charging Discharge application. Therefore, the smaller the internal resistance is, the better the battery life and rate performance will be. There are usually AC and DC test methods for measuring the internal resistance of batteries.

6. Battery self-discharge

Refers to the phenomenon of voltage drop during the open circuit static process, also known as the charge retention capacity of the battery

Generally speaking, battery self-discharge is mainly affected by manufacturing process, materials, and storage conditions.

Self-discharge is divided into two types according to whether the capacity loss is reversible: the capacity loss is reversible, which means that the capacity can be recovered after recharging; the capacity loss is irreversible, which means that the capacity cannot be recovered.

At present, there are many research theories on the causes of battery self-discharge, which can be summarized into physical reasons (storage environment, manufacturing process, materials, etc.) and chemical reasons (electrode instability in the electrolyte, internal chemical reactions, active substances are consumed) etc.), battery self-discharge will directly reduce the capacity and storage performance of the battery.

7. Battery life

It is divided into two parameters: cycle life and calendar life. Cycle life refers to the number of times a battery can be charged and discharged. That is, under ideal temperature and humidity, charge and discharge with the rated charge and discharge current, and calculate the number of cycles experienced when the battery capacity decays to 80%.

The calendar life refers to the time span during which the battery reaches the end-of-life condition (capacity decays to 80%) under specific operating conditions under the environmental conditions of use. If the calendar life is closely combined with specific use requirements, it is usually necessary to specify specific use conditions, environmental conditions, storage intervals, etc.

Cycle life is a theoretical parameter, while calendar life is more practical. However, the calculation of calendar life is complicated and time-consuming, so generally battery manufacturers only give the data of cycle life.

These 9 parameters can be called the "key" of the power battery, do you know it all?

The figure above shows the charge and discharge characteristics of a ternary lithium battery. It can be seen that different charge and discharge methods have different effects on the life of the battery. As shown in the figure above, the life of the charge and discharge at 25%-75% can reach 2500 times. That is what we call shallow charging and shallow discharging of the battery. The topic of battery life will be discussed in depth later.

8. Consistency of battery pack

This parameter is quite interesting. Even if the battery cells of the same specification and model are grouped, the performance of the battery pack in voltage, capacity, internal resistance, life, etc. is very different. When used in electric vehicles, the performance indicators often fail to reach to the original level of the single battery.

After the single battery is manufactured, due to process problems, the internal structure and material are not completely consistent, and there are certain performance differences.

The initial inconsistency accumulates with the continuous charge and discharge cycles of the battery during use, and the use environment in the battery pack is also different for each single battery, resulting in greater differences in the state of each single battery. During use, it is gradually amplified, which in some cases accelerates the degradation of the performance of some single cells, and eventually causes premature failure of the battery pack.

It should be pointed out that the performance of the power battery pack depends on the performance of the battery cells, but it is by no means a simple accumulation of the performance of the single cells. Due to the inconsistency of single battery performance, when the power battery pack is used repeatedly in electric vehicles, various problems will occur and the life span will be shortened.

In addition to the requirement to strictly control the process and keep the consistency of the single battery as much as possible in the process of production and assembly, the industry generally uses a battery management system with a balancing function to control the consistency of the batteries in the battery pack to prolong the life of the product. service life.

9. Formation

After the battery is made, it is necessary to charge the cell with a small current to activate the positive and negative materials inside, and form a passivation layer on the surface of the negative electrode—SEI (solid electrolyte interface) film to make the performance of the battery more stable. In order to reflect its true performance, this process is called formation.

The sorting process in the formation process can improve the consistency of the battery pack and improve the performance of the final battery pack. The formation capacity is an important indicator for screening qualified batteries. The picture below shows the SEI film, which looks like a black rose.

Through the above performance parameters related to power batteries, I hope it will be helpful to everyone, and I will not be a "layman" when talking about batteries in the future.