What is the performance evaluation index of power lithium battery

Lithium ion battery monomer is composed of positive and negative, electrode, electrolyte and membrane, which is the basic structural unit of battery module and battery pack. As a kind of electrochemical power source, battery has the characteristic parameters of voltage, internal resistance, capacity, energy and power.

In the power lithium battery system, each parameter can represent the different performance of the system. This paper lists each parameter of lithium battery.

Lithium battery monomer

Lithium ion battery monomer is composed of positive and negative, electrode, electrolyte and membrane, which is the basic structural unit of battery module and battery pack. As a kind of electrochemical power source, battery has the characteristic parameters of voltage, internal resistance, capacity, energy and power.

People mainly want to measure and evaluate the battery parameters for two purposes.

One is to achieve the purpose of active control. For example, the battery voltage is inconsistent, which reduces the energy storage capacity of the system.

The other is for the sake of safety. The parameters of the battery have a fixed range. The detection of battery parameters and the implementation of monitoring its boundary can play a role in characterizing the battery safety state.

voltage

The monomer voltage mainly depends on the type of monomer positive and negative electrode materials. Generally, lithium cobalt oxide, ternary positive electrode and graphite negative electrode can obtain a full charging voltage of about 4.2v, while lithium iron phosphate can only reach 3.6v at most. The voltage here, or rather the potential depending on the material properties, is going to be equal to the open circuit voltage of the battery after it's been sitting there long enough.

The voltage of the single terminal in the closed circuit is the voltage value detected by the external instrument, which is equal to the battery potential minus the internal resistance. However, the internal resistance of the battery is not constant and can be affected by a variety of factors, which are discussed in the following section.

In addition to being determined by the material, the voltage of a single cell will change with the change of charge, and the relationship is one-to-one correspondence. Therefore, in many cases, the battery charge (SOC) that cannot be directly and simply measured is often speculated by the open circuit voltage of the battery.

The monomer voltage is related to the activity level of the active material inside the battery, so it can affect the temperature of the activity level and the level of the monomer voltage in a small range.

The higher the monomer voltage, the more energy the battery of the same capacity contains. Therefore, on the premise of ensuring safety, increasing the monomer voltage upper limit is a technical route to improve the energy density of the system.

Internal resistance

Inside a lithium battery, the internal resistance of a lithium ion battery is made up of factors that prevent the ions from moving from one pole to the other. The main part includes the physical internal resistance of the conductive element; Inherent impedance of electrochemical substances such as motor materials, membranes and electrolytes; There is a temporary increase in the flow of current through the battery, which is an obstacle to the movement of lithium ions. These three parts together constitute the main body of internal resistance.

Internal resistance is most sensitive to temperature, and can vary greatly at different temperatures. One of the important reasons for the decline of battery performance at low temperature is the excessive internal resistance of the battery at low temperature.

Lithium battery as a power supply, from the outside, the internal resistance is certainly the smaller the better. Especially in power applications, small internal resistance is necessary.

capacity

The capacity of a lithium battery, the capacity that can be measured, is the maximum amount of charge that can be charged and discharged within a reasonable maximum and minimum voltage range of the battery. The capacity of the cell can be measured by charging and discharging before being loaded onto the vehicle. Once in the car, battery capacity can only be estimated by algorithms. In BMS, the accurate estimation of SOC is an important indicator of the design level.

At present, it is well known that the ampere-hour integral of the loop current is used to check the battery quantity with the battery open circuit voltage under the dynamic working condition. Although there are many other methods, they are either not stable or have too much computation, and few of them are really applied in batch.

The capacity of the monomer is obviously affected by the degree of aging. As we all know, the capacity decayed to an extreme value is when the battery is out of use, so it can be seen that there is an absolute correlation between the two. Secondly, the capacity is also affected by the temperature. At low temperature, the activity of the active material decreases and the available ions become less, so the capacity will inevitably decline.

power

The power here, to be precise, is the charge-discharge power capability of a single cell or the charge-discharge power capability of a cell per unit mass or per unit volume.

Whether a lithium battery can be charged or discharged at high power is determined by the time it is designed. The same lithium iron phosphate material or ternary material, the use of technological means, change the electrode thickness or add additives, adjust the structure of the active material, electrolyte properties, electrode SEI film properties, can play a role in adjusting the battery power performance. Generally, power performance and energy density cannot coexist. For the same material, the pursuit of high power will partially sacrifice the energy density.

Once the cell is produced, its reasonable maximum charge and discharge current has been determined. In addition to adjusting the cooling conditions of the battery and changing its maximum charge and discharge power in a small range, there is almost no room for further adjustment.

In addition to specific power, there are several parameters that can be converted into unit mass or volume to better understand the battery level.

Specific capacity, specific energy

Volume to volume is volume divided by battery volume, and mass to volume is volume divided by mass. Extending from here, the battery cost is converted to the unit capacity, that is, from the perspective of the battery charging and discharging capacity to talk about the price: the cost performance ratio of the battery unit capacity is calculated as the price ratio capacity, that is, the amount of electricity the battery can emit per unit price. However, this method is rarely used.

Similarly, the energy per unit mass ratio of a battery is energy divided by mass, that is, the energy released by the battery per unit mass; Volume specific energy, that is, energy released per unit volume of battery or active material; It is a common way to define the battery price from the unit energy price.

Lithium ion battery module

The battery module is formed by a series of battery monomers in parallel, which is the element of the battery pack. The battery module is rarely evaluated as a single subject in actual operation, and occasionally its voltage value is detected in some systems.

In fact, people tend to think of a module as a big battery. The difference is that the module has the problem of monomer consistency, and the internal cell voltage difference is the focus of the balance function. The performance of a battery module is often subject to the lowest performance of the battery monomer components of the battery module, and mainly reflected in the capacity of this indicator. When charging, the cell with high voltage is the first to be filled; When discharging, the low voltage cell is the first to go.

It is likely that the two cells are not the same. Therefore, the consistency of cell parameters within the module has a decisive impact on the module performance. Consistency is one more parameter to consider at module level than monomer level. At the beginning of module grouping, this parameter is guaranteed by screening the cell by various means. After the module is produced, consistency is an important index for its acceptance. In the process of operation, it can only rely on the balanced function of BMS to ensure.

Lithium ion battery system

A battery pack usually consists of modules in series. In addition to inheriting all the parameters of the module, the total voltage of the battery pack determines the voltage platform of the power system of the electric vehicle, which is a very important parameter.

The battery pack as a whole has several safety indicators that will be continuously monitored. Battery pack output positive and negative electrode to ground resistance, system leakage current, high voltage interlock signal, system maximum and minimum temperature, system maximum temperature difference, system maximum temperature rise rate, system maximum and minimum single voltage and so on.

The page contains the contents of the machine translation.