Explain the various parameters of the lithium battery

The lithium ion battery cell is composed of positive and negative electrodes, an electrolyte and a separator. It is a basic structural unit constituting a battery module and a battery pack. As an electrochemical power source, the battery has natural characteristics such as voltage, internal resistance, capacity, energy and power.

In the power lithium battery system, each parameter can characterize the different performance of the system. This article lists the various parameters of the lithium battery.

Lithium battery cell

The lithium ion battery cell is composed of positive and negative electrodes, an electrolyte and a separator. It is a basic structural unit constituting a battery module and a battery pack. As an electrochemical power source, the battery has natural characteristics such as voltage, internal resistance, capacity, energy and power.

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

One is to achieve the purpose of active control, for example, the battery cell voltage is inconsistent, so that the energy storage capacity of the system is reduced. If the cell voltage of the two poles can be actively adjusted, the effect of amplifying the system capacity can be achieved.

The other is for safety reasons, the battery parameters have a fixed range, the detection of battery parameters, the implementation of monitoring its boundaries, can play a role in characterizing the battery safety status.

Voltage

The monomer voltage mainly depends on the type of the positive and negative materials of the monomer. Generally, the lithium cobaltate, the ternary positive electrode and the graphite negative electrode can obtain a full charge voltage of about 4.2V, and the lithium iron phosphate can only reach 3.6V. The voltage here, to be precise, should be that the potential depends on the material properties, and the potential value is equal to the open circuit voltage of the battery after standing for a long time.

The voltage at the cell terminal in the closed loop is the value of the voltage we detected with an external instrument. The value is equal to the battery potential minus the internal resistance of the battery. The internal resistance of the battery is not constant and will be affected by many factors, which are described in the next section.

Continuing with the voltage, the cell voltage changes in accordance with the change in the charge amount, and is a one-to-one relationship. Therefore, in many cases, the battery charge (SOC) that cannot be directly measured is often Predict the battery open circuit voltage.

The cell voltage is related to the activity level of the active material inside the battery, so the temperature affecting the activity can also affect the level of the cell voltage in a small range.

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

Internal resistance

Inside the lithium battery, lithium ions move from one pole to the other, and the factors that hinder the movement of ions in the process together constitute the internal resistance of the lithium battery. The main part includes the physical internal resistance of the conductive member; the inherent impedance of the electrochemical material such as the motor material, the diaphragm and the electrolyte; and the internal increase of the lithium ion movement caused by the presence of current in the battery; these three parts together constitute the inner The main body of resistance.

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

Lithium battery as a power source, from the outside, the internal resistance must be as small as possible. Especially in the case of power applications, small internal resistance is a necessary condition.

Capacity

Lithium battery capacity, measurable capacity, is the maximum amount of charge that can be charged and discharged within the reasonable maximum and minimum voltage range of the battery. The capacity of the unit can be measured by charge and discharge before being mounted on the vehicle. Once on the train, battery capacity can only be estimated by algorithm. In the battery management system BMS, accurate estimation of the state of charge of the battery is an important indication of its design level.

It is now well known that for dynamic operating situations, the loop current is integrated, and in the non-operating state, the battery open circuit voltage is used to check the battery charge. Although there are many other methods, it is not that the stability is not good or the calculation is too large. It is rare to use it in batches.

The capacity of the monomer is obviously affected by the degree of aging. As we all know, when the capacity is attenuated to a limit value, it is obvious that the two have an absolute correlation. Secondly, the capacity is also affected by the temperature. At low temperatures, the activity of the active substance decreases, the amount of ions that can be supplied decreases, and the capacity necessarily decreases.

Power

The power here, which is exactly the specific power, is the charge/discharge power capability of a single unit or the charge/discharge power capability per unit mass or unit volume.

Whether the lithium battery can perform high-power charge and discharge, which was decided when it was designed. The same is the lithium iron phosphate material or ternary material, using the process means, changing the electrode thickness or adding additives, adjusting the structure of the active material, electrolyte properties, electrode SEI film properties, can all be used to adjust the battery power performance. In general, power performance and energy density often cannot coexist. The same material, in pursuit of high power, will partially sacrifice energy density.

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

In addition to the specific power, there are several equivalents to the unit mass or volume, which can better see the battery level parameters.

Specific capacity, specific energy

The volumetric specific capacity is the capacity divided by the volume of the battery, which is the capacity divided by the mass. From here to expand, the battery cost is converted to the unit capacity, that is, from the perspective of battery charge and discharge capacity to talk about the price: the battery unit capacity cost-effective calculation method is the price ratio capacity, that is, the unit price of the battery can release the power. However, this method is generally less useful.

Similarly, the mass-to-cell mass ratio energy is the energy divided by the mass, that is, the energy that the unit mass of the battery can emit; the volumetric energy, that is, the energy that can be emitted per unit volume of the battery or active material; the battery price is defined from the unit energy price. It is a more common way in the industry to talk about the battery price, which is 1kWh.

Lithium-ion battery module

The battery module is formed by a combination of a plurality of battery cells in series and in parallel, and it is an element constituting the battery pack. The battery module is rarely evaluated as a main body in actual operation. Occasionally, in some systems, the voltage value is detected.

In fact, people tend to think of a module as a large battery. The difference is that the module has a single cell consistency problem, and its internal cell voltage difference is the focus of the equalization function. The performance of the battery module is often limited by the lowest performance battery cells in the battery module, and is mainly reflected in the capacity index. When charging, the high voltage monomer is first filled; when discharging, the low voltage battery is discharged first.

It is likely that the two batteries are not the same. Therefore, the consistency of the internal cell parameters of the module has a decisive influence on the module performance. Consistency is a parameter that needs to be considered at the module level. At the beginning of the module grouping, this parameter will be checked by various means to ensure the battery core; the module is finished, the consistency is an important indicator of its acceptance; in the running process, it can only rely on the BMS equalization function. Guarantee.

Lithium-ion battery system

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

There are several other indicators on the battery pack that are subject to safety and are continuously monitored. Battery pack output positive and negative ground resistance, system leakage current, high voltage interlock signal, system maximum and minimum temperature, system maximum temperature difference, system maximum temperature rise rate, system highest minimum cell voltage and so on.

The above are all from the electrical performance, as a structural whole, the battery pack has many parameters that need constant attention, as well as environmental tolerance performance, abuse tolerance performance, not listed here.

The page contains the contents of the machine translation.