Power lithium-ion battery performance evaluation index

In the lithium power battery system, each parameter can characterize the different battery performance of the system, this paper lists the lithium battery parameters.
Table of Contents

Battery performance evaluation index – lithium battery cell

Lithium-ion battery monomer consists of cathode, anode, lithium ion battery electrolyte and diaphragm, which are the basic structural units that make up the battery module and battery pack. Battery as a kind of electrochemical power supply naturally has voltage, internal resistance, battery capacity, energy, power and other characteristics of the parameters, these parameters are also lithium-ion manufacturers used as the main basis for testing battery performance.

There are two main purposes for which people want to measure and evaluate the parameters that affect battery performance. One is to realize the purpose of active control, for example, the inconsistency of the voltage of the battery cells makes the system energy storage capacity lower, and the battery performance is reduced, if it can actively regulate the voltage of the cells at the two poles, then it can have the effect of amplifying the system capacity.

Another is for safety considerations, the battery parameters have their own fixed range, detection of battery parameters, the implementation of monitoring its boundaries, can play a role in characterizing the safety status of the battery and maintain good battery performance.


Battery performance evaluation index – voltage

Monomer voltage as one of the main parameters for testing battery performance, it depends on the type of monomer anode and cathode materials, general lithium cobalt cobalt oxide, ternary lithium battery cathode with graphite negative electrode can get about 4.2V full charge voltage, while lithium iron phosphate can only reach a maximum of 3.6V.

Here the voltage, accurately should be the potential depends on the material properties, potential numerically equal to the open circuit voltage of the battery after a long enough time of rest. The single terminal voltage in the closed loop is the voltage value detected by external instruments, and its value is equal to the battery potential minus the internal resistance of the battery to account for the voltage. And the battery internal resistance is not constant, will be affected by a variety of factors and changes.

The cell voltage, in addition to being determined by the battery materials, follows the change in charge and is in a one-to-one relationship. Therefore, in many cases, the cell charge (SOC), which cannot be measured directly and simply, is often inferred by using the open-circuit voltage of the cell.


The monomer voltage is related to the active degree of the active substance inside the battery, so the temperature that affects the active degree can also affect the monomer voltage level in a small range, which indirectly affects the battery performance.

The higher the monomer voltage, the more energy is contained in the same capacity of the battery, therefore, under the premise of ensuring safety, increasing the upper limit of the monomer voltage is a technical route to improve the energy density of the system and battery performance.

Battery performance evaluation index – internal resistance

Lithium batteries, lithium ions from one pole to the other pole, the process of hindering the movement of ions in the process of the factors together constitute the internal resistance of lithium batteries.

Its main parts include the physical resistance of the conductive parts; motor materials, diaphragms and electrolyte and other electrochemical substances inherent impedance; the battery through the presence of current inside the temporary increase in the movement of lithium ions generated by the obstruction; Together, these three components form the main body of internal resistance and are the reference elements for testing battery performance.

Internal resistance is most sensitive to temperature, and the value of internal resistance can change greatly at different temperatures. One of the most important reasons for battery performance degradation at low temperatures is that the internal resistance of the battery is too large at low temperatures.

Lithium battery as a power source, want to ensure that the battery performance is excellent, from the external point of view, the internal resistance is certainly the smaller the better. Especially in power applications, small internal resistance is a necessary condition.


Battery performance evaluation index – capacity

Li-ion battery capacity is also one of the most important reference factors for battery performance. The measurable capacity is the maximum amount of power that can be charged and discharged within a reasonable maximum and minimum voltage range of the battery. The capacity of a single unit can be measured by charging and discharging before it is mounted on a vehicle.

Once on board, the battery capacity can only be estimated by algorithms. In the battery management system BMS, accurate estimation of the battery state of charge SOC is an important indicator of its design level. Currently, the familiar practice is to integrate the circuit current ampere-time for dynamic operation, and to calibrate the battery charge with the open-circuit voltage of the battery in the non-operating state.

Other methods, although diverse, are either unstable or too computationally intensive, and not many of them have been applied in real quantities. The capacity of the monomer is obviously affected by the degree of aging, as we all know, the capacity decay to a limit value is the battery performance decline to the time of elimination, see the absolute correlation between the two.

Secondly, the capacity also receives the influence of temperature, under low temperature, the activity of active material activity decreases, can provide less ions, the capacity is bound to follow the decline. Therefore, there is a difference between battery performance in winter and summer.


Battery performance evaluation index – power

The power here, to be precise, should be the specific power, which is the charging and discharging power capacity of a single unit or the charging and discharging power capacity per unit mass or per unit volume of the battery cell.

Whether the performance of lithium battery is good enough for high power charging and discharging, this has been decided when it was designed. The same lithium iron phosphate material or ternary material, the use of process means, change the thickness of the electrode or additives, adjust the structure of the active substance, the nature of the electrolyte, electrode SEI film properties, can play the purpose of adjusting the power performance of the battery.

In general, the battery performance in the power performance and energy density often can not coexist, the same material, the pursuit of high power, part of the energy density will be sacrificed. Once the battery cell has been produced, its reasonable maximum charging current and discharging current has already been determined, and there is almost no room for further adjustment, except for adjusting the heat dissipation conditions of the battery to be able to change its maximum charging and discharging power to a small extent.

In addition to specific power, there are a couple of other parameters folded into the unit mass or volume that give a better indication of the level of the battery performance.


Battery performance evaluation index – Specific capacity and specific energy

Specific capacity and specific energy can also be used as a reference for testing battery performance. Volume to capacity is capacity divided by battery volume, and mass to capacity is capacity divided by mass.

Extended from here, the cost of the battery folded into the unit capacity, that is, from the battery charging and discharging ability to talk about the price: the battery unit capacity cost-effective way of calculating the price to capacity, that is, the unit price of the battery can be discharged by the amount of electricity. However, generally this way is less used.

Similarly, the mass specific energy of a battery monomer is the energy divided by the mass, i.e., the energy that can be released per unit mass of the battery; and the volume specific energy, i.e., the energy that can be released per unit volume of the battery or active substance; Defining the price of a battery from the price per unit of energy is the more common way to talk about battery prices in the industry, which is how much 1kWh costs.

Lithium-ion battery module

A battery module is formed by combining a number of battery cells in series and parallel and is the element that makes up a battery pack. The battery module is seldom evaluated individually as a body in actual operation, and occasionally in some systems, its voltage value is examined.

In fact, people tend to think of a module as a large battery. The difference is that the module has the problem of monomer consistency, and its internal cell voltage difference is the focus of the equalization function. Battery performance is often subject to the composition of the battery module in the lowest performance battery monomer, and is mainly reflected in the capacity of this indicator.


When charging, the monomer with high voltage is the first to be filled; when discharging, the cell with low voltage is the first to be discharged. And it is likely that these two cells are not the same one. Therefore, the consistency of the cell parameters within the module has a decisive impact on the battery performance.

Consistency is a parameter that needs to be considered at the module level more than the monomer. This parameter will be ensured through various means of screening the electric core at the beginning of module formation; After the module is produced, consistency is an important indicator for acceptance; during operation, it can only be guaranteed by the equalization function of BMS. For further information, you can also check top 10 two-wheeler battery management system manufacturers.

Lithium-ion battery systems

Battery packs are generally composed of modules connected in series. In addition to inheriting all the parameters of the modules, the total voltage of the battery pack determines the voltage plateau of the electric vehicle powertrain and is a very important parameter for testing the battery performance.

The battery pack as a whole also has several other safety-related battery performance indicators that will be continuously monitored. Battery pack output positive and negative pole 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 individual voltage and so on.

All of the above are from the battery performance, as a structure as a whole, the battery performance there are many parameters need to continue to pay attention to, as well as environmental resistance, abuse resistance.

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