Battery Management system (BMS) SOH

The concept of battery health is an abstract concept that attempts to reduce the complex phenomena that combine to produce battery degradation to a simple indicator of the degree of battery development from the beginning to the end of its life. The definition of end of life varies from application to application, and there may be several possible definitions, but in general, a battery system needs to be repaired or replaced when it is no longer able to provide the minimum power, energy, and standby time required by the application. Many internal processes lead to three significant externally observable effects that contribute to a reduction in battery health, namely capacity decay, impedance growth, and increased self-discharge. The capacity of a battery vanishes with changes in the number of charge and discharge cycles (often called cycle life) and total service time, called calendar life. Capacity decay is a reduction in the amount of energy available and charging capacity of a battery over time. Internally, the decline in capacity is due to one of two root causes: the inability of lithium ions or electrons to reach the location of the active substance. These problems may be due to a number of other effects, including damage to the electrode structure at the micro or macro level (the active material may fall off the electrode surface and no longer come into contact with the collector). A common industry standard for end-of-life capacity is considered to be 80% of initial capacity, but this can be significantly higher or less than what is required for a particular application.

An increase in impedance results in a decrease in the rate capacity of the battery. Many of the same phenomena that lead to reduced capacity also contribute to increased impedance. Most lithium-ion batteries using carbon anodes experience soil-electrolyte interphase (SEI) growth, which increases impedance during battery aging. The loss of the active substance leads to a reduction in the surface area of the reaction, which leads to a higher impedance. Electrolyte degradation and increased interfacial resistance also contribute to the increase in impedance. Assuming that the limit voltage is fixed throughout the aging process (the validity of this assumption may vary), this will result in a corresponding reduction in the allowable charge and discharge rate up to this limit voltage. Allow power attenuation to vary more than capacity attenuation, with some applications reducing available power by 50%.

The rate of self-discharge may increase as lithium ion batteries age. As the rate of self-discharge increases, the available standby time of the battery decreases. Assuming that the size of the battery management system is the nominal lifetime of the self-discharge start, increasing and changing the self-discharge rate between cells will reduce the compensation capability of the battery management system, and ultimately reduce the battery performance.

However, as we have seen, it can be difficult to identify specific parameters that cause capacity loss, so we look for parameters associated with capacity loss that also change with aging, often using empirical relationships. For example, the rise in internal impedance is related to the loss of capacity of a lithium-ion cell. This correlation can also be driven by major degradation mechanisms and, if correctly identified, can be used to correlate related parameters that may change with aging and are relatively easy to identify with ability. Thus, online estimates of these parameters can be associated with SOH even if SOH is not directly estimated. Of course, degradation mechanisms may also depend on use, especially temperature and current rate. In a practical SOH estimation algorithm, the environment and battery usage must be considered.

These three factors were used to calculate a single metric, battery system health (SOH). SOH =SOH (Capacity,Impedance,Self Discharge). An ideal SOH value is between 1 and 0, usually expressed as a percentage. When the battery is new, SOH should be 100%. SOH is typically defined as zero once the battery has reached the point where it is marginally capable of providing the power, capacity, and standby time required by an application. Ideally, if the battery is cycled under the same environmental conditions (using the same charge-discharge curve (so that the charge-discharge curve is effectively the same for each cycle), then the SOH should decrease in a linear manner relative to the total cycle count. In fact, operating at more extreme rates, temperatures, and charging states generally reduces battery health more quickly than operating under mild conditions, and reported SOH does not decline evenly across different types of cycles. The most accurate and straightforward way to measure capacity is to discharge completely at a very slow (C/10) rate followed by a very slow full charge. The battery capacity can be determined by the current count. However, this is very time consuming and impractical for most applications. Full charge and discharge at higher C rates may also be capacity dependent, provided that changes in internal impedance are properly considered. Again, this test is not practical for an HEV application. One can use empirical methods by calculating current cycles, but getting the required map is time consuming, especially given the changing shape of pulse charge-discharge and environmental conditions. If there is a degradation model, it can be simulated on a vehicle to estimate SOH. However, these models can be computationally demanding and may drift from the actual SOH without battery feedback. An ideal battery management system must be able to dynamically estimate these three parameters using only the same voltage, temperature, and current inputs during the operation of the battery system. In laboratory tests, reference performance (RPT) tests are performed during the full cycle and calendar life tests to assess the capacity and impedance of the battery.

The performance of a battery depends on how long it is used and the environment in which it is used. SOH estimates include distinguishing between temporary performance changes caused by extreme temperatures and long-term performance degradation associated with aging. Therefore, it is important to characterize how model parameters change with temperature and aging. Some parameters may change significantly with age, but not much with temperature, and may be good indicators of SOH. Other parameters can be assumed to be constant or their variation independent of aging, so no computational power should be consumed to estimate them. There may be parameters that vary with aging and temperature, but temperature effects can be calibrated by measured temperature and experience or Arrhenius relationships.

The ultimate symptom of capacity loss and increased impedance are the externally-observable effects of the many complex interactions that occur within the battery. A basic understanding of how lithium-ion batteries lose capacity will help develop battery management systems for such battery systems. During the charging process, lithium atoms inserted in the active cathode material must be inserted and oxidized, losing an electron. The electrons must travel from the active material, through the positive terminal, and eventually to the positive terminal of the collector and battery. The lithium ion must travel from the site of the active material, through the electrode material, to the electrolyte, where it must travel through the separator to the anode. The ions must reach the particles of the active anode material, recombine with electrons that reach the same position through the negative collector and terminal, and re-insert themselves into the negative material. The process is reversed during the discharge process. Considering the steps involved, capacity decay and impedance growth can be interpreted as failures or obstacles in one of the steps of the above process. Loss of active recyclable lithium ions results in a reduction in capacity. This can happen in a number of ways. The passivation film on the anode layer, known as the solid electrolyte interphase (SEI), is produced by the reaction of lithium ions with electrolytes. The exact reactions and products are not known, but lithium is consumed during the formation of the SEI layer, which occurs significantly during the first few battery formation cycles, but continues at a slower rate thereafter. The most described effect is the reduction of the solvent in the electrolyte to form lithium containing compounds. SEI growth reaction usually occurs mainly during charging. SEI formation is responsible for most of the capacity loss at the beginning of battery life. Lithium ions can also be due to metal lithium anode voltage below zero, usually due to charging too quickly at low temperatures, but this is not an appropriate way to maintain the normal capacity loss of the battery. Lithium can also be lost through other, less significant side effects, such as electrolyte breakdown that consumes lithium.

Another possibility for volume loss is loss of active substances; Without insertion sites, charge transfer cannot occur. The active substance can be degraded by side reaction with electrolytes. The most common assumptions are that the active substance is isolated from the electrolyte, preventing lithium ions from entering, or from the current collector, preventing electrons from entering. Both methods result in the loss of available insertion positions and lower battery capacity. The volume change of electrode material during charge and discharge is usually considered to be the main cause of breakdown. If the active substance is lost in the aliphatic state, so are the lithium ions. The increase in impedance is caused by the resistance to electron and ion transport in various parts of the battery. Ohmic resistance of metal collectors, labels and terminals is expected to be relatively stable and not a source of significant resistance changes. The growth of the SEI layer increases the transport resistance of lithium ions to and from the anode. The above reactions are certainly sensitive to temperature (higher temperatures lead to faster degradation) and current, especially charge rate, as well as the ultimate limit of charge and discharge chosen. These models are built to take into account the effect of all these inputs on the rate of degradation.