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Hello, everyone. Before you know it, 2020 will be more than half way past. It's really time flies! In this issue, let's talk about the SOH of the battery pack. Let's learn and communicate! introduce

The SOH of the battery pack is defined as a quantitative indicator of the battery health status, and is determined according to the end of the battery life. However, not all industry experts accept the single definition of battery life termination. Therefore, there are also different definitions in the industry, such as "calendar life" - the battery life expressed in months or years. Therefore, the end life of the battery is also considered to be based on the time cycle. However, we also know that the battery life is also affected by different use scenarios. Therefore, another definition of battery life was put forward immediately - "cycle life". In the second definition, the life of the battery pack is expressed by the use of the charging and discharging mode of the battery pack. At this time, the life calculation of the battery pack is based on the number of cycles of charging and discharging. When the load conditions are consistent and repeatable, this definition can be used to calculate the battery pack SOH.

Introduction to battery SOH of BMS algorithm design (I)

Battery health status - SOH

As mentioned earlier, the definition of SOH is different in the industry, that is, there are different ways to define it. SOH indicates the remaining battery life; However, the problem is that it is not a single definition of battery life termination that is widely accepted. To clarify this point, some general expressions in the industry are explained as follows:

Calendar Life

In this definition, the battery life and its end of life are represented by a series of months or years. We agree that, like other devices, the battery life is affected by different use scenarios. Of course, the battery life may also end earlier than the calendar forecast. Therefore, another way to define the battery life - cycle life has been proposed.

Cycling Life

In this definition, the battery life calculation depends on the number of cycles that the battery can maintain under a given condition. However, the accurate cycle is difficult to count in electric vehicles, because the driving conditions are variable, so the battery cannot cycle regularly. On the other hand, the charge-discharge rate will also significantly affect the number of cycles available. In addition, all similar cells do not necessarily have the same behavior, and the number of cycles available for different cells is also different.

SOH definition based on capacity attenuation (Capacity Fade)

With regard to the actual limit of battery cycle counting, some conclusions have been drawn: we need some other quantitative indicators to reflect the aging of the battery. For example, the capacity decay of battery has been used as an indicator of battery aging in many studies. The deterioration of lithium-ion batteries also began after the production of batteries due to the occurrence of electrochemical reactions inside the batteries. This process will lead to the deterioration of the active substances in the battery, and therefore, the internal resistance of the battery will increase, which means more loss and capacity attenuation in the battery. The estimation of battery capacity can give us some useful information. In this way, we can obtain the current attenuation degree of the battery by comparing its capacity (Cbatt) and its initial capacity value (Cinit). Generally speaking, we think that when the battery's end life is 80% of the initial capacity. SOH can be expressed by the following formula: SOH=1 - (Cinit Cbat)/0.2Cinit, 0.8Cinit ＜ Cbat ＜ Cinit Here, SOH can vary between 0-1, 0 means the end of battery life (Cbat=80% Cinit). The coefficient 0.2 in the denominator of the formula comes from Cinit － 0.8Cinit=0.2Cinit.

SOH definition based on power attenuation (Power Fade)

Another definition of battery SOH is based on "power attenuation" rather than capacity attenuation. This refers to how the aging process reduces the power of the battery. The power that the battery can transmit directly depends on the resistance inside the battery. The aging of almost all types of batteries will lead to the increase of battery internal resistance. Therefore, we can use this parameter to represent the SOH of the battery. The higher the internal resistance of the battery, the smaller the available power. The reason is that the voltage drop of the battery terminal is relatively high due to the high internal resistance. Let's assume a simple battery model, including a resistance (Ro) and a voltage source (Voc) in series. The terminal voltage (Vt) of the battery is calculated as follows: (The internal resistance directly affects the terminal voltage drop (Ro * Io)) Vt=Voc-Ro * Io

Many studies in the industry have also evaluated the impact of battery aging on its internal resistance. For example, when the ohmic internal resistance of the battery reaches twice the initial internal resistance, we can think that the EOL of the battery has reached. Using this definition, the battery SOH can be calculated using the following formula:

SOH=1 - (Rbat Rinit)/Rinit, Rinit ＜=Rbat ＜=2Rinit Here, Rinit is the initial internal resistance of the battery pack, and Rbat is the internal resistance of the current stage of the battery pack. The change of corresponding SOH between 0-1 represents the BOL and EOL of the battery.

In another study, the EOL of the battery was defined as 70% of the original (Pinit) maximum power (Pmax). The formula is shown as follows: Pmax/Pinit=Rinit/Rbat Here, Pinit and Rinit are the initial maximum power and initial ohmic internal resistance of the battery, and Pmax and Rbat are the current maximum power and ohmic internal resistance of the battery pack after a certain number of cycles.

Some literature studies show that the polarization internal resistance of the battery is not the ohmic internal resistance that reflects the aging of the battery. The figure below shows the change of polarization internal resistance and ohmic internal resistance during battery cycle. The results show that the ohmic internal resistance is more sensitive than the polarization internal resistance.

Introduction to battery SOH of BMS algorithm design (I)

According to different definitions of battery pack SOH, different technologies have been applied to the estimation of battery pack SOH. Generally, one or combination of the above mentioned definitions is used with appropriate measurement and estimation techniques. A variety of SOH estimation techniques have been proposed in many literatures, and one or more battery parameters are considered to change with the aging of the battery to obtain the estimation of battery SOH.

The above is the introduction of SOH that is mainly shared in this issue. The next article will bring you the relevant content of SOH estimation method. See you next time!

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