BMS algorithm design: battery SOH introduction (Part 1)

Whether reading thousands of books or traveling thousands of miles, it all comes from the accumulation of bits and pieces. I wish you all a happy May Day!

Hello everyone, before we know it, 2020 is almost over. Time really flies! In this issue, let’s talk about the SOH of battery packs. Let’s learn and communicate together! Introduction

The SOH of a battery pack is defined as a quantitative indicator of the battery's state of health and is determined based on the end of life of the battery. However, not all industry experts accept this single definition of the end of life of a battery. Therefore, there are different definitions in the industry, such as: "calendar life" - the life of the battery is expressed in months or years. Therefore, the end of life of the battery is also considered to be based on a time period. However, we also know that the life of the battery is also affected by different usage scenarios. Therefore, another definition of battery life was proposed - "cycle life". In the second definition, the life of the battery pack is expressed by the use of the battery pack charge and discharge mode. In this case, the life calculation of the battery pack is based on the number of charge and discharge cycles. This definition can be used to calculate the battery pack SOH when the load conditions are consistent and repeatable.

Battery State of Health - SOH

As mentioned earlier, the definition of SOH is controversial in the industry, that is, there are different definitions. SOH indicates the remaining battery life; however, the problem is that there is no single definition of battery end-of-life that is widely accepted. To clarify this, some common expressions in the industry are explained as follows:

Calendar Life

In this definition, the life of a battery, and its end of life, is represented by a series of months or years. We agree that, like any other device, the life of a battery is affected by different usage scenarios. Of course, the life of a battery may end earlier than the calendar predicts, so another way to define battery life - cycle life, is proposed.

Cycling Life

In this definition, the battery life calculation depends on the number of cycles that the battery can sustain under given conditions. But the exact number of cycles is difficult to count in electric vehicles , because the driving conditions are variable and the battery cannot be cycled regularly. On the other hand, the charge and discharge rate also significantly affects the number of usable cycles. In addition, all similar cells do not necessarily behave the same, and the number of usable cycles varies from cell to cell.

Definition of SOH based on capacity fade

Regarding the practical limitations of battery cycle counting, some conclusions have been drawn: we need some other quantitative indicators to reflect the aging of batteries. For example, battery capacity decay has been used as an indicator of battery aging in many studies. The deterioration of lithium-ion batteries also begins after the battery is manufactured due to the occurrence of electrochemical reactions inside the battery. This process causes the deterioration of the active materials inside the battery, and therefore, the internal resistance of the battery increases, which means more internal losses and capacity decay. Estimating the battery capacity can give us some useful information. In this way, we can obtain the current degree of battery degradation by comparing the battery capacity (Cbatt) with its initial capacity value (Cinit). Generally speaking, we consider the end of life of the battery when the current capacity reaches 80% of the initial capacity. SOH can be expressed as follows: SOH = 1- (Cinit-Cbatt)/0.2Cinit, 0.8Cinit < Cbatt < CinitHere, SOH can vary between 0-1, and 0 means the end of life of the battery (Cbatt = 80% Cinit). The coefficient 0.2 in the denominator of the formula comes from Cinit-0.8Cinit=0.2Cinit.

Definition of SOH based on power fade

Another definition of battery SOH is based on "power decay" rather than capacity decay. This refers to how the aging process reduces the power of the battery. The power that a battery can directly deliver depends on the resistance inside the battery. Aging of almost all types of batteries will lead to an increase in the internal resistance of the battery. Therefore, we can use this parameter to represent the SOH of the battery. The higher the internal resistance of the battery, the less power it can use. The reason is that the higher internal resistance causes a higher voltage drop at the battery terminals. We assume a simple battery model consisting of a resistor (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 drop in the terminal voltage (Ro*Io)) Vt = Voc - Ro*Io

There are also many studies in the industry that evaluate the impact of battery aging on its internal resistance. For example, when the battery's ohmic internal resistance reaches twice the initial internal resistance, we can consider that the battery has reached EOL. Using this definition, the battery SOH can be calculated using the following formula:

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

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

Literature research shows that the polarization internal resistance of the battery is not the ohmic internal resistance that reflects battery aging. The figure below shows the comparison of the changes in polarization internal resistance and ohmic internal resistance during battery cycling. The results show that ohmic internal resistance has a higher sensitivity than polarization internal resistance.

According to different definitions of battery pack SOH, different techniques have been applied to estimate battery pack SOH. Usually, one or a combination of the above-mentioned definitions are used together with appropriate measurement and estimation techniques. Various SOH estimation techniques are proposed in many literatures, taking into account one or more battery parameters that change as the battery ages to obtain an estimate of the battery SOH.

The above is the main introduction to SOH in this issue. The next article will introduce the estimation method of SOH. See you next time!

References: CNKI, Energy journals, related books, etc.