Calculating the State of Health for a Lithium Ion Battery System

Summary

In this Stopple Systems Insights video, Eric Topol discusses the concept of battery health, specifically focusing on State of Health (SoH). SoH is defined as the ratio of a battery's current capacity to its original capacity, measured in ampere-hours. The video explains how SoH declines over time due to capacity fade and impedance growth, which affects battery performance and efficiency. Topol illustrates the impact of these factors on electric vehicle range estimation and the importance of accurate SoH modeling for performance predictions.

Takeaways

• 🔋 The State of Health (SoH) of a battery is defined as the ratio of its current total capacity to its beginning-of-life (BOL) capacity, expressed in ampere-hours.
• 📉 SoH indicates the degradation of a battery over time, similar to how a cellphone's battery life decreases as it ages.
• 🔌 SoH is crucial for understanding the battery's remaining capacity and its ability to deliver energy over time.
• 📊 The capacity of a battery pack typically decreases with each charge/discharge cycle, forming a curve that represents its aging process.
• ⚡ Another aspect of SoH is the impedance, which is a measure of resistance within the battery. Impedance increases as the battery ages, affecting performance.
• 🔥 Increased impedance leads to higher voltage drop and more heat generation during battery operation, impacting the battery's efficiency and safety.
• 🚗 For electric vehicles, accurate SoH calculations are essential for estimating range and ensuring the vehicle's performance remains consistent over time.
• 🔍 Battery Management Systems (BMS) use algorithms to monitor SoH, including both capacity and impedance, to maintain optimal battery performance.
• 🏁 In high-performance applications, such as racing, understanding SoH is critical for predicting battery thermal limitations and overall vehicle performance.
• ⚙️ BMS algorithms must account for both SoH capacity and impedance to provide accurate and safe battery operation estimates.

Q & A

What is the State of Health (SoH) of a battery?

The State of Health (SoH) of a battery is defined as the ratio of the battery pack's current total capacity to its beginning-of-life (BOL) capacity, expressed in ampere-hours (Ah).

How does the State of Health (SoH) affect the performance of a battery?

A decreasing SoH indicates that the battery's capacity to hold charge is reducing over time, which results in shorter usage times or reduced range in devices like electric vehicles.

What is the relationship between SoH and the number of charge/discharge cycles a battery undergoes?

As a battery undergoes more charge/discharge cycles, its SoH typically decreases due to the natural degradation of the battery, leading to a reduced capacity over time.

What is the significance of the term 'cycle life' in the context of battery health?

Cycle life refers to the number of charge/discharge cycles a battery can undergo before its capacity significantly diminishes, indicating the expected lifespan of the battery.

What is the difference between SoH capacity and SoH impedance?

SoH capacity refers to the battery's available capacity relative to its BOL capacity, while SoH impedance refers to the increase in internal resistance (or impedance) of the battery over time, which can affect performance and efficiency.

Why is it important to monitor impedance growth in a battery?

Monitoring impedance growth is important because it helps in understanding the increase in internal resistance, which can lead to higher voltage drop, more heat generation, and reduced efficiency during charge/discharge cycles.

How does the battery's impedance affect the thermal management of an electric vehicle?

As impedance increases over time, the battery generates more heat during operation, which can impact the thermal management system's efficiency and potentially limit the vehicle's performance, especially in high-performance scenarios.

What is the role of Battery Management System (BMS) in estimating the range of an electric vehicle?

The BMS plays a crucial role in estimating the range of an electric vehicle by considering both the state of charge and the SoH of the battery, providing a more accurate prediction of the vehicle's remaining range.

How does the SoH affect the estimated range remaining of an electric vehicle?

The estimated range remaining of an electric vehicle is calculated by multiplying the current state of charge by the SoH capacity. A lower SoH capacity due to battery degradation will result in a reduced estimated range.

Why is it crucial to accurately model SoH in high-performance applications like racing?

In high-performance applications, such as racing, accurately modeling SoH is crucial because it directly impacts the battery's thermal and performance limits, which can determine the vehicle's competitiveness and reliability during the race.

Outlines

00:00

🔋 Understanding Battery State of Health (SoH)

In this segment, Eric Topol introduces the concept of battery State of Health (SoH), which is defined as the ratio of a battery's current total capacity to its beginning-of-life (BOL) capacity, expressed in ampere-hours. He explains that SoH is crucial for predicting battery performance over time, using the analogy of a cellphone battery's decreasing capacity after prolonged use. The video illustrates how SoH is represented graphically, showing a decreasing capacity over an increasing number of charge/discharge cycles. Additionally, Topol introduces the concept of SoH Impedance, which relates to the increase in internal resistance (ESR) of a battery over time, affecting the battery's efficiency and heat generation.

05:02

📉 Impact of SoH on Battery Performance and Estimations

The second paragraph delves into the significance of monitoring both SoH Capacity and SoH Impedance within a Battery Management System (BMS). These metrics are essential for estimating the remaining capacity and potential voltage drop or temperature rise during battery discharge. Topol uses an example to demonstrate how SoH affects the estimated range of an electric vehicle, highlighting the importance of accurate SoH modeling for range estimation. He also discusses the impact of SoH Impedance on the thermal performance of high-performance electric vehicles, explaining how increased impedance can lead to earlier thermal limitations during races. The paragraph concludes with a call to action for understanding and quantifying these values to ensure accurate performance estimations of battery packs over time.

Keywords

💡State of Health (SOH)

State of Health (SOH) refers to the measure of a battery's current capacity compared to its original capacity at the beginning of its life (BOL). In the video, it is described as a percentage that helps monitor battery health over time, providing insights into how much energy the battery can store as it ages. For example, a new battery would have a SOH of 100%, but after several charge/discharge cycles, it might drop to 60-70%, affecting its performance and lifespan.

💡Capacity Fade

Capacity fade describes the gradual reduction in a battery's ability to hold a charge as it undergoes repeated charge and discharge cycles. This concept is central to understanding SOH, as the battery's capacity diminishes over time. In the video, it is shown through a curve, where a battery might start with 50 amp hours and reduce to 44 amp hours after 600 cycles, leading to decreased performance over time.

💡Equivalent Series Resistance (ESR)

Equivalent Series Resistance (ESR) refers to the internal resistance of a battery, which affects how efficiently it can deliver energy. The video explains how ESR, measured in milli-ohms, increases over time as the battery ages, contributing to greater voltage drops and heat generation during use. This resistance growth is a key indicator of the battery's health and efficiency.

💡Battery Management System (BMS)

A Battery Management System (BMS) is an essential system that monitors and manages the state of health, charge, and other critical parameters of a battery to ensure optimal performance and safety. In the video, the BMS is responsible for calculating SOH based on factors like capacity fade and impedance, helping users understand how much energy their battery can deliver at any given time.

💡Impedance Growth

Impedance growth refers to the increase in resistance within the battery over time due to repeated charge and discharge cycles. The video emphasizes that this growth impacts the thermal efficiency and performance of the battery, as a higher impedance leads to greater heat generation and voltage drops during usage. Monitoring impedance growth is crucial in predicting the battery's lifespan and performance limits.

💡Cycle Life

Cycle life is the number of complete charge/discharge cycles a battery can undergo before its performance significantly deteriorates. The video uses this concept to explain how a battery’s capacity diminishes after hundreds of cycles, which is directly tied to the state of health. For example, a battery might perform well for the first 200 cycles but show reduced capacity after 600 cycles.

💡Beginning of Life (BOL)

Beginning of Life (BOL) refers to the initial state of a battery when it is new, with full capacity and minimal internal resistance. In the video, BOL is used as a reference point to calculate the State of Health (SOH), where a new battery starts at 100% SOH and deteriorates over time as it undergoes more cycles.

💡Thermal Output

Thermal output refers to the heat generated by a battery during operation, especially as it ages and its internal resistance increases. The video highlights how an increase in impedance due to battery aging leads to higher thermal output, which can affect the battery’s cooling requirements and overall performance, particularly in high-demand applications like electric vehicles.

💡State of Charge (SOC)

State of Charge (SOC) indicates the current charge level of a battery, typically expressed as a percentage of the full charge. In the video, SOC is combined with SOH to estimate how much range remains in an electric vehicle. For instance, if the SOC is 70% and the SOH is 80%, the remaining range can be calculated by applying these percentages to the vehicle's initial range.

💡Estimated Range Remaining

Estimated range remaining is the calculated distance or time a battery-powered device, such as an electric vehicle, can operate before it needs to be recharged. The video explains how this estimate depends not only on the State of Charge (SOC) but also on the State of Health (SOH), as battery degradation over time reduces the maximum possible range. For example, a vehicle with an 80% SOH would have a reduced maximum range compared to when it was new.

Highlights

State of Health (SoH) is defined as the total capacity of the battery pack today over the beginning of life (BOL) capacity.

SoH is expressed in units of amp hours, indicating the battery's charge capacity.

SoH can be observed in the reduced battery life of devices like cellphones over time.

A new cellphone typically has a 100% SoH, reflecting its full BOL capacity.

After 300 to 600 charge/discharge cycles, SoH may decrease to 70% or 60%, indicating capacity loss.

SoH is crucial for understanding the battery pack's energy output over time.

Cycle life refers to the battery's capacity decrease over charge/discharge cycles.

Battery capacity, measured in amp hours, decreases with each cycle, forming a decreasing curve.

SoH is not just about capacity fade; it also includes SoH impedance, which relates to resistance.

Impedance, or resistance, increases over time as the battery charges and discharges.

SoH impedance is important for predicting voltage drop and temperature rise during battery use.

BMS (Battery Management System) algorithms use SoH to estimate remaining capacity and performance.

SoH is critical for estimating the range of electric vehicles accurately.

An electric vehicle's range can decrease by up to 40 miles if SoH is not modeled correctly.

SoH impedance is vital for high-performance electric vehicles where battery thermals are a limiting factor.

Understanding SoH can prevent underestimations of a battery pack's performance over time.

Transcripts

[Music]

welcome to the stopple systems insights

video series I'm Eric Topol

president of Stoffels systems the topic

of today's video his state of health as

it's calculated by a battery management

system so what is the state of health of

the battery

well state of health or Soh is most

commonly defined as the total capacity

of the battery pack today

over the total beginning of life

capacity or what's called Bol capacity

and these are given in units of amp

hours or units of charge so for example

the state of health describes the effect

that you may have experienced when you

have a cellphone that you purchased

recently that seems to last all day and

has excellent battery life but after

about a year or two you'll notice that

the amount of time that you can use your

cell phone on a given battery charge is

decreasing over time so that would be

explained by the state of health so for

example when you initially purchase your

cellular telephone with your your

battery it would have a state of health

of a hundred percent because you just

got it and its total capacity is

basically equal to the beginning of life

capacity but after say 300 or 600

charge/discharge cycles after about a

year or two you would see that perhaps

it's gone down to 70 maybe even 60%

state of health and so it's very

important to understand what this means

because it gives you an understanding of

how much time or how much energy you can

discharge out of the system over time of

the battery pack

so let's look at how this actually works

in practice so over here on the y axis

I'm gonna describe capacity again in

units of amp hours and then in the

x-axis we're gonna use psycho life or

just battery cycles rather so for

example say this is 100 cycles

charge/discharge cycles this is 200

etcetera etc maybe up to 600 cycles in

this particular system some battery

packs have longer cycle life other

battery packs have less cycle life but

what do we mean by cycle life well over

time as you charge discharge of battery

generally speaking the capacity of that

battery pack will decrease over time so

for example when you initially purchased

that battery pack and it had zero charge

discharge cycles this would have a

capacity say of 50 amp hours after maybe

200 charge/discharge cycles now maybe

you see a capacity of 47 camp hours and

then maybe once you get to 600 cycles

you see a capacity of say 44 amp hours

so this forms a curve a decreasing curve

of the total amount of capacity that you

can store in that battery pack on each

cycle as you age the pack and so that is

the primary definition of state of

health however there are a few other

definitions or flavors of state of

health that are very important to

understand as well so from now on I'm

going to refer to this state of health

what we just talked about of state of

health capacity so this is the effect of

capacity fade over time but there's also

another effect that can be calculated in

an algorithm in a BMS that's very

helpful as well

and I'm gonna refer to that as state of

health impedance

so what is impedance meet impedance is

just a fancy way of saying resistance so

any sort of lithium-ion battery cell or

any cell in general can be modeled as a

2/2 device model basically here's your

ideal lithium-ion cell and then here's

what's called ESR the equivalent series

resistance and so typically for a cell

this might be say 20 milli ohms at the

beginning of life at a certain

temperature so for example I'll say that

this is 20 milliamps now over time as

you charge and discharge that battery

you will expect that this ESR or

equivalent series resistance will

increase over time so I'm gonna switch

to a different color here to red and now

in red the y-axis is going to refer to

impedance and this is given in units of

ohms or milli ohms depending on the

scale so what you would expect to see is

you would start with a relatively low

impedance at the beginning of life but

as you charge and discharge the cell

you're impedance will start to grow and

this is called impedance growth so as

you age the cell and use it your

impedance increases so we'll call that

impedance growth

[Music]

so these two factors are very important

to monitor within a BMS because it gives

you an estimated understanding of how

much capacity you have left to discharge

at any given cycle in the cycle life of

the system as well as as you discharge

it are you going to have more voltage

drop than before are you gonna have more

temperature rise than before because as

you can imagine as you charge or

discharge the battery over an increasing

resistance over time you're going to

have more thermal output for that

specific battery system so the battery

in general will bill will run coolest at

the beginning and over time it will

start to be more resistive and thus less

efficient and generate more heat over

discharges so it's very important in the

BMS modeling algorithms to be able to

understand what this looks like as the

battery pack ages so going back to an

important concept to think about when

you're talking about Soh so just in

summary this is Soh impedance and this

is Soh capacity so as you can imagine if

you're trying to come up with an

estimated range remaining algorithm for

example for an electric vehicle and you

want to say so I'm gonna say estimated

range remaining

okay

so how do we actually calculate that

well in a previous video we discussed

state of charge so say for a simple

approximation we want to say the state

of charge of this system is 70% and the

state of health capacity of the system

is 80% and when the vehicle was

initially purchased when the battery was

brand-new

we had a total Bol range of say 200

miles so what is the total range of the

vehicle now that it's aged due to the

capacity fade effect well if the soh

capacity is now 80% then you have your

current max range of the vehicle is now

160 miles so not an insignificant

difference it's 40 miles reduction so

very important to know that and then if

you wanted to know range remaining then

you would need to take 70% multiplied by

160 and that gives you your range

remaining I'll do the math later so it's

very important to understand that if you

didn't model your state of health

capacity correctly and you assumed that

just your state of charge was going to

give you an accurate range estimation

you would be off by potentially up to 40

miles of range Delta so it's very

important to understand this this

feature and then going to Soh and pedis

why is that so important well if you can

imagine that the cooling performance of

your vehicle

is a limiting factor for example say

that you have a race electric vehicle

it's high-performance and the battery

thermals are one of the limiting factors

that prevents you from going to the end

of the race with the maximum speed well

now you might have thermal say that the

beginning of life you had thermal

limiting that occurred say that happened

say 10 minutes into into a race that

would be at the beginning of life now

assuming that you've gone through a few

races maybe you've have 50 to 100 cycles

under your belt

they were pretty high-intensity cycles

which means that the battery degradation

was worse than expected so therefore

your impedance growth say was about 50

percent then you potentially would have

a situation where now your thermal

limiting instead of 10 minutes is now

after 5 minutes so a pretty significant

decrease in performance so it's very

important to understand and quantify

these values so that when you are making

estimations about the ability of your

battery pack to perform over time you're

able to always give accurate estimations

so that's all for today's video we'll

see you next time thank you

[Music]

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