Calculating the State of Health for a Lithium Ion Battery System
Summary
TLDRIn 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
🔋 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.
📉 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.
Mindmap
Keywords
💡State of Health (SOH)
💡Capacity Fade
💡Equivalent Series Resistance (ESR)
💡Battery Management System (BMS)
💡Impedance Growth
💡Cycle Life
💡Beginning of Life (BOL)
💡Thermal Output
💡State of Charge (SOC)
💡Estimated Range Remaining
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]
you
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