Calculating the State of Charge of a Lithium Ion Battery System using a Battery Management System
Summary
TLDRIn this Stifle Systems Insights video, Eric Staal discusses the concept of State of Charge (SOC) in battery packs, emphasizing its definition as the remaining capacity relative to the total capacity in amp-hours. He illustrates the difference between SOC based on capacity and SOC based on energy, highlighting the importance of understanding the energy available for applications like electric vehicles. Staal also explains the method of Coulomb counting for calculating SOC, the challenges of current sensor drift, and the role of open circuit voltage (OCV) lookup in ensuring accurate SOC estimation for reliable battery performance.
Takeaways
- 🔋 The state of charge (SOC) is defined as the remaining capacity in amp-hours or coulombs that can be discharged over the total capacity of the battery pack.
- 📉 SOC is measured in units of amp-hours, not energy, which is an important distinction because the voltage of a battery changes as it discharges.
- 📊 The discharge curve for lithium-ion batteries typically shows a varying downward slope, with higher voltage in the initial stages and lower voltage towards the end of discharge.
- 🚗 In electric vehicle applications, it's more useful to consider the energy available (SOC II) rather than just the capacity remaining, as it provides a more accurate fuel gauge algorithm.
- 🔍 The 50% SOC point on the discharge curve does not necessarily correspond to 50% of the energy remaining; it's where 50% of the amp-hours have been discharged.
- 🔄 Coulomb counting is the primary method for calculating SOC, which involves integrating the current over time to estimate the amp-hours discharged.
- 🛠 Current sensors used in Coulomb counting can have drift and integration errors, which is why open circuit voltage (OCV) lookup tables are used to correct and recalibrate the SOC estimation.
- 🔄 Depth of discharge (DOD) is the inverse of SOC and represents the percentage of the battery's capacity that has been used.
- 🔄 The BMS uses OCV lookup to determine the SOC when the battery is at rest, ensuring an accurate starting point for Coulomb counting and reliable SOC estimation.
- ⚙️ Accurate SOC and SOC II algorithms are crucial for the reliable and predictable operation of battery packs, preventing issues like sudden drops in estimated charge.
Q & A
What is the state of charge (SOC) of a battery pack?
-The state of charge (SOC) is defined as the capacity remaining over the total capacity in amp-hours or coulombs that you can discharge over the total capacity of the battery pack.
How is the SOC percentage calculated?
-The SOC percentage is calculated by dividing the remaining capacity by the total capacity of the battery pack. For example, if a battery pack has a total capacity of 100 amp-hours and 70 amp-hours are left, the SOC is 70%.
Why is it important to distinguish between SOC and energy units?
-It's important because SOC is measured in amp-hours, which is a measure of capacity, whereas energy is measured in watt-hours. This distinction is crucial for applications like electric vehicles where the available energy, not just capacity, determines the range.
What is the significance of the discharge curve for lithium-ion batteries?
-The discharge curve for lithium-ion batteries typically shows a varying downward slope in voltage as capacity is discharged. This curve is significant because it illustrates the relationship between voltage, capacity, and energy, which is essential for understanding battery performance.
What is the difference between SOC based on capacity (SOC I) and SOC based on energy (SOC II)?
-SOC I is based on the remaining capacity in amp-hours, while SOC II is based on the available energy. SOC II provides a more accurate representation of the expected runtime or range, as it accounts for the varying energy output at different states of charge.
Why is Coulomb counting a primary method for calculating SOC?
-Coulomb counting is a primary method for calculating SOC because it involves integrating the current over time to estimate the amp-hours discharged, providing a direct measure of the battery's state of charge.
What challenges does Coulomb counting face in accurately estimating SOC?
-Coulomb counting faces challenges such as current sensor drift and integration error, which can lead to inaccuracies in the estimation of discharged amp-hours. To mitigate this, additional methods like open circuit voltage (OCV) lookup are often used.
What is open circuit voltage (OCV) lookup and how does it help in SOC estimation?
-OCV lookup is a method where the BMS compares the actual voltage of the battery at rest with a lookup table to determine the corresponding state of charge or depth of discharge. This helps in recalibrating the SOC estimation, ensuring accuracy and reliability.
What is the relationship between depth of discharge (DOD) and SOC?
-Depth of discharge (DOD) is the inverse of SOC. While SOC represents the remaining capacity, DOD represents the amount of capacity that has been used. For example, at 70% SOC, the DOD would be 30%.
Why is it crucial to have an accurate SOC estimation in applications like electric vehicles?
-In electric vehicles, an accurate SOC estimation is crucial for predicting the remaining range and ensuring reliable operation. Inaccurate SOC estimation can lead to unexpected battery depletion, causing inconvenience and potential stranding of the vehicle.
Outlines
🔋 Understanding State of Charge (SOC) and Capacity
In this segment, Eric Staal, the president of Stoffels Systems, introduces the concept of State of Charge (SOC) in battery packs. SOC is defined as the remaining capacity of a battery pack expressed as a percentage of its total capacity. Using a 100 amp-hour battery pack as an example, if 70 amp-hours are left to discharge, the SOC is 70%. It's emphasized that SOC is measured in amp-hours, not energy, which is a critical distinction. The video also explains the voltage discharge curve of lithium-ion batteries, highlighting how voltage decreases as capacity is discharged. The importance of differentiating between energy and capacity is discussed, especially in applications like electric vehicles where energy available is more relevant than raw capacity. The concept of SOC II, which is energy-based, is introduced as a more accurate representation for fuel gauge algorithms.
🔬 Calculating SOC: Coulomb Counting and Open Circuit Voltage (OCV) Lookup
The second paragraph delves into how SOC is calculated, focusing on Coulomb counting as the primary method. Coulomb counting involves integrating the current over time to determine the amp-hours discharged, which is essential for calculating SOC. The discussion points out the challenges of current sensor drift and integration error, which can affect the accuracy of SOC estimation. To mitigate these issues, the Battery Management System (BMS) uses an open circuit voltage (OCV) lookup. This lookup compares the integrated current data with the actual cell voltage to recalibrate the SOC. The concept of depth of discharge (DOD) is also introduced as the inverse of SOC, and its importance in recalibrating the SOC after the battery has been at rest is explained. The video concludes by emphasizing the importance of accurate SOC and SOC II calculations for reliable battery pack operation, particularly in applications like electric vehicles where inaccurate readings could lead to stranded vehicles and safety concerns.
Mindmap
Keywords
💡State of Charge (SOC)
💡Battery Pack
💡Capacity
💡Coulomb Counting
💡Discharge Curve
💡Energy-Based SOC (SOC II)
💡Open Circuit Voltage (OCV)
💡Depth of Discharge (DOD)
💡Fuel Gauge Algorithm
💡Battery Management System (BMS)
Highlights
Definition of State of Charge (SOC) as the remaining capacity relative to total capacity in amp-hours or coulombs.
Example illustrating SOC calculation with a 100 amp-hour battery pack.
Clarification that SOC is measured in amp-hours, not energy units.
Explanation of the typical discharge curve for lithium-ion batteries.
Importance of understanding the varying voltage slope during battery discharge.
Misinterpretation risks when conflating SOC with fuel gauge algorithms.
Introduction of SOC II, which is based on energy rather than capacity.
Practical example of calculating SOC II based on the area under the discharge curve.
Differentiation between energy-based SOC (SOC II) and capacity-based SOC.
Discussion on the importance of SOC II for accurate fuel gauge algorithms in applications like electric vehicles.
Introduction to Coulomb counting as the primary method for calculating SOC.
Description of how a BMS uses current sensing to perform Coulomb counting.
Challenges with current sensor drift and integration error in Coulomb counting.
Use of open circuit voltage (OCV) lookup for accurate SOC estimation.
Explanation of depth of discharge (DOD) as the inverse of SOC.
Process of recalibrating SOC using OCV lookup after the battery has been at rest.
The necessity of accurate SOC algorithms for reliable battery pack operation.
Transcripts
[Music]
welcome to the stifle systems insights
video series I'm Eric Staal president of
Stoffels systems the topic of today's
video is the state of charge of a
battery pack as estimated by a BMS so
what is the state of charge or SOC so
it's very simple the state of charge is
defined as the capacity remaining so
that total capacity in amp hours or
coulombs that you can discharge over the
total capacity of the battery pack so
let's give an example
so if we had a battery pack that was say
100 amp hour battery pack total capacity
and we had 70 amp hours left to
discharge that would give us a state of
charge of 70% so this would mean that if
we fully charge the battery pack up to
the 400 amp hours and then discharge 30
amp hours we would have 70 amp hours or
70 percent of the capacity remaining now
it's very important to note that this is
in units of amp hours not in units of
energy and why is that important well if
we look at the discharge curve for a
lithium-ion battery cell or a battery
pack for that matter with voltage here
on the exit y-axis and on the x-axis we
have amp hour discharged
what does the curve typically look like
so for most lithium-ion batteries for
example like an MMC or lco type
chemistry you would expect to see a
curve like this and it falls off towards
the end and so typically this is about
4.2 volts per cell and the end of
discharge is 2.5 volts per cell this
could be a little different for
different chemistry's but the point is
the same in general you have a varying
downward facing slope for the voltage as
you discharge capacity out of the pack
so for example if I took the 50% point
for the amp hours discharged so if we
took the example above and said that
this was a hundred amp hours or 100% and
this was 50 amp hours this is zero then
this would be the 50% state of charge
point which means that we've discharged
50 amp hours out of a hundred amp hours
and we have 50 amp hours left to go
before the battery reads reaches its
termination voltage so one thing to
notice is that look at the areas under
these relative curves one side is bigger
than the other so for example there's a
lot more energy on the left side of this
line than on the right side of the line
and why is that important because when
you confuse state of charge with a fuel
gage algorithm this happens which is
typically if you want to get a fuel gage
if you want to use a fuel gage for
example from electric vehicle
application or most applications you're
more interested in the energy available
as opposed to the capacity so for
example if we were doing an electric
vehicle design and we were trying to
determine okay this is a 200 mile range
at what state of charge would you have a
hundred miles of range remaining well
look at this the balance between this
side and this side is clearly imbalanced
because the voltage is higher in the
lower than the higher states of charge
and the voltage is lower in the lower
state to charge so it's important to
introduce the concept of soe II or SOC
based on energy so we denote this as
follows
I use the black pen for this so SOC c4
capacity or SOC II for energy and these
are different and this is actually what
most applications are interested in
because this is actually the more
accurate fuel gauge algorithm that tells
you how much expected runtime use
distance range you ever made so let's
look at look at this example again if we
say okay where is the actual 50% SOC II
point on this well that would be where
we would have approximately 50% of the
area under the curve on the left side of
the line as on the right side that would
be somewhere more like here say that
this corresponds to a point of 42% SOC
see but this equals 50% SOC II so it's
very important to understand the
distinction between energy state to
charge fuel gauge algorithms and
capacity states of charge in a future
video we'll discuss how the state of
charge is actually calculated it
typically has a number of more
sophisticated elements but for the
purposes of today I do want to discuss
Coulomb counting which is the primary
way that state of charge is calculated
so as I mentioned earlier in this
example we have a hundred amp hours and
we discharged 30 amp hours to have 70
amp hours remaining which means we're at
a c2 charge of 70% but how did we
determine that we discharged 30 amp
hours well from the first video we can
remember that a BMS typically has a
current sensor either a shunt or a Hall
effect device that can monitor the
current flowing in or out of the battery
pack and what are you doing to determine
amp hours since amp hours are in units
of current times time we are actually
performing an integration
called Coulomb counting so this is your
current and this is time so say that we
have a curve that looks like this
that is how much charge that's how much
current at any given time is coming out
of the pack the area under the curve
corresponds to the actual capacity
removed so this is in units of amp hours
and this is what Coulomb counting does
Coulomb counting is basically looking at
every single slice in a time slice
integration fashion multiplying the
current times the time interval and
summing that up to get an approximation
of the integral of this function and
what that does is that gives us an
accurate estimate of amp hours which
gives us a basis for SoC
now one of the things to note about
Coulomb counting and current sensing is
that the current sensor has drift and
integration error so you're not going to
get perfect alignment of all your
sensing with the actual current spikes
itself so it's important to note that
oftentimes you also need what's called a
ocv or open cell voltage lookup to
compare what you're integrating with
your actual voltage so we'll look over
here I'll draw on this plot voltage
times I'm gonna introduce a new term
called depth of discharge depth of
discharge is the inverse of state of
charge so for example at 70 percent SOC
you would have a depth of discharge of
30% so for example 30% SOC right depth
of discharge 60% depth of discharge and
say this is a hundred percent your
voltage is gonna go like this so when
the PAC has been at rest for a
considerable period of time what you do
is the BMS will look up with a lookup
table or something similar to see what
the open circuit voltage of the cell is
for a given temperature
and then it will equate that or
determine what the corresponding state
of charge or depth of discharge is for
that and then it will re cede the soc
function so that you have a basis upon
which to get an accurate understanding
of where you need to start Coulomb
counting again and this is very
important because you don't want to have
a fuel gauge algorithm that gets off so
you can imagine how frustrating it would
be if you had say you're driving along
and all of a sudden you went from 30% to
0% state of charge immediately because
there is an inaccurate estimation it
would leave you stranded at least
anxiety all sorts of problems like that
so the benefit of having both open
circuit voltage lookup and accurate
Coulomb counting is that you can
actually ensure a high degree of
accuracy for the state of charge
algorithm and the state of charge energy
algorithm such that your results are
expected in a reliable and predictable
operation of your battery pack that's
all for today thanks for watching see
you next time
[Music]
you
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