Pre-Charge Circuits for Lithium-Ion Battery Packs

00:13:24
https://www.youtube.com/watch?v=dlGTonFXSrI

Resumen

TLDRThe video explains the critical role of pre-charge circuits in high voltage lithium-ion battery packs. It details how these circuits, which include a smaller disconnect switch and a resistor, help manage inrush current when connecting to systems with significant capacitance, such as motor inverters. The absence of a pre-charge circuit can lead to excessive inrush current, causing potential damage to components. The video also covers the RC time constant, which determines how quickly capacitors charge, and emphasizes the importance of selecting appropriate resistor values to manage heat dissipation. Additionally, it discusses the safety features of pre-charge circuits, including their ability to detect faults and prevent damage to the battery pack.

Para llevar

  • 🔋 Pre-charge circuits enhance battery safety.
  • ⚡ They limit inrush current during connection.
  • 🕒 RC time constant determines charging speed.
  • 🌡️ Resistor value affects heat generation.
  • 🚦 Fault detection improves system reliability.
  • 🔌 Essential for systems with high capacitance.
  • 💡 Avoids damage to battery components.
  • 📏 Typical resistor values can vary.
  • ⏳ Pre-charge timeout faults indicate issues.
  • 🔧 Proper design is crucial for longevity.

Cronología

  • 00:00:00 - 00:05:00

    The video discusses the importance of a pre-charge circuit in high voltage lithium-ion battery packs, which enhances safety and longevity. It explains that the pre-charge circuit, consisting of a smaller disconnect switch and a resistor, is necessary to manage inrush current when connecting to external systems with significant capacitance, such as motor inverters. Without a pre-charge circuit, closing the main disconnect switch can lead to dangerously high inrush currents, potentially causing damage to components.

  • 00:05:00 - 00:13:24

    The video further explains how the pre-charge circuit operates by first closing the pre-charge switch, allowing current to flow through a resistor to charge the capacitor gradually, thus limiting the inrush current. It introduces the concept of the RC time constant, which determines how quickly the capacitor charges. The presenter provides calculations for different resistor values and their impact on charging time, emphasizing the need for appropriate resistor ratings to handle power dissipation and the importance of monitoring for faults in the system.

Mapa mental

Vídeo de preguntas y respuestas

  • What is a pre-charge circuit?

    A pre-charge circuit is a safety feature in high voltage battery packs that limits inrush current when connecting to external systems.

  • Why is a pre-charge circuit necessary?

    It prevents excessive inrush current that can damage components and ensures safe operation of the battery pack.

  • How does a pre-charge circuit work?

    It uses a resistor to limit current flow when charging capacitors, allowing for a controlled voltage rise.

  • What is the RC time constant?

    The RC time constant is the time it takes for a capacitor to charge to approximately 63% of the battery voltage.

  • What happens if there is no pre-charge circuit?

    Without a pre-charge circuit, closing the main switch can cause a large inrush current, leading to potential damage.

  • How can a pre-charge circuit detect faults?

    It can detect faults by monitoring the voltage rise across the capacitor and raising a fault flag if the expected behavior is not observed.

  • What resistor value should be used in a pre-charge circuit?

    The resistor value affects the charging time; lower values reduce time but increase heat generation.

  • What is the maximum power dissipation in a resistor during pre-charge?

    Power dissipation is calculated as V squared over R, which can be significant at the start of the pre-charge cycle.

  • What is a pre-charge timeout fault?

    It indicates a fault if the capacitor does not charge to the expected level within a specified time.

  • What are the safety benefits of a pre-charge circuit?

    It helps prevent damage from inrush currents and allows for fault detection, enhancing overall safety.

Ver más resúmenes de vídeos

Obtén acceso instantáneo a resúmenes gratuitos de vídeos de YouTube gracias a la IA.
Subtítulos
en
Desplazamiento automático:
  • 00:00:00
    foreign
  • 00:00:03
    [Music]
  • 00:00:09
    systems insights video series I'm Eric
  • 00:00:11
    stoffel president of stoffel systems
  • 00:00:14
    today's topic is pre-charge as used in a
  • 00:00:17
    Lithium-Ion battery pack
  • 00:00:22
    it is very important to have a pre-chart
  • 00:00:24
    circuit
  • 00:00:25
    in any high voltage battery pack
  • 00:00:29
    it greatly increases the safety of the
  • 00:00:31
    battery pack and the longevity of the
  • 00:00:33
    battery and the external components
  • 00:00:35
    connected to the battery let me show you
  • 00:00:37
    how
  • 00:00:39
    so a simplified schematic of a high
  • 00:00:42
    voltage lithium-ion battery pack
  • 00:00:45
    consists of a stack of cells
  • 00:00:49
    a main disconnect switch
  • 00:00:52
    and external terminals
  • 00:00:56
    where does the pre-chart circuit live
  • 00:00:58
    it lives in parallel with the main
  • 00:01:01
    disconnect switch
  • 00:01:03
    fundamentally
  • 00:01:05
    it consists of
  • 00:01:07
    a smaller disconnect switch
  • 00:01:10
    and a resistor
  • 00:01:13
    sometimes it fuse as well but I have
  • 00:01:15
    omitted that for Simplicity
  • 00:01:18
    so why is a pre-charge circuit necessary
  • 00:01:20
    in the first place
  • 00:01:22
    well typically
  • 00:01:24
    it's because the external system
  • 00:01:26
    connected to a Lithium-Ion battery pack
  • 00:01:30
    has a rather large or significant
  • 00:01:33
    capacitance
  • 00:01:34
    C
  • 00:01:36
    this can be a three-phase motor inverter
  • 00:01:38
    this can be a grid tie system for
  • 00:01:42
    charging a discharging there's a variety
  • 00:01:44
    of Power Electronics and typically they
  • 00:01:46
    have a filter capacitance for their high
  • 00:01:48
    frequency switching
  • 00:01:50
    and this capacitance makes it absolutely
  • 00:01:53
    necessary to have a pre-chart circuit
  • 00:01:55
    so what let's start with the case if we
  • 00:01:57
    did not have a pre-chart circuit what
  • 00:01:59
    would occur
  • 00:02:01
    well when the battery
  • 00:02:04
    is off
  • 00:02:06
    all the switches are open and say that
  • 00:02:08
    we're sitting at a voltage of 400 volts
  • 00:02:12
    and the system has been off for a while
  • 00:02:14
    so the voltage of the capacitor is zero
  • 00:02:17
    volts
  • 00:02:18
    so effectively in this situation there
  • 00:02:22
    are 400 volts
  • 00:02:23
    across this disconnect switch
  • 00:02:27
    and if you did not have a pre-charge
  • 00:02:29
    system
  • 00:02:30
    and you close this switch in this
  • 00:02:32
    condition
  • 00:02:33
    you would have a very large what's
  • 00:02:35
    called inrush current
  • 00:02:37
    because the fully charged battery would
  • 00:02:40
    be discharging into
  • 00:02:42
    the fully discharged capacitor
  • 00:02:45
    and you can have very very high levels
  • 00:02:47
    of current with this in Rush upwards of
  • 00:02:49
    thousands of amps
  • 00:02:51
    and that can lead to all sorts of
  • 00:02:53
    problems including
  • 00:02:55
    sparking type failures for the main
  • 00:02:57
    contactor here
  • 00:02:59
    so it could weld shut for example
  • 00:03:02
    or it could lead to excess heating or
  • 00:03:04
    damage to the capacitor Bank itself
  • 00:03:08
    so we want to avoid all that
  • 00:03:10
    and the pre-charge circuit is the best
  • 00:03:12
    way to do that
  • 00:03:13
    so let's look at the alternative case
  • 00:03:15
    where now we have a pre-chart circuit
  • 00:03:17
    that helps us avoid this inrush current
  • 00:03:20
    how does that work
  • 00:03:22
    well instead of just closing this main
  • 00:03:24
    disconnect contactor right away what we
  • 00:03:28
    do is first close this pre-charge switch
  • 00:03:30
    so that the current flow goes through
  • 00:03:33
    the battery
  • 00:03:34
    cross the switch through the resistor
  • 00:03:37
    and then to the capacitor
  • 00:03:39
    so now instead of having a very very
  • 00:03:42
    large amount of current flowing our
  • 00:03:44
    current is limited by the resistance of
  • 00:03:45
    this resistor
  • 00:03:47
    so how does that work
  • 00:03:53
    so let's give an example of the case
  • 00:03:56
    where we have a battery voltage
  • 00:04:00
    of 400 volts
  • 00:04:02
    and a resistor
  • 00:04:05
    of a thousand ohms foreign
  • 00:04:09
    typical values you might see in a
  • 00:04:12
    pre-chart system
  • 00:04:13
    remember the amount of current flowing
  • 00:04:16
    through a circuit
  • 00:04:17
    is the voltage divided by
  • 00:04:20
    the resistance
  • 00:04:22
    Ohm's law
  • 00:04:23
    so in this case we would have 400 volts
  • 00:04:27
    divided by
  • 00:04:29
    1000 ohms
  • 00:04:32
    so we would have 400 milliamps
  • 00:04:36
    of current flow
  • 00:04:39
    that is a far more gentle situation than
  • 00:04:43
    thousands of amps in an uncontrolled
  • 00:04:45
    inrush
  • 00:04:46
    so already we're doing well
  • 00:04:49
    but what does it look like after we
  • 00:04:51
    close that switch what happens then
  • 00:04:54
    [Music]
  • 00:04:55
    so if we go on over here and we plot
  • 00:05:00
    the voltage
  • 00:05:02
    of the capacitor
  • 00:05:05
    I'll use blue for this
  • 00:05:10
    eventually
  • 00:05:11
    we want to charge it up to the same
  • 00:05:13
    battery voltage so that we have no
  • 00:05:16
    current flow through this circuit
  • 00:05:19
    and the way that works
  • 00:05:20
    is that
  • 00:05:22
    T equals zero
  • 00:05:24
    we close the pre-charge circuit switch
  • 00:05:27
    and we start charging the capacitor
  • 00:05:30
    and it follows an RC time constant
  • 00:05:33
    resistor capacitor time constant
  • 00:05:36
    until it gets to a close enough bound
  • 00:05:38
    with the battery voltage that we can now
  • 00:05:41
    close the main disconnect relay or
  • 00:05:45
    contactor and have minimal inrush
  • 00:05:48
    current and minimal sparking and minimal
  • 00:05:50
    thermal and high current flow damage
  • 00:05:53
    issues that we mentioned before
  • 00:05:56
    so how long does this curve need to
  • 00:06:00
    charge how long does this capacitor need
  • 00:06:02
    to charge for us to feel comfortable
  • 00:06:04
    closing the main contactor switch
  • 00:06:07
    while RC
  • 00:06:09
    curves
  • 00:06:11
    are dictated by their time constants or
  • 00:06:14
    Tau in a time constant is determined by
  • 00:06:20
    the resistance times the capacitance so
  • 00:06:23
    this is called an RC time constant
  • 00:06:26
    one towel or one time constant is
  • 00:06:30
    approximately 63 percent of the battery
  • 00:06:33
    voltage two time constants is
  • 00:06:35
    approximately 87 and then three time
  • 00:06:39
    constants is approximately 95 and on and
  • 00:06:42
    on and on it gets closer asymptotically
  • 00:06:44
    to the battery voltage
  • 00:06:46
    so using our example again
  • 00:06:49
    say that we have a capacitance
  • 00:06:55
    of 400
  • 00:06:57
    microfarads
  • 00:06:59
    this is a fairly typical capacitance you
  • 00:07:01
    might see an electric vehicle inverter
  • 00:07:04
    so if we have a thousand Ohm resistor
  • 00:07:07
    and 400 microfarads of capacitance
  • 00:07:11
    so we'll multiply
  • 00:07:13
    400 microfarads
  • 00:07:15
    times 1000 ohms
  • 00:07:21
    what is the time for that
  • 00:07:23
    well a thousand times 400 micro
  • 00:07:26
    that is 400 milliseconds or four tenths
  • 00:07:29
    of a second
  • 00:07:31
    so in this situation
  • 00:07:33
    one time constant of this pre-charged
  • 00:07:36
    circuit
  • 00:07:38
    would be 400 milliseconds two time
  • 00:07:40
    constants would be 800 milliseconds and
  • 00:07:42
    three time constants would be 1200
  • 00:07:44
    milliseconds or 1.2 seconds
  • 00:07:47
    so in this situation we would expect
  • 00:07:50
    that in 1.2 seconds our voltage would be
  • 00:07:54
    within five percent of the battery
  • 00:07:57
    voltage so in this case 95 percent of
  • 00:08:00
    the total battery voltage
  • 00:08:03
    and generally speaking that is
  • 00:08:04
    considered enough to close the main
  • 00:08:08
    contactor for a system
  • 00:08:10
    some systems you might want to select
  • 00:08:12
    the Threshold at 90 percent or 97
  • 00:08:14
    percent there's some Advanced
  • 00:08:16
    calculations there but generally
  • 00:08:17
    speaking three time constants or 95
  • 00:08:19
    percent is considered sufficient to
  • 00:08:22
    close your main switch
  • 00:08:25
    okay
  • 00:08:27
    so now that we've determined all this
  • 00:08:30
    we have selected a resistor of a
  • 00:08:33
    thousand ohms because of this time as
  • 00:08:36
    you can imagine if we want the
  • 00:08:38
    pre-charge time to be reduced we have to
  • 00:08:40
    select a smaller resistance value so for
  • 00:08:43
    example if we wanted to select 500 ohms
  • 00:08:45
    instead of a thousand ohms that would
  • 00:08:47
    take our time constant from 400
  • 00:08:50
    milliseconds to 200 milliseconds
  • 00:08:53
    and likewise if we have an application
  • 00:08:55
    that could accommodate a larger
  • 00:08:56
    pre-charge time we could choose a higher
  • 00:08:58
    resistance value
  • 00:08:59
    now you might ask why not just choose a
  • 00:09:02
    very small resistance value
  • 00:09:03
    well the challenge is the thermal
  • 00:09:06
    limitation
  • 00:09:08
    so the heat generated in a resistor or
  • 00:09:12
    power dissipation of that resistor
  • 00:09:15
    is calculated by V squared over r
  • 00:09:19
    so if we have a 400 volt battery system
  • 00:09:24
    and a thousand Ohm resistor
  • 00:09:26
    the amount of power dissipation we would
  • 00:09:29
    see at the beginning
  • 00:09:31
    of this pre-charge cycle
  • 00:09:33
    would be 400 volts
  • 00:09:37
    squared
  • 00:09:39
    over
  • 00:09:41
    1000 ohms
  • 00:09:44
    and that would be equal to
  • 00:09:47
    160 Watts
  • 00:09:49
    now that is a very
  • 00:09:52
    high power dissipation
  • 00:09:54
    through a resistor not just the normal
  • 00:09:56
    board Mount resistor would work for this
  • 00:09:59
    so best practices to use a dedicated
  • 00:10:01
    pre-charge rated or high pulse rated
  • 00:10:04
    resistor or a chassis Mount resistor
  • 00:10:07
    that can have significant overage power
  • 00:10:09
    dissipation without being damaged
  • 00:10:12
    now the good news is the heat generated
  • 00:10:15
    as you can imagine is only maximized at
  • 00:10:18
    this very first point as the voltage
  • 00:10:20
    Delta between
  • 00:10:24
    the battery and the capacitor
  • 00:10:27
    decreases so does your current flow and
  • 00:10:31
    so does your heat generation
  • 00:10:33
    so if I were to plot current flow in
  • 00:10:36
    this color
  • 00:10:37
    we would see a pulse of current flow
  • 00:10:38
    here
  • 00:10:40
    and then it would decay
  • 00:10:42
    according to a very similar RC curve
  • 00:10:44
    down to essentially zero
  • 00:10:49
    so the heat generation is the multiplied
  • 00:10:51
    amount by the voltage Delta and the
  • 00:10:55
    amount of current flow
  • 00:10:57
    so
  • 00:10:59
    a couple other things to think about
  • 00:11:00
    with pre-charged design
  • 00:11:03
    you want to enable
  • 00:11:05
    some checking in a fault system of your
  • 00:11:07
    BMS or battery management system to
  • 00:11:10
    ensure that everything's working as
  • 00:11:11
    expected
  • 00:11:12
    so one case to consider
  • 00:11:15
    is the case where there is now actually
  • 00:11:19
    a short circuit
  • 00:11:22
    across this capacitor or some sort of
  • 00:11:24
    other fault with this external system
  • 00:11:27
    this actually can be a real occurrence
  • 00:11:30
    if the polarity of the system is flipped
  • 00:11:32
    so for example if someone accidentally
  • 00:11:34
    connects the negative terminal to the
  • 00:11:36
    positive terminal and vice versa
  • 00:11:38
    then you can essentially have a short
  • 00:11:40
    circuit from positive to negative
  • 00:11:42
    and what's helpful about a pre-chart
  • 00:11:44
    circuit is it will actually allow you to
  • 00:11:46
    detect that when properly coupled with a
  • 00:11:48
    BMS you can actually raise a fault flag
  • 00:11:50
    to indicate that something's wrong like
  • 00:11:52
    that so for example if in this scenario
  • 00:11:56
    we had a short circuit instead of the
  • 00:12:00
    current flow going like this and the
  • 00:12:01
    voltage going Rising like this you would
  • 00:12:04
    expect to see a current flow staying
  • 00:12:07
    flat
  • 00:12:08
    and then your voltage level
  • 00:12:11
    would also stay flat or relatively flat
  • 00:12:15
    and that would indicate to the BMS that
  • 00:12:17
    something's wrong because the voltage is
  • 00:12:19
    not rising in the manner that's expected
  • 00:12:23
    and that alone can yield numerous safety
  • 00:12:25
    benefits because now the battery pack
  • 00:12:28
    will not close its main High current
  • 00:12:30
    capable switch
  • 00:12:31
    unless it has assessed that the output
  • 00:12:35
    capacitor or the capacitor connected to
  • 00:12:38
    the output of the battery pack is rising
  • 00:12:40
    at the level expected
  • 00:12:42
    so you can detect short circuits you can
  • 00:12:45
    also detect situations where the
  • 00:12:46
    capacitance is significantly different
  • 00:12:48
    than you would expect
  • 00:12:50
    so you can have what's called a
  • 00:12:51
    pre-charged timeout fault so that if
  • 00:12:54
    this does not occur in the expected 1.2
  • 00:12:56
    seconds perhaps if it takes more than
  • 00:12:58
    two seconds to get above 95 percent the
  • 00:13:01
    BMS would raise a fault flag and say
  • 00:13:03
    something's wrong with the system
  • 00:13:05
    there are a number of advanced topics
  • 00:13:07
    with the pre-charge system but these are
  • 00:13:09
    the basics to consider when reviewing a
  • 00:13:11
    pre-charged design
  • 00:13:13
    thanks for watching see you next time
  • 00:13:17
    [Music]
  • 00:13:19
    thank you
  • 00:13:20
    [Music]
Etiquetas
  • pre-charge circuit
  • lithium-ion battery
  • inrush current
  • safety
  • RC time constant
  • resistor
  • fault detection
  • battery management system
  • capacitor
  • high voltage