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How does a BMS (Battery Management System) work? | Passive & Active cell balancing Explained - YouTube
Channel: Foolish Engineer
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Hey guys, last time we saw why we need a BMS
and what it does?
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This time we will see
what is a BMS made of? and
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how does it work?
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Without wasting any time, buckle up and let's
go for a ride.
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A battery management system is made of many
hardware and software functional blocks which
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has:
cutoff MOSFETs,
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fuel gauge monitor
Cell voltage monitor / Cell voltage balancing
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circuit
real-time clock (RTC) and
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temperature monitor system
Now let鈥檚 take a look at the purpose and
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the technology behind each block
Cutoff FETs-
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These are basically transistors, mostly MOSFETs
are used, MOSFETs are nothing but controlled
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switches.
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These are used for connection and isolation
of the battery pack between the load and charger.
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MCU of the BMS measures the cell voltages,
current in real-time and based on that it
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Switches these FETs.These can be configured
in 2 types.
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Here it has a single connection to the Charger
and load in our case motor controller and
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Motor.
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2nd diagram has 2 different connections for
the Charger and Load.
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Generally 1st configuration is used in the
BMS system, because it reduces the hardware
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where just one bus controls the charging and
discharging.
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Initially both MOSFETs are turned off, when
a charger is connected, the current is not
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flowing because FETs are off.
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BMS senses voltage at the input and then it
turns on the CFET which charges the battery.
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If the voltage at the input pin is not present
then BMS determines that Motor is Connected
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then it turns on the DFET.
fuel gauge / current monitor-
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This block keeps the track of the charge coming
in and going out of the battery pack.
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Charge is the product of current and time.
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For measuring the current, a current sense
amplifier and a MCU which has an Analog to
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digital converter is used.
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A very low value current sense resistor is
connected in series with the battery line,
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voltage drop across this resistor is measured
by this amplifier then it amplifies the signal
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and delivers it to the ADC of the microcontroller.
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The microcontroller measures the voltage and
calculates the charge according to time.
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The discharging and charging current is sensed
by the direction of current, if the current
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is going out of the battery then battery is
discharging and if the current coming inside
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of the battery that means the battery is charging
or there are certain fuel gauge ICs are available
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in the market, which takes care of SOC and
SOH estimation of the battery.
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Many times using a fuel gauge IC adds additional
cost to the BMS design and when the load current
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is erratic as in Electric vehicle case, using
a shunt resistor is always the best option.
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This configuration also helps for overcurrent
protection, when current rises above safety
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level, the fuel gauge circuit senses it and
gives the signal to turn off the DFET or CFET.
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Cell voltage monitoring / cell balancing -
Monitoring the voltage of each cell is very
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important.
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Generally, Li-Ion batteries are used in EVs,
the voltage of a Li-ion cell ranges from 2.5V-
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4.2V and it depends on the chemistry.
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As we saw last time, operating the battery
outside the voltage range significantly reduces
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the lifetime of the cell.
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These cells are connected in series and parallel
combination to form a battery pack.
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When cells are connected in parallel combination
it increases the current of the battery pack,
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while a series connection increases the overall
voltage.
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Even if the cells are created by the same
manufacturer and in the same lot, physical
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properties such as internal resistance, the
nominal voltage and temperature imbalance
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will slightly differ from each other, while
charging and discharging not every cell behaves
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the same.
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While charging, If the battery pack has a
weaker cells than an average cell, this would
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result in the weakest cell to reach out it鈥檚
limit first and the rest of the cells are
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still charging, so this weaker cell heats
up and its lifespan decays,on the other hand,
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a weaker cell discharges faster than rest
of the cells.
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The weaker cell trips the discharge limit
first leaving the rest of the cells with some
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charge remaining inside of them.
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This can be overcome by cell balancing,
This cell balancing is derived into 2 parts
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Passive balancing
Active balancing
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Lets see the passive balancing -
It is the simplest and cost effective method.
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In this method a dummy load like a resistor
is used to discharge the excess voltage and
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equalize it with other cells.
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These resistors are known as bypass resistors.
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Each cell connected in series in a pack will
have its own bypass resistor connected through
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a switch.
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While charging the weaker cell charges very
fast so the MOSFET connected across the weaker
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cell is turned on, the charge of this cell
is removed and dissipated through this resistor,
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this process minimizes the charging rate of
the weaker cell, whenever charge level of
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the weak cell tends to go near to its full
capacity the MOSFET is turned on, so all cells
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along with weaker cell charges at the same
time but this method is not useful while discharging
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the weaker cell may reach to its minimum cut
off voltage earlier than the other cell
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This balancing technique is inexpensive and
technically easy to implement, but is not
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very efficient because electrical energy is
dissipated as heat in the resistors and FET
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also has switching losses.
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Another drawback is that the entire discharge
current flows through the MOSFET which is
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mostly built into the controller IC and hence
the discharge current has to be limited to
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lower value upto the ICs limits which increases
the discharging time.
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Active balancing
Unlike Passive balancing, in active balancing
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the excess charge from one cell is transferred
to another cell which has low charge to equalize
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them.
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So this can be done by charge storing components
like Capacitors and Inductors.
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Well, there are many methods to perform an
Active cell balancing.
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Let's discuss the method which is commonly
used.
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The Charge Shuttles / Flying Capacitors is
the most commonly used method in the active
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balancing, In this the capacitors are used
to transfer the charge from high voltage cell
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to low voltage cell.
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The capacitor is connected through SPDT switches,
initially the switch connects the capacitor
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to the cell which has high voltage and once
the capacitor is charged then the switch connects
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it to the cell which has lower voltage and
the charge from the capacitor flows into the
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cell.
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These capacitors are called the flying capacitors.
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Well, the problem with this method is that
the charge can be transferred only between
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adjacent cells.
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Also it takes more time because the capacitor
has to be charged and then discharged to transfer
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the charges, but still it is better than passive
balancing.
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Just like this there are many active cell
balancing techniques, which are out of the
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scope of this video (ref) (ref)
3.
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Next important parameter is Temperature Monitoring
- Li-ion batteries provide a lot of current
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while maintaining a constant voltage, which
can lead to a thermal runaway condition that
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causes the battery to catch fire.
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The construction of a battery is highly volatile.
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Well, this temperature measurement is not
just for safety, it can also suggest if the
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temperature of the battery is suitable for
charging or not.
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To measure the temperature of these batteries
a temperature sensor is used, generally the
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thermistors are used as a temperature sensor,
the thermistor is basically a temperature
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dependent resistance, whenever there is change
in temperature the resistance of the thermistor
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changes and BMS calculates the temperature
rise accordingly.
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As we saw in the last video, the BMS acts
as a logbook, to calculate the SOH and other
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parameters of the battery so for that purpose
the BMS has to take the data according to
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time, so it should be working even if the
vehicle is not on, but it might consume the
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excessive power from the battery pack itself,
so. to avoid that a Real time clock is integrated
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with the BMS which needs very small power,
and it does our job.
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(Ref)
Well that鈥檚 all about the BMS of an Electric
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vehicle, if you think there is something missing
please let me know in the comment section.
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Hit the like button if you liked this video,
subscribe to my channel and finally, thanks
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for watching.
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