(00:01) Hey guys, last time we saw why we need a BMS and what it does? This time we will see what is a BMS made of? and how does it work? Without wasting any time, buckle up and let’s go for a ride. A battery management system is made of many hardware and software functional blocks which has: cutoff MOSFETs, fuel gauge monitor Cell voltage monitor / Cell voltage balancing circuit real-time clock (RTC) and temperature monitor system Now let’s take a look at the purpose and the technology behind each block Cutoff FETs-
(00:56) These are basically transistors, mostly MOSFETs are used, MOSFETs are nothing but controlled switches. These are used for connection and isolation of the battery pack between the load and charger. MCU of the BMS measures the cell voltages, current in real-time and based on that it Switches these FETs.These can be configured in 2 types.
(01:24) Here it has a single connection to the Charger and load in our case motor controller and Motor. 2nd diagram has 2 different connections for the Charger and Load. Generally 1st configuration is used in the BMS system, because it reduces the hardware where just one bus controls the charging and discharging.
(01:55) Initially both MOSFETs are turned off, when a charger is connected, the current is not flowing because FETs are off. BMS senses voltage at the input and then it turns on the CFET which charges the battery. If the voltage at the input pin is not present then BMS determines that Motor is Connected then it turns on the DFET.
(02:26) fuel gauge / current monitor- This block keeps the track of the charge coming in and going out of the battery pack. Charge is the product of current and time. For measuring the current, a current sense amplifier and a MCU which has an Analog to digital converter is used. A very low value current sense resistor is connected in series with the battery line, voltage drop across this resistor is measured by this amplifier then it amplifies the signal and delivers it to the ADC of the microcontroller.
(03:00) The microcontroller measures the voltage and calculates the charge according to time. The discharging and charging current is sensed by the direction of current, if the current is going out of the battery then battery is discharging and if the current coming inside of the battery that means the battery is charging or there are certain fuel gauge ICs are available in the market, which takes care of SOC and SOH estimation of the battery.
(03:30) Many times using a fuel gauge IC adds additional cost to the BMS design and when the load current is erratic as in Electric vehicle case, using a shunt resistor is always the best option. This configuration also helps for overcurrent protection, when current rises above safety level, the fuel gauge circuit senses it and gives the signal to turn off the DFET or CFET.
(03:59) Cell voltage monitoring / cell balancing - Monitoring the voltage of each cell is very important. Generally, Li-Ion batteries are used in EVs, the voltage of a Li-ion cell ranges from 2.5V- 4.2V and it depends on the chemistry. As we saw last time, operating the battery outside the voltage range significantly reduces the lifetime of the cell.
(04:26) These cells are connected in series and parallel combination to form a battery pack. When cells are connected in parallel combination it increases the current of the battery pack, while a series connection increases the overall voltage. Even if the cells are created by the same manufacturer and in the same lot, physical properties such as internal resistance, the nominal voltage and temperature imbalance will slightly differ from each other, while charging and discharging not every cell behaves the same.
(04:47) While charging, If the battery pack has a weaker cells than an average cell, this would result in the weakest cell to reach out it’s limit first and the rest of the cells are still charging, so this weaker cell heats up and its lifespan decays,on the other hand, a weaker cell discharges faster than rest of the cells.
(05:11) The weaker cell trips the discharge limit first leaving the rest of the cells with some charge remaining inside of them. This can be overcome by cell balancing, This cell balancing is derived into 2 parts Passive balancing Active balancing Lets see the passive balancing - It is the simplest and cost effective method.
(05:37) In this method a dummy load like a resistor is used to discharge the excess voltage and equalize it with other cells. These resistors are known as bypass resistors. Each cell connected in series in a pack will have its own bypass resistor connected through a switch. While charging the weaker cell charges very fast so the MOSFET connected across the weaker cell is turned on, the charge of this cell is removed and dissipated through this resistor, this process minimizes the charging rate of the weaker cell, whenever charge level of
(06:15) the weak cell tends to go near to its full capacity the MOSFET is turned on, so all cells along with weaker cell charges at the same time but this method is not useful while discharging the weaker cell may reach to its minimum cut off voltage earlier than the other cell This balancing technique is inexpensive and technically easy to implement, but is not very efficient because electrical energy is dissipated as heat in the resistors and FET also has switching losses.
(06:52) Another drawback is that the entire discharge current flows through the MOSFET which is mostly built into the controller IC and hence the discharge current has to be limited to lower value upto the ICs limits which increases the discharging time. Active balancing Unlike Passive balancing, in active balancing the excess charge from one cell is transferred to another cell which has low charge to equalize them.
(07:33) So this can be done by charge storing components like Capacitors and Inductors. Well, there are many methods to perform an Active cell balancing. Let’s discuss the method which is commonly used. The Charge Shuttles / Flying Capacitors is the most commonly used method in the active balancing, In this the capacitors are used to transfer the charge from high voltage cell to low voltage cell.
(08:00) The capacitor is connected through SPDT switches, initially the switch connects the capacitor to the cell which has high voltage and once the capacitor is charged then the switch connects it to the cell which has lower voltage and the charge from the capacitor flows into the cell. These capacitors are called the flying capacitors.
(08:25) Well, the problem with this method is that the charge can be transferred only between adjacent cells. Also it takes more time because the capacitor has to be charged and then discharged to transfer the charges, but still it is better than passive balancing. Just like this there are many active cell balancing techniques, which are out of the scope of this video (ref) (ref) 3.
(08:46) Next important parameter is Temperature Monitoring - Li-ion batteries provide a lot of current while maintaining a constant voltage, which can lead to a thermal runaway condition that causes the battery to catch fire. The construction of a battery is highly volatile. Well, this temperature measurement is not just for safety, it can also suggest if the temperature of the battery is suitable for charging or not.
(09:16) To measure the temperature of these batteries a temperature sensor is used, generally the thermistors are used as a temperature sensor, the thermistor is basically a temperature dependent resistance, whenever there is change in temperature the resistance of the thermistor changes and BMS calculates the temperature rise accordingly.
(09:39) As we saw in the last video, the BMS acts as a logbook, to calculate the SOH and other parameters of the battery so for that purpose the BMS has to take the data according to time, so it should be working even if the vehicle is not on, but it might consume the excessive power from the battery pack itself, so.
(10:06) to avoid that a Real time clock is integrated with the BMS which needs very small power, and it does our job. (Ref) Well that’s all about the BMS of an Electric vehicle, if you think there is something missing please let me know in the comment section. Hit the like button if you liked this video, subscribe to my channel and finally, thanks for watching.bu yazının hepsini türkçeye çevir