BMS solutions for electric bicycles and electric motorcycles under the new national standard for electric vehicles

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BMS solutions for electric bicycles and electric motorcycles under the new national standard for electric vehicles [Copy link]

 

Author: Andrew Su

On May 15, 2018, in accordance with the national standard management procedures, the Ministry of Industry and Information Technology organized the revision of the mandatory national standard "Safety Technical Specifications for Electric Bicycles" (GB 17761-2018), which was approved and issued by the State Administration for Market Regulation and the National Standards Administration, and officially came into effect on April 15, 2019.

The new national standard has made major adjustments to the technical requirements for electric bicycles:

1. Added anti-tampering requirements to prevent products from being illegally modified after leaving the factory;
2. The maximum speed increased from 20 km/h to 25 km/h;
3. The motor power is increased from 240W to 400W;
4. The vehicle mass including the battery is adjusted from 40 kg to 55 kg
5. The riding ability requirements have been strengthened, and electric bicycles must have pedaling function.

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Data source: SMM Ministry of Industry and Information Technology

This standard emphasizes the non-motor vehicle attributes of electric bicycles in terms of speed, weight, etc., especially whether they have the ability to pedal, which completely distinguishes the previously confused electric bicycles and electric motorcycles. In terms of usage, electric bicycles and electric motorcycles will be subject to stricter traffic regulations than electric bicycles. For example, electric motorcycles must be equipped with motor vehicle license plates, drivers are required to hold a driver's license, and can only drive on motor vehicle lanes. This will greatly help improve traffic order and reduce traffic accidents. From this perspective, the new national standard for electric bicycles has laid a norm for the long-term and healthy development of the electric bicycle market.

The new national standard for electric bicycles limits the maximum weight of the vehicle to no more than 55 kilograms. Although there is no clear prohibition on the use of lead-acid batteries, it actually limits lead-acid batteries to electric bicycles with shorter mileage, which objectively encourages everyone to use lithium batteries. The harm of lead-acid batteries to the environment is already a consensus. Last year, some deputies to the National People's Congress proposed to ban the use of lead-acid batteries. At present, China produces more than 32 million electric bicycles each year, but due to the high purchase cost of lithium batteries, the current penetration rate is less than 12%. The release of the new national standard will inevitably accelerate the application of lithium batteries in electric bicycles. In fact, since the service life of lithium batteries is more than 2 to 3 times that of lead-acid batteries, the average cost of lithium batteries is already comparable to that of lead-acid batteries. With the release of lithium battery production capacity, its cost will still have room for a 30-40% decline in the next three years. Therefore, it can be expected that lithium batteries will occupy most of the electric bicycle market within 5 years.

Compared with lead-acid batteries, lithium batteries are easily damaged under conditions such as overvoltage, undervoltage, overcurrent, and overtemperature. Therefore, lithium batteries must be equipped with corresponding charging management circuits and monitoring protection (BMS) circuits to be used safely. The extensive use of lithium batteries in electric bicycles and electric motorcycles will inevitably drive the rapid growth of demand for lithium battery BMS systems. The new national standard for electric bicycles stipulates that the nominal voltage of the battery is ≤48V and the maximum output voltage is ≤60V, which stipulates the maximum number of cells in series for electric bicycles: 48/3.0=16 (the nominal voltage of lithium iron phosphate battery cells is usually 3.0V or 3.2V). If ternary or manganese lithium batteries are used, the maximum number of cells in series is 48/3.7=13. The commonly used number of cells in series on the market is 7, 10, 13, 15, and 16. The pure hardware lithium battery protection solution has the advantages of low cost and flexible use. It is currently the mainstream solution for electric bicycle BMS systems.

Due to the lack of pedal riding ability, according to the new national standard for electric bicycles, the previously popular electric scooters will be classified as electric motorcycles. Therefore, the market share of electric motorcycles will increase significantly in the future. From the perspective of the development of the battery management system, combined with the national recommended standard for lithium-ion batteries for electric motorcycles and electric mopeds (GB/T 36672-2018, not yet enforced), which was released in September 2018 and implemented on April 1, 2019, it is stipulated that electric motorcycles should have the voltage and temperature of single cells, the total voltage, current, charge and discharge times, and the maximum charging current data collection and limited storage. This requires the BMS system of electric motorcycles to have the ability to collect voltage, current, and temperature. Pure hardware protection solutions cannot meet this requirement. Therefore, battery monitoring solutions with analog front-end and protection functions will be more widely used. The national recommended standard for lithium-ion batteries for electric motorcycles and electric mopeds requires that the battery management system communicate with the vehicle controller, motor controller, and charger using the CAN communication protocol. Therefore, electric motorcycles will use more CAN drivers and isolators in the future, and corresponding isolated power supplies will be required for this purpose. The new national standard for electric bicycles does not have these requirements, so electric motorcycles will use a more complex BMS system than electric bicycles. At present, most electric scooters and electric motorcycles still use lead-acid batteries. Although GB/T 36672-2018 does not restrict the use of lead-acid batteries in terms of weight, the current lead-acid battery management system usually does not have the ability to collect voltage, current, temperature and communication. In terms of meeting national standards, lithium batteries still have advantages. GB/T 36672-2018 also recommends the rated voltage of electric motorcycle batteries, 48V/60V/72V/84V/96V/144V. The mainstream on the market is 48V/60V/72V, so the commonly used number of battery cells in series is 13, 16, 17, 20, and 24.

To sum up, the impact of the implementation of the new national standards for electric bicycles and motorcycles on the market is mainly reflected in:

1. Lithium batteries will replace lead-acid batteries and become the mainstream of electric bicycle and motorcycle batteries
2. Electric scooters will be included in the category of electric motorcycles, thereby increasing the total market size of electric motorcycles
3. Due to the requirements of the new standard for electric motorcycles (not yet enforced), lithium battery management solutions with the ability to sample single cell voltage, temperature and other information will become mainstream
4. Due to the requirements of the new standard for electric motorcycles (not yet enforced), the CAN bus will be widely used, and the corresponding isolation and low static power consumption high-voltage DC-DC will also be widely used

The following is a typical system block diagram of the BMS system for electric bicycle and motorcycle battery packs. In addition to the gray-marked batteries, temperature and current detection resistors, and fuses (the yellow-marked chips are optional functions), TI can provide complete solutions for electric bicycles and motorcycles, and has been recognized by a large number of brand customers.

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As can be seen from the figure above, the BMS system can be divided into the following parts from the functional point of view:

1. Battery information collection, measurement and protection circuit
2. Power Management Circuit
3. CAN bus isolation drive circuit (for electric motorcycles)
4. MCU (not described in detail in this article)

The following is a detailed introduction to the related chips that TI recommends.

1. Battery information collection, measurement and protection circuit

This circuit includes the analog front end and protection chip BQ76930 or BQ76940 for battery voltage sampling , the battery power measurement chip BQ78350 or BQ34Z100 , the secondary protection chip BQ7718 , and the high-voltage side MOS driver chip BQ76200 .

1. Analog front end and protection: BQ76930 , BQ76940

BQ76930 supports the application of 6~10 battery cells cascaded, and BQ76940 supports the application of 9~15 battery cells cascaded. They can accurately collect the voltage of each level of battery cells, the current of the battery pack, and the temperature of the battery cells or MOS, and control the charging and discharging MOS to protect the battery in the case of overvoltage, undervoltage, overtemperature, overcurrent and short circuit faults, and notify the controller through an interrupt signal. After the thresholds of voltage, current, and temperature protection are set by the controller through I2C when the system is powered on, BQ76930 and BQ76940 can provide complete hardware protection functions. BQ76930 and BQ76940 integrate a 5mA internal balancing circuit. If a larger balancing current is required, an external balancing circuit can be used to provide a balancing current greater than 50mA. Customers can design their own balancing control strategy through software (two adjacent batteries cannot be balanced at the same time). BQ76930 and BQ76940 integrate 14-bit high-precision voltage and temperature sampling ADCs and independent 16-bit high-precision current sampling ADCs. These data can be transmitted to the MCU or the matching fuel gauge chip BQ78350 via I2C to obtain high-precision battery power information. The MCU can also provide software protection for the system by setting the parameters of the BQ78350 .

BQ76930 and BQ76940 can also support more battery cell cascading applications through cascading, as shown in the following figure:

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Some electric motorcycles use 16 or 17 battery strings, which can also be achieved by using an external circuit to expand the number of channels of the BQ76940 using a low quiescent current op amp TLV2401 / LM2904 .

Related Links:

1) BQ76930 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=bq76930#q=bq76930&t=everything&linkId=1

2) BQ76940 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=bq76940#q=bq76940&t=everything&linkId=1

3) TLV2401 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=TLV2401#q=TLV2401&t=everything&linkId=1

4) LM2904 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=LM2904#q=LM2904&t=everything&linkId=1

5) BQ76930 cascade application: 查看详情

6) Low power consumption 16S/17S BMS solution based on BQ76940 : 查看详情

2. Battery metering: BQ78350 , BQ34Z100 , BQ34110

When using electric bicycles and motorcycles, customers often need to know the battery status in order to plan their trips reasonably and prevent sudden battery power outages during driving. A high-precision fuel meter can effectively improve the user experience.

BQ78350 communicates with Q76930 or BQ76940 via I2C bus to obtain the voltage of each cell, the temperature of the cell and the current of the battery pack, and uses CEDV algorithm to accurately calculate the battery pack's state of charge and health. Based on the hardware protection of BQ76930 and BQ76940 , BQ78350 can further provide software protection and secondary protection functions for more convenient balancing control. In addition, BQ78350 also has black box, SHA-1 encryption and LED indication functions. BQ78350 also provides SMBus interface for communication with external systems.

BQ34Z100 supports the application of 1~16 battery cells cascade. Unlike BQ78350 , BQ34Z100 does not detect the voltage of each battery cell, but uses voltage divider resistors to detect the average voltage of the entire battery pack, and uses TI's patented impedance tracking algorithm to accurately calculate the battery pack's power state and health status. In addition, BQ34Z100 also provides SHA-1 encryption and LED indication functions. BQ34Z100 is relatively simple to use and can be used with various analog front-ends or hardware protection solutions.

For lithium iron phosphate batteries, since the discharge curve of the battery cell is relatively flat, the accuracy of voltage acquisition has a greater impact on the measurement accuracy of the impedance tracking algorithm. BQ34110 is similar to BQ34Z100 , but uses the CEDV algorithm and is suitable for lithium iron phosphate battery applications.

Related Links:

1) BQ78350-R2: http://www.ti.com.cn/tool/cn/BQ78350-R2-DEVICE-FW?keyMatch=bq78350&tisearch=Search-CN-Everything

2) BQ34Z100-G1 : 查看详情

3) BQ34110 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm= bq34110 #q= bq34110 &t=everything&linkId=1

3. Secondary protection: BQ7718

In order to ensure the safety of lithium batteries, in addition to the basic overvoltage, overcurrent and other protection circuits, the battery management system also requires the use of an independent secondary protection circuit. When the primary protection circuit fails, the fuse is blown to prevent lithium batteries from causing safety accidents under harsh conditions such as overvoltage. Among them, the most important and common is the secondary overvoltage protection.

BQ7718 supports 2~5 cells in series, but can also be cascaded using the following method, supporting applications with up to 20 cells in cascade. BQ7718 has a voltage accuracy of up to 10mV, power consumption of less than 1uA, and high reliability, which has been recognized by a large number of customers.

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Related link: http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=bq7718#q=bq7718&t=everything&linkId=1

4. High-voltage side MOS switch driver: BQ76200

As shown in the block diagram above, the battery protection system protects lithium batteries by controlling the charge and discharge switches. Usually, the charge and discharge switches (usually MOS) are placed at the negative pole (ground) of the battery pack. This method has advantages in wiring and heat dissipation. However, for applications with fuel gauges and battery status monitoring functions, when the charge or discharge switch is turned off, the ground connection is disconnected, and the external system cannot communicate directly with the battery's internal management system. Therefore, an isolation circuit must be used to read fault information, etc. If the charge and discharge switches are placed at the positive pole of the battery pack as shown in the figure below, no isolation circuit is required, and the external system can communicate with the battery's internal management system. This requires a drive circuit for the charge and discharge switch MOS (usually N-MOS, low impedance, low cost), and BQ76200 is a simple and reliable choice.

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BQ76200 has a maximum withstand voltage of 100V and can be applied to 3~15S lithium-ion battery packs or 3~17 series lithium iron phosphate battery packs. BQ76200 also integrates P-MOS drive function, which can easily realize the pre-charge or pre-discharge function of the battery pack.

Related link: http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=bq76200#q=bq76200&t=everything&linkId=1

5. Power management circuit

a. High voltage step-down circuit LM5164 / LM5165

In addition to the analog front end or secondary protection chip, the battery management system usually has battery metering, MCU and isolated communication circuits. These devices usually require a power supply voltage of 2.5~5V, so a step-down circuit is needed to convert the battery voltage into a low-voltage power supply of 2.5~5V for them to power. Since the battery pack is often not charged for a long time during storage and transportation, the lithium battery management circuit will continue to consume power, which may cause the lithium battery to over-discharge or even damage. However, for safety reasons, the lithium battery management circuit cannot be turned off. Under the condition of long-term storage, in order to reduce the power consumption of the system, the MCU of the lithium battery management system is usually set to sleep, and the quiescent current of the power system needs to be reduced to less than 0.1mA, or even lower. The quiescent current of the high-voltage power supply in the switching mode is usually high, so customers usually use LDO (such as TPS7A4001 ) to power the MCU, but because the efficiency of LDO is very low (~10~15%), this will cause problems such as heating. TI recently launched a high-voltage synchronous buck power solution LM5164 / LM5165 with a quiescent current as low as 10uA, while providing >80%/90% high efficiency. LM5165 has a withstand voltage of 65V and a maximum output current of 150mA, which can be used to power MCU, power meter chip and communication interface; LM5154 has a withstand voltage of 100V and a maximum output current of 1A, and can also provide power for external USB interfaces, making it convenient to provide emergency power for mobile phones, etc.

b.Isolated power supply SN6501

Isolation of CAN, RS485 and other buses cannot be separated from isolated power supply. TI's SN6501 is a single-chip isolated power supply solution that uses a push-pull architecture to drive an isolation transformer and can provide 150~350mA of current for a 3.3V/5V bus. Compared with traditional isolated power supplies, SN6501 does not require a startup circuit and a voltage feedback circuit, and its architecture is very simple. The frequency can be as high as 360~410KHz, so a small transformer can be used, with very few external circuits, and the overall volume and cost are greatly reduced. SN6501 has an integrated dead zone protection function, and the efficiency can be as high as about 85% under 50~100mA load conditions.

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c.FLY-BUCK isolated power supply LM5018 / LM5019

Compared with the LM5164 / LM5165 + SN6501 power supply architecture, the FLY-BUCK architecture isolated power supply only requires one power chip, which has the advantage of lower cost, but is inferior in static power consumption, output voltage accuracy, and mutual interference between power supplies. LM5018 / LM5019 are both 100V withstand voltage power supply solutions that support the FLY-BUCK architecture. Its working principle is as follows:

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d. Timer TPL5010 with watchdog function

Because the safety of the system needs to be monitored, the MCU of the BMS system cannot be shut down when it is working. However, the operating current of the MCU is usually relatively large (>1mA). Therefore, in order to reduce the static power consumption when the battery is idle, the MCU must enter standby mode, but it must also be able to wake up periodically to monitor the battery status, which requires the use of a timer.

TPL5010 can provide a timer with a time interval of 100ms~7200s, which can be flexibly set through external resistors, with an accuracy of 1%, and a static power consumption of only 35nA. In addition, TPL5010 also integrates a watchdog function to avoid serious safety consequences when the MCU program runs away, which is required by standards such as EN50271.

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Related Links:

1) LM5164 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=LM5164#q=LM5164&t=everything&linkId=1

2) LM5165 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=LM5165#q=LM5165&t=everything&linkId=1

3) TPS7A4001 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=TPS7A4001#q=TPS7A4001&t=everything&linkId=1

4) LM5018 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=LM5018#q=LM5018&t=everything&linkId=1

5) LM5019 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=LM5019#q=LM5019&t=everything&linkId=1

6) SN6501 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm= SN6501 #q= SN6501 &t=everything&linkId=1

7) TPL5010 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=TPL5010#q=TPL5010&t=everything&linkId=1

6. Isolate communications

According to the requirements of the new national standard, electric motorcycles need to be equipped with a CAN bus interface. The CAN bus is a half-duplex asynchronous serial communication protocol that uses differential voltage signals. It has complex mechanisms such as priority management and arbitration to ensure the reliability of communication. The communication of the MCU usually uses high and low level logic to transmit data. Therefore, electric motorcycle batteries need to use a CAN bus transceiver, such as TCAN1042. Most electric vehicle batteries place the charge and discharge MOS on the low voltage side. When the battery enters the protection state, the charge and discharge MOS will be disconnected, which may cause a large potential difference between the ground voltage of the battery protection board and the ground level of the external device, causing damage. Moreover, for batteries with communication interfaces, there is a possibility of electric shock or damage to external devices due to the high voltage inside the battery. Therefore, the communication interface should have an isolation function. TI also has CAN bus transceivers such as ISO1042 and ISO1050 that integrate transceivers and isolation functions . In addition, as mentioned above, for batteries greater than 15S, a cascaded architecture of two BQ76930s can be used . The upper BQ76930 needs to communicate with the MCU through an I2C isolator, such as ISO1540 / ISO1541 . TI's isolation device uses capacitive isolation technology, which has the advantages of small overall solution size, fast speed, low power consumption, strong anti-interference ability, high temperature tolerance, and long life.

a.TCAN1042

The TCAN1042 series meets the ISO1188-2 standard, supports 2Mbps communication rate, and can support up to 5Mbps communication rate. The CAN bus supports 5V logic level, the differential input voltage can withstand up to +/-58V or +/-70V, the common mode withstand voltage range is +/-30V, TCAN1042 supports 3.3V/5V logic level communication with MCU, and the loop delay is less than 110ns. TCAN1042 can be set to standby mode, the minimum power consumption can be as low as 0.5~5uA, and it can be awakened through CAN bus communication.

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If the 3.3V CAN bus logic level is used, TCAN334 can achieve similar functions as TCAN1042, but the CAN bus voltage resistance value will be reduced to +/-14V.

b. ISO7721

CAN bus usually requires isolation, which can be achieved by combining ISO7721 and TCAN1042. ISO7721 supports 2.5V/3.3V/5V logic level communication, isolation level >3000V or 5000V (two different packages), and has passed UL, VDE, TUV, CSA, CQC certification. ISO7721 supports a maximum communication rate of 100Mbps and a loop delay of 11ns.

C. ISO1042

In order to reduce the size of the solution and simplify the design, the CAN bus transceiver is usually integrated with the isolation function, such as ISO1042 .

ISO1042 meets ISO1188-2 standard, has passed UL, VDE, TUV, CSA, CQC certification, supports 2Mbps communication rate, and can support up to 5Mbps communication rate. CAN bus supports 5V logic level, differential input voltage up to +/-70V, common mode withstand voltage range +/-30V, isolation level 5000V. ISO1042 supports 1.8V/2.5V/3.3V/5V logic level communication with MCU, and the loop delay is less than 160ns. ISO1042 integrates bus short circuit and other protection functions to ensure the safety of the chip.

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d. ISO1541

As mentioned earlier, for applications with >15 battery cells in cascade, a cascaded architecture of 2 BQ76930s or BQ76940s can be used. The MCU or BQ8350 needs to read the voltage and other data of the upper battery cells, but the ground level of the upper BQ76930 / BQ76940 is inconsistent with that of the MCU. Therefore, it is necessary to communicate with the MCU through an isolated I2C interface. ISO1541 is a good choice.

ISO1541 meets the UL1577 standard, has passed UL, CSA, CQC, VDE certification, and supports 1Mbps bidirectional communication rate. The CAN bus supports 5V logic level, the maximum tolerance of differential input voltage is +/-70V, the common mode withstand voltage range is +/-30V, and the isolation level is 2500V. ISO1541 supports 3.3V/5V logic level.

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Related Links:

1. TCAN1042: http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=TCAN1042#q=TCAN1042&t=everything&linkId=1

2. TCAN334 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=TCAN334#q=TCAN334&t=everything&linkId=1

3. ISO7721 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=ISO7721#q=ISO7721&t=everything&linkId=1

4. ISO1042 : http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm=ISO1042#q=ISO1042&t=everything&linkId=1

5. ISO1541 :http://www.ti.com.cn/sitesearch/cn/docs/universalsearch.tsp?searchTerm= ISO1541 #q= ISO1541 &t=everything&linkId=1

In addition to the commonly used chips mentioned above, TI also provides many related products to meet the needs of various applications. The following link will provide you with a more comprehensive solution and summarize the classic reference design and application technology articles for your reference: 查看详情