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Understanding the Power of Battery Management Systems

Sambit Sengupta
Sambit Sengupta, Associate Director – Field Applications, Avnet India

According to the Society of Manufacturers of Electric Vehicles (SMEV), electric vehicle (EV) sales in India was 56,000 units in fiscal year 2018 – which is a 124% growth year on year. Even though the sales in fiscal year 2017 was a mere 25,000 units, the Indian market shows a peculiar trend of its own. EV sales is growing in 2-Wheeler (2W) and 3-Wheeler (3W). For last mile connectivity (such as short commutes from the metro station to your home or office, or a short commute to the nearby mall),EVs, particularly 2W and 3Woffer a great solution. With government backing for Faster Adoption and Manufacturing of Hybrid and Electric Vehicles (FAME) and high subsidy for EVs,the future outlook for EV sales is very positive.There will be tremendous growth for EVs in 2W and 3W as there are lots of infrastructural incentives for the same.It is predicted that by 2030, every third vehicle will be electrically powered.

Society of Manufacturers of Electric Vehicles

Equally astonishing is the growth spurt seen in shared and connected mobility. It is common sight to see people using a shared car, rented car or bike for their urgent commute requirements. This market is ready for induction of EVs, mostly 2W and 3W first, and eventually 4 wheelers so it is the ripe time to discuss the EV safety incidents (as we see from time to time in international news reports). An analysis of some 50 cases of EV incidents that have occurred globally since 2010 shows that as much as 50% were due to battery malfunction. This is unsurprising given that battery packs, the “heart” of EVs, are made up of thousands of monolithic lithium batteries. Among all commercially viable battery technologies, lithium batteries currently exhibit the highest energy density though they are naturally volatile and could potentially explode in the event of overcharge, overtemperature, overdischargeand other operating issues, or if there are issues in manufacturing techniques and structural materials. This is where BMS (battery management system)—a solution that enables lithium batteries to power EVs while controlling their volatile nature—steps in. BMS is a new technology that progressed alongside advancements in EV development. As safety requirements for automobiles are particularly stringent, people have naturally developed high expectations for the functionality of BMSs. For example, the ideal BMS must be capable of preventing overcharge/overdischarge, regulating temperature, maintaining battery voltage and temperature equilibrium, predicting remaining battery capacity and remaining mileage etc. They must also be capable of real-time monitoring and adjusting battery management parameters while working in conjunction with other parallel subsystems. In short, BMSs are monitoring systems that must be able to monitor, analyze, control and provide feedback on power batteries to ensure efficiency and operational safety.

A Typical BMS system will be like below:

BMS

From the perspective of BMS technological frameworks, core functions include:

• Accurate prediction of battery status. This pertains to accurate measurement and prediction of parameters for battery SOC (State of Charge), SOP (State of Power) and SOH (State of Health), conducting dynamic monitoring and providing diagnosis for each battery, and building historical files on battery use, thereby providing a basis for data analysis and control decisions.

• Energy balance. Performance discrepancies between different batteries within a battery pack can negatively affect lifespan and system usage. These discrepancies must be mitigated through energy balancing measures to ensure conformity among different batteries and perform dynamic “maintenance” on the battery pack. BMS currently utilize active balance and passive balance strategies each of which offer unique advantages (see Table 1).

• Protection. BMSs are able to protect the battery pack and respond rapidly in the event of malfunction due to overcharge, overdischarge, overcurrent, or over/undertemperature.

• Data communication. Data communication mechanisms can be established using CAN BUS or other methods to transmit data to the display system, vehicle control system, charging system and other external equipment. Some BMSs are even equipped with Wi-Fi transmission capability for cloud connection.

other methods to transmit data

To support the above BMS functions, technology providers have been actively developing new products and solutions. For example, the Maxim Integrated MAX14920/MAX14921 battery measurement analog front-end devices can support high precision sampling of voltage and provide level shifting for primary/secondary battery packs up to 16 cells/+65V (max) and support passive balancing through an external FET driver. While Maxim Integrated offers high performance “products”, Texas Instruments (TI) provides full “solutions”. TI’s bq76PL455A-Q1 is an integrated 16-cell battery monitoring and protection device based on which a comprehensive passive balance battery solution can be built. It can be used to monitor and detect various malfunctions such as overvoltage, undervoltage, overtemperature and communication faults, and allows up to sixteen bq76PL455A-Q1 devices to communicate with a host via a single high-speed Universal Asynchronous Receiver/ Transmitter (UART) interface.

NXP, on the other hand, leverages its comprehensive automobile electronic product lines to provide a comprehensive BMS solution that encompasses micro controllers MCUs, analog front-end battery controller ICs, isolated network high speed transceivers, system base chips (SBC) and others functions. Customers that utilize the solution can manage high voltages of 800V and above.

Apart from the above, Avnet and various manufacturersprovide support inproviding various options for BMS ASIC device, DC relay and contactor, microcontroller (MCU) with controller area network (CAN), CAN physical, metal–oxide–semiconductor field-effect transistor (MOSFET) and connector. Aside from the above mentioned hardware capabilities, BMS system designs require corresponding software capabilities, i.e. the development of a core algorithm. A high quality algorithm enhances battery status prediction accuracy and offers strong correction capabilities, thereby mitigating the impact of hardware issues such as battery quality. This allows precise control even for batteries with a lesser degree of uniformity, hence reducing overall system cost. The advantages and pitfalls of an algorithm is also reflected in the hardware resources required. High performance algorithms require only a low amount of CPU computing resources, greatly enhancing system efficiency. Avnet and their different vendors provide support and guidance in this area. According to estimates, India’s EV battery market will reach 120 croreRrupees by 2020. Although the cost of BMS relative to the entire EV is relatively low, developers should nevertheless ensure it is tightly integrated into EV design to ensure that power batteries don’t become ticking bombs.

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