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Importance of battery management system (BMS) in EVs

Electric Vehicles
Author: Aditya Raj Verma, Co-founder and Chief Technology Officer, EVI Technologies

The Royal Swedish Academy of Sciences awarded Nobel Prize in Chemistry, 2019 to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino for development of lithium-ion batteries, a battery born in late 90’s. At that moment everyone was clueless about the power, lithium was bringing in its gauntlet. The revolution was tremendous and today from ipod shuffle to electric trucks everything carries lithium in its pocket. Recent customers of lithium are the electric vehicles, a transportation running on the same principle as of a battery toy car. 312, while Delhi’s AQI still says severe, similar are the number of EV’s running on its road as compared to 3 million vehicles registered. While the technology of electric vehicle is as old as lithium battery, still the mere number raises question on its innovation and evolution.

Why EV manufacturer chose lithium battery over other battery technology?

The beauty embodied in lithium battery packs is the energy they can carry in the required volume. High energy density of lithium batteries make it perfect for electric vehicle, lighter and efficient. Charging is always a concern for any EV owner and with faster charging rate of lithium packs outshines other technologies. Lithium charges at faster rate and reduces the refuelling time of the vehicle.

Moving on to the construction of lithium cell -the chemical structure, solid electrolyte and stable encapsulation make it suitable for application prone to mechanical vibrations and impacts. The lithium cells are found performing at same efficiency in both stationary and mobile applications. As the saying “every technology stays ideal only in text books”, lithium brings a good amount of constraints and restrictions in its usage.

Why lithium battery needs a manager?

Any organization or team outperforms when it has a best manager. Lithium batteries with birth deficiencies need proper monitoring and control to utilize its full potential. Thermal instability has always remained a greatest hurdle in its design and application. The efficiency falls considerably at higher operating temperature. VRLA (Valve regulated Lead Acid) batteries still prove a good choice for low end EV’s because of robust nature, thermal stability and natural cell balancing. Though lithium brings higher energy density, faster charging rate, low self discharge and longer life cycle but these features only shine when the manager is around. Over charging is another prime concern with lithium batteries usage therefore charging need to be closely & accurately monitored.

Lithium battery packs comprises of multiple cells connected in series and parallel configuration, hence cell balancing is the important aspect for efficient power delivery. Imbalance cell causes internal loading of battery reducing state of charge and increasing failure rate. Cell balancing mechanism is required for proper battery pack operation and higher cell life. Battery management system or BMS plays the role of a manager for lithium batteries maintaining and enhancing its performance with protection against hostile conditions to prevent irreversible failure.

What is battery management system (BMS)?

 As the name is self-explanatory, a system designed to manage and control a battery unit for proper operation. Initial designs only incorporated protection components i.e. fuses, FET switches with analog comparators sensing deviation and pulling the completely battery unit down in case of abruptions. With due of course of time, lithium batteries found application in electronic devices, storage system, power backup system and need of more sophisticated BMS arouse rapidly. A modern BMS comprises of multiple blocks integrated together to build a complete lithium battery unit:

  1. Power Delivery Unit (PDU), comprises of FET switch and gate driver circuit which control the outgoing and incoming power in the battery. Cell balancing with individual current bypass FETs are also part of the PDU. Few specific PDU also manages regenerative braking power
  2. Sensing and measurement, comprises of on chip or external ADCs to compute cell voltages, junction temperature and charge/discharge current. SAR based ADC provide high sampling rate and resolution. Modern BMS chip have SAR-ADC based AFE integrated in it.
  3. Communication block, comprise of CAN transceiver, RS232 transceiver for communicating with parallel battery packs or with the vehicle control unit
  4. Central processor manages the above blocks with embedded algorithm taking decision as per inputs and battery state. It also manages external and internal communications of BMS.

“To make a best lithium battery pack just manage its voltage, current & temperature and this is what a BMS does”

Why BMS is important for an EV?

 “EV uses lithium battery pack and BMS is ought to become the integral part of EV”

600+ words you have read, before the article steps into the importance of BMS in EVs, as stated in the title. Well similar are the number of feature and functions BMS brings to EV power train. The anatomy of an electric vehicle state battery as a fuel and better is fuel to energy conversion, smoother will be the ride.

Loading is critical aspect of battery technology and managing the power delivery becomes the prime task of the BMS. Constant load as in case of electronic devices, power requirement is rather uniform and never stresses the BMS and the battery. Electric vehicle are non uniform load or to be precise an erratic loading system, where battery needs control over the power delivery. BMS acts as power controlling unit preventing sudden abruption in load current to affect lithium cells. Further, the loading transients range in milliseconds and require an effective management to avoid high discharge rate.

Electric vehicle is a subset of the vehicle therefore needs a fuel gauge. Coulomb counting, a part of BMS precisely measure the state of charge and compute the drive range. This mitigates the range anxiety of any EV owner. Further, refuelling or recharging battery bank from any charging station requires BMS integrated battery unit for information exchange with the charger. It is because of BMS, you can refuel your vehicle with same rate and efficiency at any public charging station.

Diving deep into the EV architecture, various small auxiliary loads as headlamps, dashboard, and cooling unit also draws power from main battery bank. Motor controller can communicate with BMS and efficiently manage the power demand. Auxiliary loads are not intelligent to communicate with BMS therefore it independently manages these loads without reducing efficiency on the power train.

Existing BMS design primarily focused on managing lithium cells and has outperformed on every benchmarks. Still, achieving best efficiencies in transportation remain a dream requiring a more advance and upgraded BMS. Briefly stating in bullets the characteristics of EV BMS:

  • EV BMS should have a variable power control and delivery system rather than digital switch based PDU shutting system down at low SOC
  • EV BMS should be able to communicate with motor controller and override motor controller decisions in cases of abruptions and failure
  • EV BMS should have a TMS (Temperature Management System) for battery bank rather than having only temperature probes. Low power cooling unit can be the part of TMS
  • EV BMS should have PDU designed to support regenerative braking
  • EV BMS should have higher processing and faster ADCs for better response to transient load
  • EV BMS should have auxiliary load management component to power EV auxiliaries
  • EV BMS should also have telematics module for enabling battery swapping technologies
  • EV BMS should support all global charging standards CCS, Chademo and GB/T.

 How will be an EV BMS?

As the electric vehicles are evolving so is the battery technology. Future BMS will be smarter and faster, with on chip analytics to compute accurate drive range and adapt to load changes with no comprise on power delivery. The BMS will support any kind of charging protocols with no firmware or hardware reconfiguration. The BMS will support infinite battery stacking and any cell configuration. The EV BMS will enable vehicle to grid capability, authorize charging transactions, book charging slots and standardize BMS design for an electric vehicle.

Wrapping up with a remark from Elon Musk, “A product is broken if it requires a manual”, where battery management system (BMS) can eliminate the need of a manual to operate a battery bank.

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BiS Team

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