Smart BMS for a post-subsidy world | Flexibility is key

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Introduction

The sudden tapering of the FAME-II subsidy in June 2023 has disrupted the nascent e2W industry in India. While there are differing views on whether tapering was the right decision at the right time, the industry has been unanimous in increasing prices. This has consequently had a moderating effect on sales that is expected to persist in the near term. Demand for e2Ws in commercial applications such as last-mile delivery and shared mobility is expected to be strong, with battery swapping picking up pace, due to favorable economics. Consumer demand for personal ownership will eventually bounce back as well on the strength of new launches and growing awareness of the lower total cost of ownership. 

This article by Narsimh Kamath looks at some opportunities in the BMS for Tier1s and OEMs to mitigate the impact of subsidy taper.

Future-proofing BMS to multiple chemistries & pack mechanical designs

In a post-subsidy world, OEMs are racing to launch new lower-cost vehicle models. A BMS platform that can provide flexibility to support different pack mechanical designs and battery chemistries would help accelerate the time to market on new launches. Here, it is crucial to consider a couple of factors:

• For OEMs who are clear in their choice of NMC chemistry, a 14S BMS may be the most cost-optimized design.

• For OEMs who have either decided to use only LFP or want the flexibility to support LFP in the future, the obvious choice at first glance might be a 16S BMS. However, it is important to consider potential mechanical constraints in the vehicle and pack design that may introduce a split battery pack in some models and, associated with it, one or two busbars. Here, an 18S BMS that supports busbar measurement may provide greater flexibility in supporting a wider range of battery pack mechanical designs in addition to supporting both NMC and LFP chemistries.

Figure 1: An ideal BMS is flexible to support different pack mechanical designs in addition to supporting multiple cell chemistries | Image credit: Narsimh Kamath

Optimizing battery size for lower cost

In a post-subsidy world, OEMs are actively re-evaluating the battery capacity and, in most cases, are planning to downsize the battery. Rightly, since the FAME subsidy was tied to the battery capacity and with subsidy tapering, so does the motivation to offer large batteries. Here, it is vital to consider the problem of oversizing battery packs to ensure the consumer can achieve the promised range. Typically, the battery cells are kept between 10% and 90% of their full capacity, meaning only 80% of the battery capacity is usable for a normal driving range. Now, if there is a cell voltage measurement error of 5%, the cells must be kept between 15% and 85%, which means that to achieve the same range, the battery must be oversized by close to 18%.

Additionally, suppose the cell voltage measurement error is not specified in terms of a maximum error; then, a further margin must be added to ensure that cells are not over-charged or over-discharged. This means the battery must be further oversized to provide the promised range for the consumer. This oversizing of the battery can add significant costs. It is crucial for the Tier1s and OEMs to evaluate whether the BMS AFE specifies a maximum measurement error applicable across the entire operating temperature range and the whole cell voltage range that is relevant to their battery chemistry. Tier1s may sometimes to tempted to use low-cost BMS AFEs to reduce their BOM. However, a BMS AFE that provides accurate cell voltage measurements within an error that is specified across both the operating temperature range as well as the entire cell voltage range (different range, depending on LFP or NMC chemistry) can save significant costs at the vehicle level for the OEM in terms of optimized battery capacity.

Figure 2: Visualization of cost savings for OEMs with a selection of a BMS AFE that truly guarantees measurement accuracy | Image credit: Narsimh Kamath

Leveraging smart BMS to offer value-added services

In a post-subsidy world, OEMs can benefit from adopting a ‘Smart BMS’ platform that allows them the ability to leverage digital & AI technologies to offer value-added services to end consumers. Following are a few examples of value-added services that are personalized for each vehicle and user:

• Battery extended warranty
• User-specific insurance
• Optimized charging profiles
• Drive mode recommendations
• Charger recommendations

Smart BMS

A smart BMS is a BMS that employs intelligence at the edge & works in conjunction with a digital twin in the cloud to maximize battery life, ensure battery safety, improve range, and reduce charging time.

A smart BMS interprets the raw sensor data (measurements of cell voltages, temperatures, and of the current) and exchanges information with other systems in the vehicle (e.g., charger and VCU) to generate timely insights that help in autonomously protecting the battery, accurately estimating its state of charge and state of health and in constantly optimizing battery operation through drive and charging cycles.

When accompanied by a battery digital twin in the cloud, the smart BMS offers digital services that provide powerful efficiency gains for OEMs and the ability to offer differentiated features and SW updates for enhanced value to consumers.

At the heart of the smart BMS is a powerful and scalable safety microcontroller. The microcontroller must be powerful to execute various software modules for algorithms, control loops, safety monitoring, and safety decisions. It must be scalable to support software updates that allow fast adaptation to changing regulations, bug fixes, and new features such as machine learning at the edge. And finally, it must be a safety microcontroller so that random hardware faults do not cause unintended malfunction.

Figure 3: A smart BMS + battery digital twin can increase efficiencies for the OEM & enable value-added services for consumers. Image credit: Narsimh Kamath

Figure 4: An example of a Smart BMS that is safe, scalable, secure, and flexible. Image credit: Narsimh Kamath

Don’t forget safety!

In a post-subsidy world, it will be necessary not to drop the ball when it comes to increasing the safety levels of the battery pack. Two examples of where there is an opportunity to improve safety levels:

Active paralleling of swappable batteries

Tapering of FAME-II subsidy creates a more level playing field for fixed and swappable batteries. Indeed, swappable batteries offer the benefit of a lower upfront cost of acquiring an EV alongside the convenience of near-zero downtime due to charging. As such, swappable batteries will likely gain traction in a post-subsidy world. For certain use cases, OEMs prefer to use two battery packs that are connected in parallel. The total capacity of the battery pack is then the sum of the capacities in the individual packs. Using this scheme is attractive, for e.g., in models with removable battery packs for swapping or convenience of charging. Using two smaller battery packs means the user can load and unload the individual packs more easily during battery charging or swapping. This configuration can also be visualized as a single battery pack comprising two parallel strings (of battery cells). It introduces unique challenges, such as the possibility of large inrush currents and eddy (circulating) currents when the two individual strings have differing capacities. If left unaddressed, these challenges could pose a risk to battery safety.

Indeed, item 6 in Annexure 8K of AIS-156 Amendment 3 states, ‘REESS shall have Active paralleling circuits for the parallel connection of strings to eliminate circulating currents…’.

Here, an option is to consider using two separate diode-protected DC buses, one for charging; the other for discharge, with software-controlled MOSFETs (part of the SmartBMS) that allow only one path at a time. This configuration eliminates circulating (eddy) currents.

Figure 5: A concept for active paralleling of multiple removable (swappable) batteries in an electric two-wheeler. Image credit: Narsimh Kamath

Early detection of thermal runaway

A key consideration is the prevention and detection of thermal runaway. Indeed, criteria (b) in clause 6.11.4.1 of AIS156 Amendment 3 states that ‘REESS shall have an audio-visual warning for early detection of thermal event/gases in case of thermal runaway of cells. This warning shall be activated at least 5 minutes prior to thermal propagation such as fire and explosion occurs.’

Here, it is important to understand early indicators of a thermal runway to determine ways to provide early warning efficiently. The figure below compares various indicators during the thermal runaway process. While gas sensors respond quickly, different cell chemistries may require different gas detection. Gas sensors may also be prone to providing false alarms in case of pollution near the battery pack. On the other hand, pressure sensors offer a fast and reliable reaction to thermal runaway independent of their position in the battery pack. The smartBMS block diagram in an earlier section shows the use of a pressure sensor interfaced with the safety MCU to respond to the AIS156 requirement in an efficient and scalable manner across different battery pack designs.

Figure 6: Comparison of different types of sensors in detecting thermal runaways. A small, low-power battery pressure sensor can offer fast and reliable detection independent of its location in the battery pack. Image Credit: ‘Fast Thermal Runaway Detection for Lithium-Ion Cells in Large Scale Traction Batteries’, Koch et al., Batteries (ISSN 2313-0105).

Conclusion

A post-subsidy world calls for a smart BMS that is flexible and capable of supporting multiple chemistries and battery pack mechanical designs. The BMS should be a conduit for providing digital value-added services to increase revenue generation opportunities for the OEM. Swappable batteries will likely gain traction, so BMS developers should pay attention to additional considerations, such as active paralleling. And finally, the industry should not let down its guard when it comes to safety. Additional safety measures, such as battery pressure sensors, can help with the early detection of thermal runaways.

About the author

Narsimh Kamath graduated with a B. Tech in Electronics & Communication Engineering from the National Institute of Technology Karnataka and has 15 years of experience spanning multiple roles across design and applications in the semiconductor industry. He is currently the business development manager – electrification for India, Southeast Asia, and ANZ regions at NXP, a global leader in automotive and industrial semiconductors. He has previously served as lead systems architect on a battery management system solution and holds multiple US patents. Reach him at narsimh.kamath@nxp.com.

Also Read: Why is a BMS needed in electric vehicles?