Wireless battery management systems

Customized parts unique to a single vehicle work fine when volumes are low. General Motors (GM) can afford to use unusual parts on the Chevrolet Corvette (21,626 sales in 2020) that would become prohibitively expensive on the Silverado pickup line (594,094 sales).

As electric vehicles (EVs) move from Corvette-like niche numbers into high-volume, automakers must standardize as much as possible to simplify engineering, design, and manufacturing. Early simplification candidates are battery pack wiring harnesses.

Today’s EVs have customized cabling systems to carry data to vehicle battery management systems (BMS), the sophisticated computers that track temperature and state-of-charge in each battery cell to estimate vehicle range and ensure batteries don’t overheat.

To simplify BMS connections, several suppliers have begun offering wireless options (wBMS). Technology providers and automakers say wireless systems are more uniform and can scale easily from packs for giant commercial vehicles to ones used in compact cars. GM plans to use wBMS to manage its Ultium battery packs, starting with the GMC Hummer EV pickup late this year.

“Scalability and complexity reduction are a theme with our Ultium batteries – the wireless battery management system is the critical enabler,” says Kent Helfrich, GM executive director of Global Electrification and Battery Systems.

GM’s wBMS for Ultium comes from supplier Analog Devices. Texas Instruments and other technology companies are working with other original equipment manufacturers (OEMs) on similar technology. The goals – simplify battery connections, share components across as many vehicles as possible, and remove design constraints.

Design freedom

GM calls its wBMS nearly wireless because batteries will still need high-voltage cables to carry power from batteries to electric motors. Removing data cables, however, frees designers to create oddly shaped or placed battery packs.

Gina Aquilano, technology director for automotive, Analog Devices, says designers can customize battery pack sizes and shapes. Most new EVs integrate battery packs into their floors, but alternative mounting designs might make more sense for some vehicles.

“Instead of thicker connectors for wired connections, you get a chip-sized wireless device,” Aquilano says. Eliminating connectors slightly shrinks each module, the same logic Apple used in eliminating the 3.5mm headphone jack to make the iPhone thinner.

Also, engineering a battery pack specific to one vehicle means designing a unique wiring harness to go along with it, creating a product lifecycle management (PLM) challenge. Mark Ng, system engineering manager at Texas Instruments specializing in BMS for hybrids and EVs, says sharing components instead of using custom harnesses will create economies of scale for EVs, even if individual model volumes are low.

“An OEM isn’t going to design a BMS for just one model of car,” Ng says. “It’s going to be a platform for the entire fleet.”

Analog and TI engineers say their systems are proprietary and won’t go into great detail of how they operate, but the systems use similar principles. OEMs tend to package EV battery cells into modules. So, a pack may have 12 modules, each with dozens of individual cells. TI’s system uses cell supervisor units (CSUs) within each module that report data to the BMS via wireless transponders. Aquilano says Analog’s system uses chip-sized wireless units within the battery pack to monitor cells and communicate with the BMS.

Photo credit: Texas Instrument

Security

EV cyber attacks could be catastrophic – a signal to discharge all battery cells simultaneously could generate enough heat to cause a fire or explosion – so OEMs and suppliers must focus on cybersecurity.

TI and Analog use closed networks, wireless units within the battery communicate only with the central battery controller. Vehicle electronic control units (ECUs) that manage other systems communicate with the wBMS, not the individual cells. Limited access to one entry/exit point limits points of attack on the network.

Aquilano says each hardware and software component in the system includes security and encryption systems to protect data.

“We considered security up front using a top-down approach,” Aquilano explains. “We considered every threat possibility, every point of weakness where something could interfere, and we addressed it during that design phase.”

At TI, Ng says the company applied decades of experience in cybersecurity, wired and wireless, into its wBMS, starting with the moment the vehicle starts.

“When your BMS is booting up, you have to be sure it's booting your actual code. So, if an imposter tries to reprogram it to run his operating system, the BMS has to verify the actual program it’s supposed to run,” Ng says.

Manufacturing/service

Aquilano says when Analog began working with GM, they expected to reduce system complexity and make battery systems more scalable. The biggest surprise, however, was how excited manufacturing engineers were about going wireless. Auto plants juggle different sizes and shapes of wiring harnesses, and industrial engineers dread the outcomes if the wrong wiring gets installed.

“When you eliminate all of the individual battery module connections, you eliminate the wiring harness, and robotic assembly becomes possible,” Aquilano says.

Vehicle designers have incorporated battery packs into structural components on several upcoming EVs. Drilling holes can weaken structural pieces that must absorb crash damage, so battery packs that don’t need wiring holes drilled become more attractive.

TI’s Ng says wBMS also makes vehicles easier to service. Wired BMS tend to wire modules in series, meaning if the fifth of eight modules has a problem, technicians must disconnect the wiring to five modules.

“With wireless, the BMS determines where the flaw is, and you can really just swap things out,” Ng says. Independent communication between the wBMS and each module eliminates the serial communication problem. Aquilano adds that wireless systems also support used battery reuse cases at the end of an EV’s life.

Analog Devices https://www.analog.com

General Motors https://www.gm.com

Texas Instruments https://www.ti.com

About the author: Robert Schoenberger is editor of Today’s eMobility. He can be reached at rschoenberger@gie.net or 216.393.0271.