It’s not an iPhone
An EV lasts much longer
BMS is key
We’ve recently passed a quarter century since the lithium-ion battery first powered a battery electric vehicle (BEV). With the demonstration of its superior power density over nickel-metal hydride and lead-acid chemistries, lightweight Panasonic laptop cylindrical cells were destined for mobility applications with the Tesla Roadster in 2008. This was followed by popular vehicles like the pouch-celled Nissan Leaf in 2009, and subsequently the cylindrical-celled Tesla Model S in 2012 and the Tesla Model 3 in 2017.
Quick Change to Supercharging
The Model S, first prototyped in 2009, was designed for quick battery swaps. However, a revolutionary paper on ultra-fast DC charging of lithium batteries at elevated temperatures, coupled with Tesla Model S’s onboard charger that was designed to be stacked for higher output power levels, produced the Tesla Supercharger, introduced in September 2012, which was capable of quarter megawatt class charging levels.
To be able to pull off this fast-charging parlor trick meant provisioning of a sophisticated, onboard, thermal management of the high-voltage battery (HVB) by a system known as a “BMS” (battery-management system). The BMS was normally responsible for reading cell-group voltages, battery-module temperatures and pack current. To be able to supercharge, though, the battery had to first be “pre-conditioned" by heating the battery coolant circulating through the pack’s modules to a high temperature sufficiently ahead of time to account for the thermal mass of a pack weighing in at over half a tonne.
The brilliance here was in geolocating the vehicle and using the vehicle navigation system to determine ETA, which was then back-calculated to determine when to have the BMS start heating the pack. Arriving at the Supercharger station, the BMS negotiated the amount of charge current with the Supercharger, with the BMS following an optimized charging-current profile until constant-current charging mode reached the target cell voltage. At that point, the BMS adjusts the current requested from the charging station by holding the voltage until the current tapers off to a minimum current cutoff.
Keepin’ It Chill
The charging of the pack at high charge current meant high power dissipation in the battery cells. Thus, another mode of the thermal system is invoked by the BMS—using the vehicle’s air conditioner to cool the pack coolant, creating a sink for the heat being produced during Supercharging. That was state-of-the-art a dozen years ago. Fast charging and battery longevity depend entirely on the design of the vehicle’s thermal system and the BMS architecture.
Vehicle electrification, since the early days of the NiMH-powered GM EV1, hasn’t changed a whole lot on the propulsion side. The EV1 weighed in at almost 3,000 pounds, was powered by an AC motor, and achieved an efficacy of almost 6 miles/kWh at 60 mph, whereas a Chevy Bolt EV or Tesla Model 3 have an efficacy of about 4 miles/kWh.
Motor efficiency has risen from high 80%’s to mid 90%’s, primarily as a result of using rare-earth permanent-magnet rotors in place of a caged induction rotor, but also through the use of high-speed ceramic bearings and, since last April when Remy’s patent expired, hairpin stator windings. Inverter technology has advanced markedly in the technology being used, recently by using IGBTs and now by using wide-bandgap devices that include silicon-carbide (SiC) MOSFETs. Nothing much more to see here folks, so move along.
Despite these powertrain advances creating efficacies that make BEVs extremely compelling, what EV users see and feel is determined exclusively by the BMS. It determines how much whip gets cracked, how fast the vehicle can charge, and how far the driver is being told the vehicle can go.
Automotive Electrification Week
I’ve been recently working on an Automotive Electrification project, which has launched today, that has resulted in five articles on battery-management systems, written by subject matter experts from National Instruments, Electrified Veronika, Electronic Design’s James Morra and myself, and the sponsor of this Automotive Electrification Week, Texas Instruments (thank you!).
The perspective provided by these technical articles will leave readers with an understanding of how much further BMS advances can go in delivering accurate “fuel gauging,” in reducing BMS characterization times, which now take years, through simulation using digital twins; and the importance of precise sensing of voltage to better balance battery cells (a pair of 0.5% SMT resistors aren’t good enough) thereby providing battery pack longevity and precision state measurement (SOH, SOC).
The articles also go into how making every EV battery’s BMS accessible can create a circular economy where each EV battery can displace combustion vehicles and fossil-fueled power plants for a half century before needing recycling, as well as providing insight into what capabilities are coming for future onboard BMS computing power through the use of onboard edge-AI and by using vehicle resources to perform impedance spectroscopy on the battery pack.
Dino is Dying
As stellar as they presently are, EVs will get much better and more accurate in coming years, with the current crop of BEV offerings looking like The Flintstones car by comparison. And most of those improvements, what owners and manufacturers will see and value, will likely be in the battery- management system, not in solid-state batteries, not in motor advances, not in alternative battery chemistries.
Electric vehicles will go farther, will perform better, will charge faster, will have their range extended—all through BMS advancements.
All for now,
-andyT
Andy Turudic
Technology Editor
Electronic Design
AndyT's Nonlinearities blog arrives the first and third Monday of every month. To make sure you don't miss the latest edition, new articles, or breaking news coverage, please subscribe to our Electronic Design Today newsletter. Please also subscribe to Andy’s Automotive Electronics bi-weekly newletter.
