It's been five years since the Tesla Roadster became the first modern EV sold in the U.S. and three years since the Chevrolet Volt and Nissan Leaf came to market, followed by Fisker’s Karma a few months later.
Each of these radical products has shown weakness in its infancy; the Volt burned down after crash-testing at NHSTA; Leaf customers saw their driving range dwindle in hotter climates; and the Karma has made headlines for a variety of issues.
Tesla is not immune to problems, but aside from some unfortunate customers bricking their batteries, the company has maintained a solid track record considering its humble and recent beginnings. If larger automakers weren’t paying attention before, a slew of “Car of the Year” awards is starting to turn heads. Tesla’s first few years on the market has provided the company with a unique position and mindset from which to innovate, pushing the firm ahead of the big OEMs in its mastery of several electric vehicle technologies, particularly the battery. The major OEMs (GM, VW, Nissan, etc.) have taken a markedly less rigorous approach than Tesla with regards to the ultimate safety of their battery systems; the source of this disparity is deeply rooted within each of these organizations, and the effects are not yet readily apparent, despite their significance.
The incumbent OEMs set out to build their EVs by demanding an “automotive” battery cell. Typically, writing specifications and encouraging competition among vendors pushes the existing automotive supply chain to deliver robust and advanced components at low cost and with minimal effort from the OEMs themselves. It seemed logical to apply the same strategy to the battery world. Strict requirements such as low-temperature performance, high cycle life, strict abuse tolerance, and larger format (to reduce total parts count) were issued to the cell makers. Cell makers invested heavily in congruent technologies with the belief that sizable production orders were coming to offset the costs. The unfortunate reality has been a chicken-or-egg problem in which these development costs keep battery prices high and bump EVs out of the high volume markets.
A123 tried to break this stalemate by blindly ramping production, but the massive orders never came. Fisker and Tesla started life without purchasing power (generally ignored by suppliers) and with no influence on cell design. Rather than seeking to customize a “perfect” cell, the companies chose from what was available on the market. Fisker employed an “outsourced engineering” business model similar to the OEMs, and chose a conservative and “safe” A123 cell for its battery system. Tesla opted instead for the 18650 “laptop” cell, a choice inherited from the company’s early technology licenses from powertrain builder AC Propulsion.
It was a good choice because the 18650 cell format was already a commodity. The PC market had driven the price of cells down and manufacturing processes were mature, so the chicken-or-egg problem did not exist. Competition also drove the 18650 cell to have the highest energy density in the industry, further helping Tesla to build a compelling car. Naturally, laptop cells were not designed for automotive use. Decade-long lifetimes, high power output and sub-zero operation were never priorities for the compact-and-low-cost segment of the electronics market, and the inherent safety risks of a massive battery were not relevant. While the major carmakers would not tolerate weaknesses at the cell level, Tesla designed a battery pack that could effectively compensate for the 18650’s shortcomings, clinging to the belief that the cell’s advantages in cost and energy density would more than offset the extra design effort. That strategy drove inventive thinking toward sophisticated and comprehensive battery management in the form of pack balancing, monitoring, thermal control and, most importantly, fault tolerance.
Any form of stored energy comes with the inherent risk of that energy being released in undesired ways, and a battery cell is no exception.
A curious problem for massive multi-cell packs is what is known as thermal runaway propagation -- the sudden heat generated by a single catastrophic cell failure can potentially overheat nearby cells and create a chain reaction of failures. The released energy from a massive battery pack in runaway cannot be practically contained on a vehicle, just as a gasoline fire originating outside of the engine block is problematic for a traditional automobile. The commodity 18650 cell is often regarded as dangerous because it can be abused to the point of catastrophic failure more readily than other cell types. “Automotive” cells are held to a higher standard, using a regimen of abuse tests with strict pass/fail criteria, as part of the automakers’ specification for a cell that is compatible with their automotive environment.
The two different approaches to cell/pack safety standards yield very different results for the overall vehicle. Tesla compensated for the safety shortcomings of their more delicate cell by designing a battery pack capable of handling catastrophic cell failures. Alternatively, by pushing stringent safety regulations onto the cell manufacturers and using an automotive cell, our OEMs did not face the same urgency; they justified to themselves more relaxed responsibility at the system level. As a direct result, Tesla has the only vehicles currently on the market that are explicitly designed to tolerate cell failures safely.
The approach taken by Tesla (and others including my company, BRD Motorcycles) assumes catastrophic failures will occur. This necessitates a battery pack designed to absorb such failures and prevent the entire battery from becoming unsafe. Conversely, the OEM attitude has been to assume that catastrophic cell failures will not occur, and this conclusion has led them to eschew that same requirement.
A few of the high-profile electric vehicle fires reveal the danger in the latter approach. Returning to that Chevrolet Volt NHTSA fire, it was determined that a side impact test performed by NHTSA caused a coolant leak in the battery compartment. Eventually, the coolant level rose high enough to submerge a still-running voltage sensor, which promptly short-circuited and ignited electrolyte that was leaking out of a damaged battery. The runaway event propagated from cell to cell and the entire vehicle was eventually consumed. GM blamed NHTSA for not following the prescribed post-crash procedure to discharge the pack, completely sidestepping the fact that a single cell failure destroyed the entire vehicle.
GM “fixed” the car by reinforcing that section of the car, which enabled them to pass the test -- without ever addressing the core issue.
Another example is the A123 Prius: an aftermarket Prius conversion using “safe” A123 cells caught fire while in use, and burned to the ground a few minutes later. The ensuing investigation found that a loose electrical connection generated intense heat, causing a nearby cell to fail catastrophically. The propagation from cell to cell was not contained and the entire vehicle burned down. Hopefully Fisker has been more diligent with the protection of its A123-based battery system.
Electric vehicles have the potential to be much safer than gas vehicles due to their simplicity and controllability.
Gas vehicles have been evolving for over 100 years to cope with the rapid detonation of highly toxic chemicals, and the EV movement will take a few years to perfect its collection of nuances. The few notorious EV fires that have occurred and are not indicative of any widespread issues.
All battery designs are not created equal, especially under extreme conditions, and as of this writing no standards or requirements exist around anti-propagation requirements. I hope that a specific test will be adopted, but I suspect that Big Auto will continue to push safety concerns to cell makers and favor traditional crash simulations that can mask internal weaknesses of the battery itself. At the very least, this is an area where regulation is lacking, and a safety compromise of which the public should be aware.
Rob Sweney is the "Battery Wizard" at BRD Motorcycles.