Controlling temperatures is relatively simple when the batteries are in a fixed location, say, next to a wind farm, but it becomes harder when they are placed in a car or bus.
"The biggest thing that people become concerned about [for batteries in cars] is the ability to be able to tolerate abuse," said Joe Redfield, principal engineer at the Southwest Research Institute, a nonprofit engineering research and development group.
In a car, a battery is exposed to a wide range of humidities, temperatures and electrical loads. All of these factors influence the battery's reliability, and if they get too extreme, they can cause a thermal runaway condition.
New problem for firefighters
The problem is compounded by the fact that newer lithium-ion batteries store more electricity than other electrochemical storage systems. "The lead-acid battery has been around a long time" and is a mature technology, said Redfield. "The energy levels of lithium-ion batteries are much, much, much greater than that of lead-acid storage."
This becomes a major problem for firefighters and first responders in the event of an accident involving lithium-ion batteries. Water can't always be used to extinguish an electrical fire, since water can conduct electricity.
In addition, in the case of a thermal runaway, it's usually not the batteries that catch fire but their fumes, though lithium itself is flammable. Even after the fire is extinguished, the batteries can still generate tremendous amounts of heat and reignite fumes, hampering rescue efforts.
One solution is to separate batteries into modules, making it easier to isolate a failed battery from the rest. Another trick is to have a master kill switch, a mechanism that quickly disables the electrical system and discharges the batteries.
The Department of Energy and the National Fire Protection Association are working together to train firefighters and rescue workers to identify these switches in vehicles and grid storage systems as well as in how to respond to battery fires, according to the NHTSA.
Redfield said that the best way to prevent such incidents is with a battery management system that evenly distributes electrical loads and controls temperatures. "It's not just for safety; it's primarily there to provide performance and battery life," he said.
Electrics get high marks in crash tests
"As the operating temperature increases, the lifetime diminishes dramatically. You want to ensure the longest battery life, and if you achieve that, then you're clearly in the safety limits of the operating environment," he added.
Overall, Redfield expects that energy storage systems will help increase renewable energy use and curb fossil fuel dependence in the United States. The bumps along the road are significant, but they do not result from an inherent flaw in the idea.
"Failures in new technology have almost always been the result of design shortcuts that were made in putting the new technology into progress. Every now and then, you have some uncharted territory -- things we haven't seen before -- but typically, they are few and far between," said Redfield.
"It really is going down the same path we've gone down many times before. We don't need to make the same mistakes we've made with liquid fuels." After the earlier testing, NHTSA gave the Volt a five-star crash test rating -- the agency's highest -- and it did the same for Nissan's all-electric Leaf.
Meanwhile, a second testing agency, the Insurance Institute for Highway Safety, has given the Chevrolet Volt a "G," the highest safety score possible, after side crash tests on the front, side, rear and rollovers.
Research by an affiliate of the insurance group, the Highway Loss Data Institute, estimates that overall chances of being injured in a crash are 25 percent lower in hybrids because their large batteries make them heavier than similar gasoline-powered cars.