The company hopes that, by adding steel to the plates protecting the batteries, it will ease worries about the car's safety. Three Volt batteries caught fire after government crash tests last year, prompting a federal investigation and sending GM engineers scrambling to find a fix.
Eligible for the free repairs, announced Thursday, are 8,000 Volts on U.S. roads and another 4,400 still for sale. The cars are covered by a "customer satisfaction program" run by GM, which is similar to a safety recall but allows the carmaker to avoid the bad publicity and federal monitoring that come with a recall.
GM and federal safety officials believe last year's fires were caused by coolant leaking from damaged plastic casing around the batteries after side-impact collisions. The coolant caused an electrical short, which sparked battery fires seven days to three weeks after the crashes. No owners have reported fires after crashes.
GM has a huge incentive to fix the problem and protect the Volt's image. Although the car isn't a big seller — it's fallen short of sales goals — it burnishes GM's image as a greener, more innovative carmaker.
The safety stumble could make it even harder for the Chevrolet Volt to compete with rival electric cars such as the Nissan Leaf. To contain the bad publicity after the fires, GM last year offered to buy back Volts from worried owners.
Adding the steel will spread the force of a crash over a larger area, says Mary Barra, GM's product development chief. Tests by GM and the government have shown that the repairs, to start in February, will prevent battery damage and coolant leaks.
"We have made the Volt even safer," says Mark Reuss, GM's North American president.
GM has done crash tests on four reinforced Volts and found that the fix worked. The National Highway Traffic Safety Administration also has crash-tested a Volt with the added steel.
"The preliminary results of the crash test indicate the remedy proposed by General Motors today should address the issue," the federal safety agency says.
The agency will monitor the crashed car for another week as it continues its investigation.
NHTSA critics have accused the agency of going easy on GM because the government still owns 26.5 per cent of the company's shares and the Obama administration has urged more sales of electric cars to end U.S. dependence on foreign oil.
GM nearly ran out of cash and needed a $49.5 billion government bailout to survive bankruptcy protection in 2009. The government took that stake in the company in exchange for the aid.
But Clarence Ditlow, executive director of the Center for Auto Safety, a watchdog group, says he sees no evidence of a government conspiracy, adding that NHTSA frequently lets automakers use safety campaigns when there should be recalls. He blames that on the agency having too few investigators to regulate such a large industry.
NHTSA spokeswoman Lynda Tran declined comment on the criticism.
NHTSA agreed to allow "safety campaigns" instead of recalls in the 1990s under pressure from the auto industry, Ditlow said. There is little difference between the two other than that NHTSA monitors recalls and makes sure more owners take their cars to dealers to have the repairs made, he says.
"Safety campaigns are a kinder, gentler form of a recall," he says. "It's trying to put a good name on a safety recall."
GM sold 7,671 Volts last year, falling short of its goal of 10,000. Its main competitor, Nissan's Leaf, sold 9,674. The Volt had its best month ever in December with 1,529 sales, but a GM executive conceded this week that the battery fires may have affected sales.
"There has been some uncertainty in the market," says Alan Batey, vice-president of GM's Chevrolet division. "We do believe that uncertainty will go away."
News of the fix helped GM stock. Shares rose $1.02, or nearly 5 per cent, to close at $22.17.
The Volt has a T-shaped, 400 pound (181 kilogram) battery pack that can power the car for about 35 miles (56 kilometres). After that, a small gasoline generator kicks in to run the electric motor. The car has a base price of about $40,000.
NHTSA began studying the Volt batteries after a test car caught fire last June. The fire broke out three weeks after a side-impact test.
At first, GM blamed NHTSA for the June fire, saying it should have drained the battery to prevent any fires after the test. But the company quickly retreated and said it never told NHTSA to drain the battery. GM executives also said there was no formal procedure in place to drain batteries after crashes involving owners.
NHTSA opened an investigation into the Volt's safety in November following that fire and two others that occurred after tests.
Now the company sends out a team to drain the batteries after being notified of a crash by its OnStar safety system.
Publicity about the fires touched off a massive effort by GM engineers to find the cause and fix the problems quickly. In December, GM CEO Dan Akerson said the company would buy back Volts from any owner who wasn't satisfied. Earlier, the company offered free loaner cars to Volt owners if they were concerned about safety.
So far, about 250 of the owners have asked for a loaner or a buyback.
Last month Akerson said GM wanted to fix the problem quickly to help its customers — and to help electric cars in general. "It's better to get it right now," he said.
Once parts for the repairs are available, GM plans to contact all Volt owners and advise them to set up an appointment for the work to be done. Reuss said the repairs should take two to three hours.
The company also will add steel in all North American Volts and European Opel Amperas produced in the future at a Detroit factory.