As U.S. gasoline prices hover above $4 per gallon and automakers race to make plug-in hybrids available in 2010, it’s no wonder that green-car enthusiasts have turned to after-market kits to convert their hybrids into plug-ins sooner.

The kits include additional batteries and plugs that allow drivers to replace some fuel with electricity from a standard wall outlet, getting more than 100 miles per gallon. It’s an alluring proposition.

But the road to plug-in hybrids hasn’t been easy, and now conversion companies may have run into another bump.

Hybrids Plus this week said a converted Toyota Prius had caught fire, causing no injuries, but “significant damage” to the car.

According to a Cooperative Research Network report, the fire – which happened June 7 – destroyed the car.

Boulder, Colo.-based Hybrids Plus has advised all of its conversion owners to stop driving the vehicles until further information is available. According to the press release, forensic examinations have not been able to conclusively identify the cause of the fire, but established that the battery cells – which, according to plug-in advocacy group came from A123Systems – were not the reason.

The company said it has begun inspecting and upgrading all of its systems to eliminate potential concerns, and would upgrade all its customers’ systems for free.

It isn’t the first instance of fires in conversions.

CalCars last month reported a failure that resulted in a meltdown of the original nickel-metal-hydride battery in the world’s first Prius plug-in hybrid conversion.

The possibility of plug-in hybrid fires have been concern ever since a series of laptop battery fires caught public attention – and led to a recall of an unprecedented 4.1 million Dell laptops with Sony batteries – a couple of years ago.

Even though neither of these fires apparently were caused by lithium-ion batteries, the news is likely to raise some concern about plug-in hybrids from consumers and automakers, said Mike Omotoso, senior manager of global powertrain research at JD Power and Associates.

“This shows [companies] need more time for testing the viability of these vehicles before they can be commercialized,” he said. “This is a good example of why some manufacturers, like Honda, have been holding off on making plug-in hybrids. It’s clear it’s not just as simple as connecting a cable to your battery and plugging it into your wall. If that were the case, it would have been done by the major manufacturers already.”

He emphasized that these fires don’t necessarily mean that plug-in hybrids are unsafe, but said they could raise the perception of a safety issue, which could impact their commercialization.

“There’s a difference between someone doing an after-market conversion in a garage, versus a manufacturer making a plug-in hybrid from the get-go,” he said. “If there are more of these examples, even if it’s just some guy in a garage, people will think, ‘Maybe plug-in hybrids aren’t the way to go because they are too unsafe.’ ”

In a newsletter released Wednesday night, indicated similar concerns.

“For several years, some opinion leaders from automakers, utilities and national labs have expressed their fears that ‘one bad accident’ could set back the progress of [plug-in hybrids],” founder Felix Kramer wrote. “We have agreed that safety must be top priority. … We hope that this and other incidents will lead to far greater emphasis on safety as well as full and rapid disclosure of incidents.”

He added that the nonprofit is aware that conversions by small companies and individuals never could be as well-designed as those by large carmakers, which is why it has encouraged automakers to bring plug-in hybrids to the market more quickly.

“The fact that carmakers can build better and safer PHEVs is self-evident, but the demand is so great that individuals and companies continue to bang down the doors of the suppliers of conversions,” he said. “We’re all impatient for the great transition to electrification of transportation to begin. The longer we have to wait, the larger will be the trend toward third-party conversions, for better or for worse.”

Still, he argued, it’s important to keep the danger in perspective. After all, the cars we already drive every day use a highly explosive fuel that could be set off by a stray spark or catch fire in accidents, he wrote.

According to the National Fire Protection Association, U.S. vehicle fires in 2006 totaled 250,000, causing 445 deaths, 1,075 injuries and $982 million in property damage.