It follows a vehicle fire originating in a recently side impact damaged Volt at the agency's storage yard last June. In November, the NHTSA conducted two similar tests and announced a full safety defect investigation after both battery units showed abnormal activity such as heating up and sparking as a result of the damage.

With no real-world data to work from, the NHTSA partnered with General Motors to conduct further testing, finding the damage had caused battery coolant to leak onto electrified components.

In response, General Motors developed a structural upgrade to protect the battery in side impact tests, and a system which monitors coolant levels for its built-in cooling system. Following four crash tests with no intrusion into the battery compartment, the manufacturer said it will fit the modifications to all newly built Volt and Ampera models, while existing cars will be retro-fitted in the near future.

The investigation has also allowed the NHTSA to develop guidance to help consumers, emergency services and towing companies work with severely damaged electric vehicles. 

In its concluding report, the NHTSA said: ‘The agency’s investigation has concluded that no discernible defect trend exists and that the vehicle modifications recently developed by General Motors reduce the potential for battery intrusion resulting from side impacts.

‘NHTSA remains unaware of any real-world crashes that have resulted in a battery-related fire involving the Chevy Volt or any other electric vehicle. NHTSA continues to believe that electric vehicles show great promise as a safe and fuel-efficient option for American drivers.’