WARREN, Michigan - General Motors has "enhanced" its controversial Chevrolet Volt with changes to the vehicle structure and battery coolant system.The reason? To further protect the battery from an electrical fire days - perhaps weeks - after a severe crash and in response to a US National Highway Traffic Safety Administration preliminary evaluation of post-crash battery performance.'ALWAYS SAFE TO DRIVE'The NHTSA opened the evaluation on November 25, 2011 after a severe-impact lab test on a battery pack resulted in an electrical fire six days later. The test was conducted to reproduce coolant leak that occurred in a full-scale vehicle crash test in May, 2011 that resulted in an electrical fire three WEEKS later.Through the first 11 months of 2011, Volt owners accumulated nearly 20-million miles without an incident.Mary Barra, GM's senior vice-president for global product development, said: “The Volt has always been safe to drive. Now we will go the extra mile to ensure our customers’ peace of mind after a severe crash."The modifications will strengthen an existing portion of the Volt’s vehicle safety structure to further protect the battery pack in a severe side collision, add a coolant level sensor in the battery coolant reservoir and add a tamper-resistant bracket to the top of the coolant reservoir to help prevent over-filling.'AMONG THE SAFEST'Barra added: "We've tested the Volt’s battery system for more than 285 000 hours, or 25 years, of operation. We’re as confident as ever that the cell design is among the safest on the market."The modifications will be applied to all Volts, including those sold in Europe.