GM cooperated fully with NHTSA during the testing and analysis period following the May event, said Mary Barra, GM senior vice president of Global Product Development. Based on this work, GM determined the fire was the result of a minor intrusion from a portion of the vehicle into a side section of the battery pack.  This intrusion resulted in a small coolant leak inside the battery of approximately 50 ml. 

As part of NHTSA’s test procedure, the vehicle was put through a “slow roll,” Barra said, where it’s rotated at 90 degree increments, holding in each position for about five minutes. During the slow roll, an additional one liter of coolant leaked. With the vehicle in the 180° position (upside down), the coolant came in contact with the printed circuit board electronics at the top of the battery pack. Three weeks later this condition, in combination with a charged battery, led to electrical activity that resulted in the post-crash fire.

The modifications:

• Strengthen an existing portion of the Volt’s vehicle safety structure to further protect the battery pack in a severe side collision.

• Add a sensor in the reservoir of the battery coolant system to monitor coolant levels.

• Add a tamper-resistant bracket to the top of the battery coolant reservoir to help prevent potential coolant overfill.

GM conducted four successful crash tests between 9-21 December 2011 of Volts with the structural enhancement. The enhancement performed as intended. There was no intrusion into the battery pack and no coolant leakage in any of the tests.

I want to be clear that today’s actions have nothing to do with the battery pack itself.  None of these changes will touch the battery cell or pack.  As a result we will not change any part of the manufacturing process at our Brownstown, Michigan, battery pack assembly plant. We have tested the Volt’s battery system for more than 285,000 hours, or 25 years, of operation.

It’s important to note, the battery cell design used in the Volt was not the cause of the incidents that prompted the investigation. We’re confident in the robustness, quality and safety of the cell chemistry used in the Volt battery, which is supplied by LG Chem.  Before we chose LG as our cell supplier, we put their battery through extensive abuse testing, including mutilation, puncture and overcharge scenarios.  We took the cell pouch and literally twisted it, crushed it and even punched nails through it.  We’re as confident as ever that the cell design is among the safest on the market.

We’ve also seen a lot of speculation regarding the Volt battery’s liquid cooling system.  Early in development we decided to use liquid cooling because of the benefit it provides for performance and overall battery life.  It’s why we were able to provide our customers with what was the longest Electric Vehicle battery warranty when it was announced—eight-years or 100,000 miles. Key competitors have now followed our lead and are offering similar battery warranties. We still believe liquid cooling is the best option for the Volt.

—Mary Barra

GM will conduct a Customer Satisfaction Program to further protect the Volt battery from the possibility of an electrical fire occurring days or weeks after a severe side crash. Volt customers will be individually notified when the modifications are available for their vehicles. The enhancements are being incorporated into the Volt manufacturing process as production resumes this month.

The Volt is a Top Safety Pick by the Insurance Institute for Highway Safety and has earned other safety awards from key third-party organizations. Through the first 11 months of 2011, Volt owners accumulated nearly 20 million miles without an incident similar to the results in the NHTSA tests.