Federal regulators have closed their investigation into the Chevrolet Volt, saying they are satisfied with the steps General Motors has taken to protect the car's lithium-ion battery and minimize the risk of a fire in the days and weeks after a severe crash.

The National Highway Traffic Safety Administration posted an explanation and summary of its inquiry on Friday and announced the conclusion of the investigation it launched Nov. 25.

"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," the feds said in a statement.

The statement adds, "Based on the available data, NHTSA does not believe that Chevy Volts or other electric vehicles pose a greater risk of fire than gasoline-powered vehicles."

The findings vindicate General Motors, which always argued the Chevrolet Volt is safe, and electric vehicle advocates who argued that the inquiry was much ado about very little.

"The National Highway Traffic Safety Administration’s decision to close its investigation is consistent with the results of our internal testing and assessment," GM said in a statement. "The engineering enhancements that General Motors announced on Jan. 5, 2012 will provide additional protection for the battery, minimizing the risk of a post-crash fire in the days and weeks after a severe crash and rollover. The voluntary action that GM is taking is intended to make a safe vehicle even safer."

The issue was never the car, but its 435-pound battery and what to do with it after a crash.

The problem came to light in June when a Volt caught fire three weeks after a side-impact crash test. The fire was caused by a coolant leak that occurred when the battery pack was punctured. The coolant leaked, eventually causing a short circuit, GM said.

Investigators conducted three subsequent tests on Volt battery packs in November., investigators intentionally damaged the 16-kilowatt-hour batteries and ruptured their coolant lines. A test on Nov. 16 did not result in a fire. A test on Nov. 17 resulted in a temporary increase in the pack’s temperature. During a test on Nov. 18, the pack began to emit smoke and sparks after being rotated 180 degrees within hours of the impact test. The pack tested on Nov. 17 eventually caught fire three days after the test.

To address the issue, GM said it will revise the structure surrounding the Chevrolet Volt’s battery pack and modify its coolant system, changes the automaker says will better protect the pack and reduce the risk of an electrical fire.

The mods amount to reinforcing the underbody of the car around the pack, installing a sensor to monitor battery coolant levels and affixing a bracket to prevent overfilling the battery coolant reservoirs. The changes add about three pounds to the weight of the car.

The feds said they are not aware of any Volts catching fire in real-world collisions.

"NHTSA continues to believe that electric vehicles show great promise as a safe and fuel-efficient option for American drivers," the NHTSA said.

General Motors sold 7,671 Volts last year, including 1,529 in December.

Photo: General Motors