The normal fire department responded to a fire at the Rivian manufacturing plant on Saturday morning. It was at least the third fire at the factory in the past seven months.

In Saturday’s incident, a battery went into thermal runaway – when a battery cell overheats and catches fire – in a battery test area on the southwest side of the plant. This is where the batteries are built for Rivian vehicles, and it forced the evacuation of the battery assembly area.

Firefighters moved the battery outside after extinguishing it and said they continued to cool it with water until it was handed over to Rivian engineers for investigation and dismantling.

The exact cause of the fire is under investigation, but the battery was in a repair area and was being tested when it ignited, the fire department said. Thermal runaway occurs when a battery cell overheats and catches fire. The lit cell releases more heat to adjacent cells in a self-sustaining chain reaction.

The fire did not involve any vehicle or production equipment. The only damage was to the battery, carrier and equipment in the test cabin, according to firefighters.

The report came in around 10:38 a.m. and firefighters said they had returned the building to Rivian’s control by 2 p.m. There are no casualties.

The normal fire department has responded to at least two other fires at the Rivian plant since production began last fall. In February, a vehicle caught fire inside the plant, but the sprinkler system prevented it from spreading. In October 2021, a small fire broke out in the automated battery assembly area.