British battery sensor firm Metis Engineering said on Tuesday it has developed a sensor to measure electric vehicle (EV) battery health that should allow insurers to properly assess damage after accidents and allow them to avoid unnecessary write-offs.

For many EVs, there has so far been no way to repair or assess even slightly damaged battery packs after accidents, forcing insurance companies to write off low-mileage cars - leading to higher premiums and undercutting gains from going electric.

As well as detecting thermal runaway - a rapid and unstoppable increase in temperature that leads to fires in EVs - Metis' Cell Guard sensor contains an accelerometer that can detect the duration and severity of collisions even while the car is switched off, CEO Joe Holdsworth told Reuters.

The sensor can also check for venting, where damaged batteries leak gas that cause explosions.

Bristol-based Metis is working with around 160 carmakers and battery storage companies on testing the matchbox-sized sensor, Holdsworth said. He declined to disclose names.

He added that a major insurance company is using the sensor for crash tests to better understand the impact on batteries.

Depending on how the sensor is configured within an EV, third parties should be able to access the sensor's data to see how badly a battery is damaged - which would address a major complaint from insurers that they lack access to data on an EV battery's health after an accident.

Holdsworth said the sensor could also help address concerns over used EVs.

"The unknown risk of poor EV battery health is one of the biggest and most significant barriers hindering the growth of the used EV market," he said.

The company has production capacity of up to 1,000 units daily, which could be easily expanded, he added.

(Reporting by Nick Carey, editing by Ed Osmond)