Retired electric car batteries are getting a second job. Companies are turning them into grid energy storage.

EV batteries are usually retired when their capacity drops to 70 to 80 percent. That is too low for drivers. But it is plenty for stationary storage, where the battery does not have to move a car.

The industry had a busy June. B2U Storage Solutions said it would repurpose used batteries from Waymo robotaxis for grid storage. Redwood Materials unveiled a plan to combine about 100 used General Motors batteries to supply 1.5 megawatts to a GM plant in Michigan. Moment Energy finished building what it calls the world’s largest battery-repurposing facility.

The idea solves two problems at once. Grid operators need storage for excess renewable energy. Carmakers need somewhere for old batteries to go.

“Batteries retired with 70 to 80 percent of their original capacity can be excellent candidates for grid storage,” Stanford professor Simona Onori told IEEE Spectrum. Used packs work best as backup power that only runs occasionally. They are a poor fit for jobs that need constant fast charging.

The first mass-market EVs arrived in the early 2010s. Their batteries are now retiring in growing numbers.