The Old EV Batteries Being Used to Make New EVs

Toyota is helping to build Mazdas. At least, its aging car batteries are.

Cooperation between rivals is nothing new in the car world. Toyota and Subaru have teamed up to build small sports cars and electric vehicles that are, underneath the skin and the logos, essentially the same. GM and Hyundai have signed a memo of understanding to share new vehicles and clean energy tech, while Honda has used GM’s Ultium platform as the basis of its Prologue EV.

In Japan, Toyota and Mazda now say they will work together to deploy Toyota’s Sweep Energy Storage System, a way to reuse old EV batteries. The “sweep” will combine all kinds of old batteries from electric cars and hybrids into a single unit that can store energy to help power Mazda’s Hiroshima car factory. It’s a clever and promising method to give those batteries a second life, where old car parts help to create new cars.

Energy storage systems are among the most interesting answers to the question of what to do with the forthcoming flood of old EV batteries. It’s true that recycling can recover many of the precious metals therein, and a new industry has arisen to do that work. But the process remains dirty and expensive. Stationary energy storage, meanwhile, is a way to extend a battery’s useful life rather than send it to the recycling yard.

Consider a unit from an older EV that has lost half its capacity, diminishing the vehicle’s range from a healthy 250 miles to a paltry 125. That would be an impractically small distance between charges for many drivers, but it doesn’t mean the battery is cooked. It takes a lot of energy to push a car that weighs several thousand pounds, so that old unit still can store plenty of kilowatt-hours for purposes other than propulsion.

Storage systems can use batteries, old or new, to save surplus solar energy during the day to be used overnight, or to stash backup energy that could be fed onto the grid to avoid blackouts in times of shortage. This application gives older batteries a less labor-intensive way to remain useful in their retirement years. The batteries can be daisy-chained together so that even older units with diminished performance can create ample energy storage.

At Mazda’s Hiroshima factory, Toyota’s system connects to 1,500 megawatts of solar capacity installed on the roof — the only power generation system in Japan run by a car company. The sweep battery, currently in a testing phase to see whether it can interface seamlessly with the plant, would help balance out the supply and demand of the renewable energy coming from upstairs.

The real key to Toyota’s system is its versatility. Most battery backup systems, like Tesla’s Powerwall, use identical batteries in the creation of a whole, which cuts down the electrical complexity. The sweep system, however, can use a mishmash of batteries from different vehicles with different capacities or battery chemistries. The technique that gives the “sweep” system its name is the software’s ability to sweep across all the batteries in the series and turn the power supply from any of them off and on within microseconds in order to control the energy output of the whole system.

It matters not whether the units came from a new all-electric car or an old Toyota Prius hybrid. Whether they are lithium-ion, nickel-metal-hydride, or lead-acid makes no difference. As CarBuzz explains, it’s as if you could combine all the extra batteries in your junk drawer and all the half-used ones around the house to seamlessly create one big unit that taps into all their energy. Even old batteries salvaged from car accidents can be used if the batteries themselves are undamaged. Toyota says it integrates the old batteries’ original inverters (the devices that transform DC into AC power) into the system, negating the need to build a new one for the system as a whole.

The giant automaker has already proven the sweep concept: In 2022, it built a sweep system for the Japanese energy giant Jera that, with its battery powers combined, could store more than 1,200 kilowatt-hours. (For a comparison, the battery in the long-range version of the current Tesla Model Y can store 75 kilowatt-hours, while the average American home uses about 900 kilowatt-hours per month.)

Toyota will need to prove that the sweep can scale up to the level of a car factory, and larger. If it can, then it’s a promising way for yesterday’s batteries to help stabilize and manage the green energy of tomorrow. Not bad for a geriatric power pack.