Proposition: Electric cars will remain mostly niche products until they have a range of 800 kilometers, or roughly 500 miles, with an affordable battery.

That’s as far as most people would want to drive in a day, and then they have all night to recharge.

That’s how we came up with a figure of 800 km—or a nice round 500 miles—as the goal for our R&D project, Battery 500. It began in 2009 at the IBM Almaden Research Center, in San Jose, Calif., and has grown since then into a multinational partnership with commercial and academic participants in Europe, Asia, and the United States. It is based on metal-air technology, which packs far more energy into a battery of a given mass than today’s state-of-the-art technology, the lithium-ion battery. We are still years away from commercialization, but we have made enough progress to predict that these batteries could be used in cars in the foreseeable future. Why are we so confident? Read on.

Electric motors are ideally suited for powering cars. They’re lightweight and extremely powerful, they achieve efficiencies in excess of 90 percent, they don’t need complex transmissions, and they churn out torque in just the right way, providing full rotational force starting with zero rpms. Internal-combustion engines, by contrast, don’t produce high torque until they’re spinning at thousands of rpms.

But even though they’re propelled by a near-ideal mechanism, electric cars have a huge drawback, which is the low energy content of the batteries. Gasoline packs about 13,000 watt-hours per kilogram; the best production lithium-ion cells store only about 250 Wh/kg. Add the mass of the ancillary battery equipment—including the bus bars, cooling system, and battery management system—and the energy density of the entire system drops by half, giving the batteries a pitiful 1 percent of the raw energy density of gasoline.

This huge gap between the energy densities of gasoline and batteries seemed to make it impossible to build competitive electric cars, but the success of theTesla Model S has shown that it can be done. One major factor in favor of the electric car is the high efficiency with which it converts battery power to motive power at the wheels—about six times as efficiently as the average for gasoline-fueled cars in the United States. Also, electric car makers put the biggest, heaviest battery they can reasonably fit into their designs. Even so, the ranges fall far short of the 500-mile target. The upshot is that electric-car batteries need to attain at least twice the energy density of Li-ion cells to achieve a range of 800 km.