Proterra’s other key innovation is building its buses out of composite materials, saving thousands of pounds to offset the weight of the batteries and make the buses more efficient.

A similar fast-charging system has been developed by Sinautec Automobile Technologies, based in Arlington, Virginia, and its Chinese partner, Shanghai Aowei Technology Development Company. Their buses use ultracapacitors that can be charged even more quickly than the lithium-titanate batteries. But they only hold enough charge to go several city blocks, requiring charging stations at many bus stops along a route. The Proterra system could allow buses to charge just once per route, reducing the number of chargers needed (which it says cost about $50,000 apiece). The Chinese automaker BYD is also marketing fast-charging buses, but its system requires 30 minutes to charge to 50 percent capacity.

The high up-front cost of electric buses continues to limit their appeal, even to transit agencies in the United States that can get the federal government to cover 80 percent of the cost of new buses, says Bill Van Amburg, senior vice president at CalStart, an organization that promotes heavy-duty hybrid and electric vehicles. Another problem is that new chemistries like lithium titanate have yet to be proved to last in buses. Finally, Van Amburg says, transit agencies, which are used to buying buses that can ply any of the routes on their system, would probably need to make some adjustments to accommodate buses that have to charge one or more times per route.

So far Proterra has made about 10 buses, and these are in use by transit agencies in states including California and Texas. The company plans  to use its new funding to scale up production capacity and drive down costs.