A network of clean, quiet, electric-powered buses may one day carry MBTA riders all across the region, but not until bus manufacturers figure out how to pack more miles into their batteries.
That’s the primary issue that transit officials discussed Monday as they reviewed the performance of five electric buses that began running on the Silver Line last year. While the batteries' charge is good for about 110 miles in nice weather, it is very sensitive to the temperature and falls precipitously in colder conditions.
During a 20-degree day, for example, the charge only lasts about 60 miles, where a Silver Line bus may travel as far as 170 miles in a day.
“We’re really constrained by the operating restrictions of the buses with the battery capacity they currently have,” chief engineer Erik Stoothoff said at Monday’s MBTA board meeting.
A related issue is that it takes upwards of six hours to charge the battery, complicating the process of getting them in and out of service. The MBTA’s hybrid buses, by contrast, take about 20 minutes to fuel, which lasts about 400 miles.
The batteries’ charge could be worse in even colder weather, but the T hasn’t had a chance to find out because Boston had such a mild winter last year, Stoothoff said.
None of this comes as a complete surprise. Battery issues in cold weather are well-documented, and since using a federal grant to buy the buses the MBTA has stressed they were being treated as a pilot program, to learn more about the technology and how it operates.
Still, state Transportation Secretary Stephanie Pollack said it was a “concern” that the buses, built by New Flyer, only lived up to their advertised mileage in good weather.
Matt Casale, an environmental advocate at the Public Interest Research Group, which has pushed the MBTA to adopt the technology, said the agency could still consider buying more electric buses to use on shorter routes.
“They’re not unexpected challenges," he said. "I don’t think we shouldn’t move forward with an electric bus program. Nobody’s asking the T to flip the entire fleet tomorrow.”
Stoothoff said battery technology has already improved significantly since the MBTA received the buses. The agency will develop long-term plans to eventually shift to electric buses, which could include some smaller purchases in the short term, he said.
But officials will wait for the technology to improve before replacing a large portion of its fleet.
“We’re only talking about timing here," Pollack said. "Nobody is saying we’re going to still be buying hybrids 20 years from now. But there’s a big difference between, ‘Do we buy enough in two years ... or do we wait five?’ And to me, that’s all about the ability of people to manufacture buses that we can put into service.”
Expanding the fleet would also require charging infrastructure and adequate electrical capacity across the MBTA’s aging bus maintenance facilities. The agency plans to build a new bus garage in Quincy that officials say will be designed to accommodate an eventual switch to electric buses.
A 2018 report ordered by the Baker administration on the future of Massachusetts transportation suggested that all buses purchased with state funds should be zero-emission vehicles by 2030. Batteries are not the only way to achieve the goal: overhead wires have long powered trolley buses in Cambridge. But the MBTA has primarily focused on battery power in recent years.