(Bloomberg) -- Broken and malfunctioning chargers, erratic charging pricing and drivers of regular cars who park in spots reserved for electric vehicles are a significant obstacle to increasing EV sales, a study led by a Harvard Business School fellow has found.

Based on a decade of data obtained from EV drivers’ reviews of charging stations, the research found that charging stations in the US have an average reliability score of just 78%, meaning about one in five don’t work. That’s less reliable, on average, than regular gas stations, lead researcher Omar Asensio said.

“Imagine if you go to a traditional gas station and two out of 10 times the pumps are out of order,” Asensio said. “Consumers would revolt.”

Spotty coverage — or “charging deserts” — are another problem, with entire counties in US states such as Washington and Virginia not having a single public charger. Meanwhile, EV drivers also routinely watch internal combustion engine car drivers park in spots reserved for EV charging.

The research is bad news at a time when EV sales growth is slowing. BloombergNEF earlier this month slashed EV sales estimates and warned the auto industry is falling further off the track toward decarbonization. The reliability issues also come despite automakers like General Motors Co. and Stellantis NV investing heavily in charging infrastructure in the US.

In North America, drivers are often finding broken equipment because “no one’s maintaining these stations,” according to Asensio. This leads to “charge anxiety,” rather than the more commonly cited range anxiety that early EV adopters faced.

A lack of functioning chargers could become an even more pressing issue should EV sales rebound. 

S&P Global Mobility estimates there’s a need for around 1.2 million Level 2 chargers — or ones that are able to charge an EV from empty to 80% in as little as four hours — across the US by 2027. 

“The transition to a vehicle market dominated with electric vehicles will take years to fully develop, but it has begun,” S&P analyst Ian McIlravey said. “With the transition comes a need to evolve the public vehicle charging network and today’s charging infrastructure is insufficient to support a drastic increase in the number of EVs in operation.”

Asensio’s research was based on an examination of more than 1 million charging-station reviews by EV drivers across North America, Europe and Asia written over 10 years.

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