Passengers had to be evacuated from the Line A train after the incident near San Giovanni metro station. Nobody was hurt in the incident.
But the line was then closed between San Giovanni and Ottaviano – a stretch of 10 stops which crosses the city centre.
Atac, the company that manages all public transport in Rome, quickly laid on replacement buses but passengers were angry at the disruption – which happened at around 8.20am at the height of rush-hour.
“Don't dare go on strike this week!” Tweeted one irate traveller, directing her anger towards the staff of Atac, who have exacerbated structural problems in the city all summer by calling frequent strikes.
— Sara Stefanini (@SaraStefanini88) September 28, 2015
“Rome dreams of the Olympics but can't even maintain its metro,” wrote another disappointed resident.
— Rosario Pipolo (@rosariopipolo) September 28, 2015
“We don't know how this happened,” Rome transport assessor Stefano Esposito told Il Messaggero.
“The battery pack is about the size of a chest of draws – either there was some kind of structural failure or someone left the battery pack compartment open after maintenance.”
This is the latest in a series of incidents to blight Rome’s metro network in recent months.
In early June, two trains collided in a tunnel on Line B, injuring 21 people. Less than a week later, a five-year-old boy died after falling into the shaft of a faulty lift at Furio Camillio stop, on Line A.
And in July passengers experienced a terrifying journey when a crowded train drove between two stops with the doors wide open.
Meanwhile, four students became trapped in a metro station earlier this month after taking the last train home, only to find the station closed.