A battery exploded this week while fire marshals were arresting an e-bike retailer, underscoring the lethal threat posed by lithium-ion batteries used to power e-bikes, according to the FDNY.

FDNY officials said the retailer, Wei Chen of Wilson’s E-Bike Shop, has been making his own uncertified batteries, called “Frankenstein batteries,” which are particularly flammable and hard to extinguish. No one was injured in Monday’s small fire.

Chen is the second person to be arrested under the city’s recent crackdown on e-bike retailers selling cheap and refurbished batteries to delivery workers. The batteries can explode while charging, making them particularly dangerous when stored in residential buildings. Eighteen people died last year in battery fires in the city, contributing to the highest number of fire deaths in nearly two decades, according to FDNY data.

Chen was criminally charged with reckless endangerment on Monday and was issued summonses for a range of violations, including illegal modification of batteries using secondhand parts and illegal storage of gasoline, the FDNY said.

Reached at his Forest Hills shop on Wednesday, Chen said: “We have not done anything. ”

The FDNY said it found 80 e-bikes and other micromobility devices, some of which were used and being repaired, plus two toolboxes with 282 lithium-ion cells that can be used to build uncertified battery packs. As the cells were being packed into hazmat drums, one of the cells exploded and caught fire, according to the FDNY.

The bike shop had been inspected twice in recent months and cited for several violations. Fire Commissioner Laura Kavanagh had previously said a battery cell caught fire and had to be extinguished during a February inspection at the same shop.

“Unregulated, tampered with, and uncertified batteries are extremely dangerous and deadly,” Kavanagh said at the time. “They kill people, they have killed people and they will kill more people if businesses continue to operate in this manner.”