No one was injured in the explosion in the 4800-block of North Broadway Street, officials said. Traffic was blocked around the building for a time, though Broadway has since reopened. The post office remains locked and closed. No one is being allowed in or out.
The box was initially reported as a suspicious package, drawing a hazmat response from Chicago police and firefighters.
It was not clear if the batteries had been sent through the mail or dropped off at the post office.
The USPS Inspection Service is leading the investigation.