Apple is pinning the blame for battery issues with some of its iPhone 6s smartphones on an error in the manufacturing process that saw a component exposed to ambient air for longer than intended.
Late last month Apple said it would replace batteries in affected iPhones 6s' after complaints of the devices shutting down without warning.
Customers affected by the problem reported that their devices would switch off unexpectedly once the battery charge fell to between 50 and 60 percent.
On Friday the tech giant revealed a component in affected batteries had been “exposed to controlled ambient air longer than it should have before being assembled into battery packs".
“As a result, these batteries degrade faster than a normal battery and cause unexpected shutdowns to occur,” it said.
“We looked for any other factors that could cause an iPhone to shut down unexpectedy. After intensive investigations, no new factors have been identified. We will continue to monitor and analyse customer reports.”
It said affected phones were made between September and October last year and came from a "limited" range of serial numbers. Apple did not detail how many devices were impacted by the battery issue.
iPhone 6s owners can find out if their device is affected by checking their serial number on Apple's support page.