More than 100 users say new software is causing Fitbits to malfunction or stop working altogether.
More than 100 Fitbit users say a software update issued by the fitness-tracker firm in December 2023 has left their devices unusable.
The Fitbit Charge 5 update was meant to add new features and fix bugs, but many users have taken to a Fitbit forum to list the problems it has caused.
One of the complaints is that batteries drain rapidly – a problem one user said had rendered their device “useless”.
The BBC has approached Google, which owns Fitbit, for comment.
“Basically, it’s useless now, the battery’s dead,” one user told the BBC.
Dean, in Essex – who did not want his full name made public – blamed the latest software update, saying previously his device was “working really well” and was “easily” able to last seven days per charge.
He said there was clearly a wider problem.
“I don’t really see why hundreds of other people would be having the same problem after installing the update if it wasn’t, but [Fitbit] is going completely quiet and not responding.”
Fitbit – which was founded in California – helped pioneer the fitness tracker market. It launched its first device in 2009, and has sold more than 120 million units worldwide since then.
It was bought by Google in 2019 for $2.1bn (£1.66bn).
But the wearable tech market has been challenged in recent years by the increasing capabilities of smartphones.
Earlier this month, Google laid off hundreds of staff at Fitbit, with co-founders James Park and Eric Friedman reportedly among those to leave.
Dean said the issue arose in his device immediately after updating it.
“I noticed the battery started discharging immediately – it wouldn’t hold its charge,” he said.
“The battery was completely dead. I contacted customer services for the online chat, and all they would do is talk me through factory-resetting the device, which wouldn’t help.
“Then I logged on to the forums and asked for help there and found that other people were having the problem and it was completely being ignored by Fitbit.”
Fitbit’s forums are full of similar complaints, some made only this week.
One person said their Charge 5 went “from perfect to non-functional overnight”, while another said in a post made on Christmas Day that the update caused their device to be “completely bricked”.
Others have criticised the “frustrating” customer service, which offered “no solution other than to buy a new Fitbit”.
One customer has demanded Google “be more accountable”, saying they intend to switch to a device provided by a rival firm.
“Everyone who has bought a Fitbit Charge 5 anywhere should be able to return these units without question and receive an upgrade to keep faith,” they said.
“The company is big enough to absorb the cost.”
Google has yet to respond.
Additional reporting by Kris Bramwell.