The Chinese company behind a palm-sized 4G smartphone has admitted that the receiver performance is below expectations, in certain circumstances.
Unihertz says the Jelly phone battery has three days of work time and lasts seven days on standby.
But chief executive Stephen Xu has told the BBC that “heavy use” may reduce the life of the battery for three or four hours.
This included keeping the free wi-fi and Bluetooth switched on all the time, he said.
“If you use a phone a lot, even an iPhone can’t last a day,” Mr Xu said.
The Jelly battery should last about a day with “normal use,” he added.
He said that the problems reported by some Jelly users with GPS capabilities in a combination of hardware and software. Unihertz hoping to release an update soon, he said.
“[With the Jelly,] we’re trying to find a balance between large phones and small wearables,” Mr Xu said.
Stuart Miles, founder of the review website Pocket Lint, said that there was a market for small phones.
“As phones have got bigger and bigger, there is a push for people who just want something small to put in the pocket,” he said.
“This phone is clearly trying to tap into that supposed need, but I wonder if a smartwatch with a 3G call to offer the same, if not more functionality.”
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The Jelly of the phone, which Unihertz calls the world’s smallest 4G phone, was developed after its creator has raised more than $1.2 m (£0.9 m) in crowdfunding.
It is available in the UNITED kingdom, the UNITED states and in many other countries, but it is not compatible with the CDMA mobile standard, which means that it will not work with some mobile networks including Verizon and Sprint.
Unihertz also decided to stop the supply of caps to the device, which is supplied with a USB cable.
“We realized that, due to the different standards of the charger products, it is more difficult for us, so we’ve removed the caps,” said Mr Xu.
“I think that most people already have a plug.”