The days of charging your iPhone every night could soon be over. British technology company Intelligent Energy has signed a deal with an unnamed smartphone maker to provide fuel cells for batteries - similar to those used in electric cars - that it claims will keep devices powered for more than a week.

The technology works by combining hydrogen and oxygen to produce electrical energy, with only heat and water emitted as by-products. As well as being extremely environmentally friendly, it is also very efficient. Intelligent Energy has not revealed the name of the smartphone maker it is working with to develop fuel cell batteries, simply describing it as an "emerging operator", but some experts are speculating that it could be Apple .

The two companies were rumoured to be working together as far back as 2014, and Intelligent Energy is known to have partnered with an “international electronics company” to purchase patents related to fuel cell technology.

Apple has also been trialling a "fuel cell system" to replace the old lithium-ion batteries found in its Macbooks - and has hinted that the innovative technology could be used in its range of phones and watches. Intelligent Energy already has a prototype smartphone with an embedded fuel cell, and it hopes that the new deal will result in the technology being licensed more widely.

"We believe embedding fuel cell technology into portable devices provides a solution to the current dilemma of battery life," said Intelligent Energy’s Julian Hughes." With consumers demanding more and more from their phones, battery innovation has not kept up. What we offer is a solution that is clean and efficient and means consumers could be truly mobile and free from the constraints of the grid."

While external batteries using fuel cell technology are already available, they tend to be expensive and bulky. Intelligent Energy recently won a gold Edison Award for its relatively small external charger called Upp - which is essentially a larger version of the firm's iPhone 6 fuel cell prototype. Should smartphone batteries last more than a day?