It takes an hour of pedalling to charge a phone completely
Two Kenyan students are hoping to market a device that allows bicycle riders to charge their mobile phones.
Jeremiah Murimi, 24, and Pascal Katana, 22, said they wanted their dynamo-powered "smart charger" to help people without electricity in rural areas.
"We both come from villages and we know the problems," Mr Murimi told the BBC.
People have to travel great distances to shops where they are charged $2 a time to power their phone, usually from a car battery or solar panel.
"The device is so small you can put it in your pocket with your phone while you are on your bike," said Mr Murimi.
It is estimated that some 17.5 million people out of Kenya's 38.5 million population own a mobile handset - up from 200,000 in 2000.
We took most of [the] items from a junk yard
Although similar devices already exist in other countries, they are not available in Kenya.
The two electrical engineering students from Nairobi University have been working on their own invention, which they are selling for 350 Kenyan shillings ($4.50) each, over the last few months during their university break.
In Kenya, bicycles are sold with a dynamo to be attached to the back wheel to power the lights.
The dynamo lead can be switched to plug into the charger instead, they explained.
Mr Katana explained it takes an hour of pedalling to fully charge a phone, about the same time it would if it were plugged into the mains electricity.
The BBC's Ruth Nesoba says after a short ride, the phone's battery display indicated that it was charging.
The cash-strapped students used old bits of electronic equipment for the project.
"We took most of [the] items from a junk yard - using bits from spoilt radios and spoilt televisions," said Mr Katana.
The dynamo is attached to the back wheel
Workers with bicycles at the campus were used as guinea pigs, including security guard David Nyangoro.
"I use a bicycle especially when I'm at home in the rural areas, where we travel a lot," he said.
"It's very expensive nowadays charging a phone. With the new charger I hope it will be more economical, as once you have bought it, things will be easier for you and no more expenses."
Mr Murimi says so far they have only made two chargers - but are making five more for people who have seen it demonstrated.
"And a non-government organisation in western Kenya wants 15 so they can test them out in rural areas to see how popular they prove," he said.
The two friends are about to start their fifth and final year at university in September.
"We are not planning to stop our studies," Mr Murimi said.
Kenya's National Council for Science and Technology has backed the project, and the students hope they will find a way of mass-producing the chargers.