Power your phone with... hydrogen?
Anyone that has used a mobile phone will know the dreaded feeling when your battery is running low.
But for those without access to a power socket and charger - keeping a phone in working order is a much more complex task.
One answer, particularly in the developing world, may be small hydrogen fuel cells.
The BBC World Service's technology correspondent Mark Gregory met Henri Winand, chief executive of Intelligent Energy, a company that specialises in the area - and manufactures its own hydrogen-powered device, the Upp.
- From the section Technology