The cells are 32 centimeters by 65 centimeter, slightly larger than a nine-volt battery, and easily fit in the palm of the hand. They are snapped into a case about the size of a smartphone for use, which outputs electricity through a USB connection.

A sample on the show floor produced some heat, but not enough to be uncomfortable to the touch.

Rohm, which aims to bring the cells to market from April of next year, says each one produces 5 watt hours of power, enough to charge a smartphone in about two hours. The company is working with the smaller firm Aquafairy on the technology.

"The cartridges will cost about the same as equivalent batteries on sale today," said Kazuhiro Tatsumi, a research manager at Aquafairy.

He declined to give a price for the reusable cases, but said they will likely be under 10,000 yen (US$130).