The carrier also touted a high-speed charging service for its smartphones, that is under development. External battery cases made for NEC Medias smartphones could be fully charged in just 10 minutes, then provided another 50 percent to the phone's original battery.

The batteries are based on lithium-titanate technology, which is quick-charging but can have lower capacity than the traditional lithium-ion batteries used in mobile phones.

DoCoMo hopes to eventually build external batteries that work with a wide range of phones, then set up quick-charging stations at convenience stores and other locations.