This pack contains four AAA size 1.2V Nickel-Metal Hydride Rechargeable Batteries (740mAh) suitable for olympus digital voice recorders. These batteries are charged in the BU-400 Charger available only in the Ni-MH Quick Charger and Battery Set (B-90SU). The charger can charge two AAA batteries at a time. The B-90SU kit comes with four AA size Nickel-Metal Hydrade batteries that are usable in most electronic devices. DM-520/DM-420/WS-510M users can also charge the batteries in the unit by connecting the device to a powered USB port.