Capacities from 700mAh to 2100mAh. In particular, the TP13204 (4-cell, 14.8V, $72.90) or the TP13203 (3-cell, 11.3V, $54.90) may be good fits for their size/capacity/voltage. ThunderPower

($99 ea.) A high capacity (2200mAh) LiIon battery from the Aibo. There's a matching $60 charger. Overall dimensions 2.83 x 2.12 x 0.83 inches (W x H x D).