These are your standard 1.5V AA alkaline batteries from Panasonic.
Experiential Robotics Platform (XRP) Kit - Beta - KIT-22230 - SparkFun Electronics
Battery Technologies - SparkFun Learn
Efficiently charge your AA batteries with this Panasonic AA battery charger. It takes up to four AA NiMH batteries and can charge them in as little as seven hours, and you can also use AAA batteries with this charger should the need arise. The Panasonic AA battery charger detects defective batteries and nonrechargeables to prevent accidents.
Panasonic K-KJ17KHCA4A Eneloop Pro Individual Cell Battery Charger with 4 AA Ni-MH Rechargeable Batteries, 4 Pack
Battery Holder - 4xAA Square from MindKits New Zealand
Getting Started with the MyoWare® 2.0 Muscle Sensor Ecosystem - SparkFun Learn
Panasonic P-P201PA/1B Type 5 Cordless Phone Battery : Electronics
Description Includes Documents The Experiential Robotics Platform (XRP) is a hands-on, comprehensive robotics platform and includes everything needed
Experiential Robotics Platform (XRP) Kit - Beta
9V Alkaline Battery - PRT-10218 - SparkFun Electronics
Alkaline Batteries Panasonic Industrial Devices
Panasonic Alkaline Battery - AA from MindKits New Zealand