Not sure if you can get this from Blueprints, but from C++ the OVRPlugin header file contains these functions which should give you access to what you want:
/// Gets the status of the system's battery or "Unknown" if there is none.
OVRP_EXPORT ovrpResult ovrp_GetSystemBatteryStatus2(ovrpBatteryStatus* systemBatteryStatus);
/// Gets the current available battery charge, ranging from 0 (empty) to 1 (full).
OVRP_EXPORT ovrpResult ovrp_GetSystemBatteryLevel2(float* systemBatteryLevel);
Hope this helps,