As I recall, that is a single byte, where each bit represents the status of each of the 8 possible wireless stations receivable by the console, and a 1 in the bit means the battery in that station is low. So 0 means all 8 stations are OK, 1 means station ID 1 battery is low, 2 means station 2 is low, 3 means stations 1 and 2 are low, etc.