MoveHub class has field voltage to subscribe to battery voltage status. Callback accepts single parameter with current value. The range of values is float between 0 and 1.0. Every time data is received, value is also written into last_value field of Voltage object. Values less than 0.2 are known as lowest values, when unit turns off.
from pylgbst.hub import MoveHub, Voltage
def callback(value):
print("Voltage: %s" % value)
hub = MoveHub()
print ("Value L: " % hub.voltage.get_sensor_data(Voltage.VOLTAGE_L))
print ("Value S: " % hub.voltage.get_sensor_data(Voltage.VOLTAGE_S))