show battery levels

def cmd_bat(self, args):
        '''show battery levels'''
        print("Flight battery:   %u%%" % self.battery_level)
        if self.settings.numcells != 0:
            print("%.2f V/cell for %u cells - approx %u%%" % (self.per_cell,
                                                              self.settings.numcells,
                                                              self.vcell_to_battery_percent(self.per_cell)))