You can't. It's only visible when you touch the satellite bars.
All TT units are really meant to be used with the included car chargers anyway. Of course, some do use them on battery power only and in 'real world' situations, the best anybody can hope for is usually in the 2-2.5 hr range, depending upon how your batttery savings preferences are set, your brightness settings, whether bluetooth is enabled (for models that have it), etc.
But, as said, wiser to use the unit with a charger.