The battery bar is still there in Joules mode. In power mode, in the space just under the battery bar, it also shows the written voltage remaining of the battery. That area shows your set temperature in Joules mode. When you charge it via USB, the bar graph only shows a lightning bolt over the battery outline, so if you're in Joules mode there is nothing to tell you what the voltage is during the charging cycle. If you charge it in power mode you can actually see the voltage reading rise as the charge progresses.
Thanks for the detailed answer! Another question, if you leave your Ni200 build atty on, and change from temp mode to wattage mode and back to temp mode again, do you have to lock in the atty resistance again?