Most UPS units will charge even when they are switched off.
It has to be charged at some time because the battery will not last forever. When the battery is too low a UPS will shut off.
With normal use a unit like that is just always connected and only switches to battery when the power fails or has another fault that will trigger the switch to battery.
All of this should be automatic(charge when you have AC power and run on batteries when you do not).
Some units also allow monitoring the battery level and estimated time remaining on the computer.