Colgeek is spot on. The only UPS's that I have direct experience with are APC brand, and the professional models (room sized) I used to maintain.
An uninterrupted power supply does more than charge a battery. First the normal house AC is converted to direct current. Then a portion of that direct current is siphoned off to charge the batteries (Trickle Charge). The rest is converted back into AC and used to power your computer, or whatever else is plugged into it. Only some of the UPS ‘s outputs are battery backed up, but all are conditioned by the AC/DC/AC conversion.
That’s what makes them so expensive, and awesome. Brown outs, surges and black outs can’t damage your equipment because the ups is conditioning your power and providing a battery backup so that you have time to shut down your computer properly, in the case of an extended black out.
Back to the tickle charge. Yes. It is necessary to constantly charge the UPS because most power interruptions that place a demand on the battery are so quick you might not even notice them. If your house power browns down (goes below 110VAC, but does not fail, your ups will discharge the battery to compensate. It may or may not inform you about this, depending on the make and model. It does this as part of the power conditioning as well.