The engines turn generators which supply the electrical power to the aircraft, including a battery charger that keeps the battery at 100%. (Normally, power coming "from the battery" on the diagrams, is actually coming from the battery charger or another component like a Transformer-Rectifier converting AC to DC current. The battery is very rarely actually being drained in flight.)
If you had an aircraft battery & went to deplete it with USB devices, it would take a lot of them & a really long time. Aircraft batteries store power way beyond what our normal cell phones & iPads do, and can power all the emergency systems on the jet for an hour or more (radios, INS, flight instruments, minimal lights, GPS, flight management system, etc). Of course, if you're down to just the battery, in-flight entertainment is NOT one of the systems being powered!