The user may want the battery charged asap but the intelligent charger might decide that now isn't the time for that and really annoy you. If the battery requires complicated charging and discharging schedule, like old ones that had to be discharged before charging, or periodically, then this has to be controlled by the user because you won't want to wake up one morning and find your charger has intelligently flattened the battery for you, or failed to charge it because it's time to let it drain all the way down.
A toggle "prepare off-grid use", nothing more complicated than that.
You could even try some implicit haptic UI ideas like having two power ports, one for operating (that only charges when the battery is dangerously low) and one for complete charging (that also runs the device, but it's clearly designated as the "charging port"). With USB power delivery that would not even waste case surface, it would be an entirely internal change to make one more USB-C power capable.
