A wall wart slow charge is low enough on the current that the battery can take being overcharged and not be damaged unless it's really excessive. That said, there's no need to charge your batteries as long as you are. A 5-6 hour charge before flying should get you up to full capacity if your wall wart is at a C/10 rate for your battery. C/10 means the current of the charger is 1/10 the capacity of your battery.