What is the ma output of the AC/USB? it should say on it. It may not be enough to charge the battery. If it is 5v and 500ma (aka .5 amp) then it is the same as a PC motherboard USB port and can be used to charge the battery. Actually if it is anything over 400mA it can probably charge the battery.
BTW I LOVE Rosewill RUC6180 and RUC6181 4-port 2amp AC to USB adapters. They can hold 4 assorted battery charger cables (or an end slot can hold a solid short battery charging adapter). I get mine at NewEgg - I think they now only have the RUC6181