While the 740 does technically have an internal battery, it is not designed to allow normal usage for an extended period of time. Rather, AP advertises it as a "Built-In Emergency Battery", which allows the device to be operated long enough that it can be transferred to another power source if the primary source fails. For example, if its current power source fails (ships power goes offline, or the external battery it's plugged into goes dead) then you have a couple of minutes to grab a spare external battery out of your flight bag, unwrap the cord for it, and plug it into the iFly.
I'm not very familiar with the 5xx device (no longer listed for sale), but I think it was the only iFly device that's ever been offered with an internal battery that was intended to be the primary power supply for the device in normal use.