Sorry to disagree Steve. The voltage out of an UNregulated battery charger will be somewhere near 17/18 volts, some are higher. This would possibly 'cook the 12 volt fan', because of the light load. But, It relies on the characteristics of the battery/load, to limit the current/voltage.
HTH.
L.
ps. If you have 240 avail., why not use a 240/9volts @ 500 mA plug pack????? (probably enough voltage, speed wise). L.