Does anybody know the cut off voltage of an X990?

[BatteryCharger]

I've managed to kill both of the 4.5ah 12v NiMh battery packs for my X990. Retail price on a new one is $118. :shakehead Does anybody know what the cut off voltage is for the X990 ballast? I want to replace the batteries with an 11.1v lithium ion pack, but will the X990 be able to drain them all the way down to 9 volts?

BTW - has anybody else lithium-ized an X990?