What do you think about this battery from Batterygeek? Specs: 130 Wh 16 or 19 volts output 1.7 lbs This should give 4.5 times the normal operating time. Should be possible to watch demanding movies for about 7 hours, or do less demanding things for 18 hours. I'm not sure if the voltage is too high or within limits. The Air magsafe seems to output 14.5 volts.