A small dry battery of zinc and ammonium chloride weighing 85 g will operate continuously through a $4-\Omega$ resistance for 450 min before its voltage falls below 0.75 V. The initial voltage is $1.60,$ and the effective voltage over the whole life of the battery is taken to be $1.00 .$ Theoretically, how many kilometers above the earth could this battery be raised by the energy delivered under these conditions?