Yes, you can buy a bigger battery pack. But you don't need to. You can fly at 4.6 volts on the meter. Make sure the meter you're using has a "load" - usually about 300 mA. Are you using a device designed to measure voltage "under load"? A typical 600 mAh pack should give you at least 60 minutes of flying for a trainer type of airplane (4 or 5 servos) on a .46 powered glow plane.