Specs for Ezee sprint:

Electric drive system:
Maximum Speed : 20 mph
Range: Up to 30 miles (with pedaling)
Weight : 65 lbs
Motor: Brushless Servo motor 700 watt peak with a continuous rating of 350 watt with planetary gears.
Controller: PWM, digital, self diagnostic indicator, overload protection 16 amps, low voltage protection - 31.5 V
Battery: 36V 9Ah NiMH battery, or 36V 10 Ah Lithium ion battery, (Li-ion)
Charger: Charge current 2 amps., maximum charge time 5.5 hours

I'd hook up a voltmeter and see whats really going on. Check the voltage of the batteries before/during/after charging and riding etc... Check actual cutoff voltage also when bike cuts out. They claim its 31.5 above. 36v at 9ah of battery with 16amp protection should last more then 8 miles unless your going straight uphill for the 8 miles. Fully charged battery should read 1.4 volts per cell resting so that comes to 42 volts charged pack.