NiMH battery for Electric Bike
A 15-cell NiMH pack will give 18 volts fully charged, instead of 24 volts. The bike's electronics is likely to see the lower voltage as a heavily discharged battery pack and refuse to operate to protect the pack from damage. Even if not and the motor will run at the lower voltage, it will be drawing more current and it or the wiring may overheat. Without knowing details of the bike's electronics, I really cannot be sure. You can try it but you are risking damaging the bike.