What your 4.8v 1800mah battery does is that it will provide 4.8 volts for 1800 miliamphours. After you use the 1800 mah up, the voltage is not guaranteed to stay at the minimum 4.8 and can fall on its nose. So, what that means is anything better than 4.8 is flyable; 4.8 and below and you are on your own.