Can an 18v battery drive a power saw?

This is not possible, because your electric chain saw it is set to work at 21 volts, you choose to use 18 volts even lithium batteries to give it power, then obviously the lithium battery voltage is too low, will make your electric chain saw it does not work up to the full load, thus affecting the function of your electric chain saw work. That's why you need a 21-volt power supply, not an 18-volt lithium battery.

As long as the size of the same and emergency use or can be, but the same capacity of the battery 18-volt working time than the 20-volt to be shorter, because the battery voltage and current determines the capacity of the battery battery, high voltage output current is the same as the capacity of the battery of high voltage is the larger, and vice versa, the smaller, chainsaw with 18-volt batteries there is also a disadvantage is that the power is also smaller.