To give a particular vehicle an adequate performance takes a particular level of power. This required power level depends on the mass of the vehicle, the top speed of the vehicle, the acceleration rate you require and the gradients it must climb. If you think of how your car behaves the above seems to be common sense.
In an electric vehicle the motive power comes from the battery. Electrical power is volts multiplied by amps so that 40 amps from a 12v battery is 480 watts. But 480 watts is also given from a 24v battery by a current of only 20 amps. For a particular power, the higher the voltage, the lower the current.
Now electrical current causes heating. Motor, wiring and controller will all get hot and waste power. The heat wasted is proportional to the square of the current multiplied by the resistance. Other things being equal, that would cause losses on 24v to be half those on 12v, but of course it’s more complicated than that.
It is clear from this that a 24v system is always better than a 12v system – provided you can physically fit two batteries. By the same token 36 or 48v would be even better – but there is little practical advantage and 48v requires different controllers which are not so readily available. Nevertheless really heavy current systems (milk floats, electric cars, fork lift trucks) often use 72v or even 96v to reduce heating.
The amount of energy in the batteries is amps X hours X volts. Consider a 12v 60 Ampere Hour battery. Clearly this is exactly the same as two smaller 12v 30 AH batteries in parallel. But the total amount of energy in these two will not change whether we connect them in parallel or in series. So a 12v 60 AH battery can store exactly the same energy as a 24v 30 AH battery.
There is another factor against 12v operation, except at low currents: MOSFETs need a good voltage to fully turn them on, so almost all of 4QD’s controllers use an internal 9v supply rail, which is adequate to ensure proper turn-on. However, there is not much difference between 9v and 12v. It does not take much current to be drawn from the battery before it drops 2v at its terminals. A small mount of extra drop in and wiring – and the 9v supply drops. After that, the available current from the controller drops quite quickly! Remember that the battery current is actually a chopped version of the motor current, see our circuits archive for more detail, so the inductance and resistance of the batteries and battery wiring all contribute to any voltage drop.
For this reason, we would generally not advise 12v operation if the peak motor current is likely to be more than around 30-50 amps.