To give a vehicle a certain performance takes a particular level of power. This 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.
In an electric vehicle the 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. The 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. Moving from 12V to 24V halves the current but halving the current reduces the heating losses to a quarter.
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 36V or 48V would be even better, heavy duty systems such as fork lift trucks often use 96V, electric cars are now using 600V!
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 the wiring and the internal 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.