I have seen threads stating that 24v starting systems are better in cold weather, my question is why? Yes, voltage is the force that pushes current but i was always under the impression that the more current you have going through an electric motor the stronger it will be. 12v with two batteries doubles the amperage going through the motor so shouldn't it be just as, if not more effective for starting an engine?
this is more of a question out of general curiosity more than anything.
Two 12V batteries in parallel will NOT put double the amperage through a starter motor.
They are capable of delivering double the amperage but the actual amperage draw is a function of the design of the motor.
P=I x E
Power = Amperage x Voltage.
Double the amperage for a given voltage and double the power.
Double the voltage for a given current flow and double the power.
How much power a motor uses is a function of the motor design so a normally functioning 12V starter motor that is designed to use say 200A on single battery will still use 200A when connected to a dozen 12V parallel batteries.
So if a motor is designed to provide x amount of torque and x RPMs at 12V and we want that motor to produce more torque and more RPMs then we need to increase the power that the motor uses. We can double the current or double the voltage. As Matt65 points out doubling the voltage is a more efficient way of increasing the power of our motor.
IOW if we have two motors that are identical in mechanical characteristics (torque and RPM) but one is 12V and the other is 24V the 24V motor will use less current and consequently smaller wires and control devices.