With regard to a DC motor, the voltage across the motor, minus IR losses, is proportional to the speed or RPMs of the motor. The current through the motor is proportional to the torque produced by the motor. Normally the back EMF created by the speed of the motor opposes the voltage being applied and limits the amount of current the motor draws and thus it's torque. As more and more load is applied to the motor, the motor slows reducing its back EMF allowing more current to flow, producing more torque. With a lower applied voltage to the motor not only will the motor be turning slower, which reduces the back EMF and allows more current to flow, the losses such as the internal battery resistance, the cable loss and the IR drop in the motor become more important. This may mean that the motor may not have the speed or torque to turn over the engine and stall. When the motor stalls there is no longer any back EMF and all the current goes towards heating up the motor windings and that can damage the motor. Pete.