andylaurence wrote:My thinking is that the batteries are a limiting factor. Even at 40mph, whilst only 10hp may be needed to maintain speed, a much larger amount will be needed to overcome the grip of the tyres. That will be limited by the batteries, unless a large number of them are used.
But I've just shown that, even for cheap batteries, this simply isn't the case. For acceleration equivalent to an ICE car you need maybe 3 times the mean power, which equates directly to 3 times the battery discharge rate. If you are running the batteries at 3C for the mean power, then this means you are running them at 9C maximum for the peak power. As this is much less than your notional 30C cells, then the batteries quite clearly aren't the limiting factor.
In practice, the motor will only have a mean to peak rating of perhaps 2.5 to 3:1. Heat in the motor will most probably be the thing that limits performance, rather than the discharge capability of the batteries. Motor heating isn't a function of power, it's proportional to torque, or more accurately, directly proportional to the cube of the current. As current is linearly proportional to torque for most motors, then it's easy to see that power isn't really that relevant.
andylaurence wrote:You need both. They are directly linked by definition. Power is equal to the torque and the speed. As such, higher torque for a given road speed equals more power. Yes, that means it takes less power at lower speeds, however, it also means large amounts of power at higher speeds.
Power equals torque x rpm, so for the case of a standing start, with the wheels loaded to the maximum torque that they will take without exceeding their ability to maintain grip, at the instant of application of throttle, the power is zero. As the vehicle accelerates the power increases to that needed to maintain the chosen speed, but the rate of acceleration is wholly dependent on the torque that is available (and can be used), not the maximum power.
andylaurence wrote: That to me says I need lots of power to make a fast EV and that's what I'm planning (comparatively). Perhaps I'll revisit this when I have got to the point where the drivetrain needs to be powered.
To match the full throttle top speed on the level with electric power will require the same power as the ICE produces, but this size of electric motor will produce massive levels of acceleration when compared to the ICE version. You need to decide what you actually want, then design for it, taking account of the very big difference in torque characteristics between the two types of motor.
The big challenge with high power electric motors is torque control. Even the heavy, low powered, Prius has much, much more motor torque than the front wheels can handle, to the extent that the electronic torque control is active all the time to stop the wheels from spinning, even under very modest throttle settings.