Jenming said:
The purpose is he already has the batteries and doesn't want to buy a new pack if these will work.
The controller steps down the voltage, perhaps that could be enough? Use a 120v controller and a 48v motor and never let the controller go over 50% pwm?
Maybe someone who knows more than me would know if that would let the magic smoke out.
As long as the controller itself can handle the battery voltage, you can always use it with a lower voltage motor. The only catch is that the motor will spin faster than you want it to at full throttle, so as you note you'd need to use lower throttle input (limit with voltage divider, etc) to have lower average motor voltage, or use a controller (or something like the Cycle Analyst) to limit speed, if the motor spins faster than you want in the wheel size you need.
Nothing should let any smoke out as long as you are not exceeding the power the motor can handle, so it doesn't overheat, and the current demanded by the controller is set to not exceed what the battery can supply (which is probably not much), regardless of conditions.
If the controller is not the type to strictly limit current to *never* exceed the battery limit you specify, it could spike current demand high enough under high load conditions (startup from a stop, hills) for long enough to cause the battery's BMS to either shutdown, or to fail (blow fuses, blow up FETs, etc) if the BMS is unable to handle it.
The controller is a type of DC-DC converter, so it can step voltage down, but how exactly it does this in response to the throttle depends on it's design and programming, any user-available settings, and conditions at the time. As long as it is the common cheap ebike controller type, it will probably use a PWM-throttle, sometimes called a speed throttle (though it isn't, actually), and you can essentially directly control the motor voltage this way.
If it uses a torque (current) throttle, or a power throttle, then you don't have any direct control over motor voltage, only torque (or power), but this type of throttle is usually easier to control, depending on the total system and conditions.
If you want to see how this would affect the system, you should go to http://ebikes.ca/simulator, and pick stuff to make a system similar to what you want (doesn't ahve to be exact). Since there arent' any 120v batteries there, you can either choose custom and "make" one, or you can simply pick a 72v system (battery and controller) and motor typically used on a 36v or 24v system to see approximately what effect you get using it at only 50% throttle in various situations (slopes, speeds, etc). Before you start playing with it, read the entire page so you know how it works and what everythign does. (otherwise it can be confusing)
