I want to build a small generator that would fit inside of a brake rotor, the larger ones with the park brake shoes inside, to see if it could possible help charge the batteries a little bit, any suggestions helpful thanks
You mean to build a generator that will use the motion of the wheels as you drive to generate power, in order to recharge the batteries on your electric vehicle? Or do you mean some sort of regenerative charging system that will act as both a brake and power recovery system?
If you mean the former, forget about it, as the generator will resist turning as it generates power, creating drag. The drag it creates will use more power from the batteries to overcome than it will produce. You will wind up with less range than if you don’t bother. Range will be reduced by both carrying more weight and trying to push that extra weight against the new resistance to turning that the generator will put on.
If you mean the latter, there are better ways to achieve this. The proper controller and an AC motor will use the drive motor as a generator. Might be able to set up a DC system this way too, but unfortunately I don’t know the details. A search here will turn up more information on this.
it will make that much of a difference?
yup
a generator is “loaded” when you hook up batteries… it does not spin freely.
I know that I just didnt think it would be so loaded that it would use more power to run then it was producing, so basically generators are really inefficient or just in the sense I was thinking by trying ot use momentum instead of an engine?
This is a very simplistic view of how a generator works. Technical accuracy may not be accurate to the level that a degreed engineer with a doctorate in magnetic theory would type, but is accurate enough to show why this won’t work. Any doctorate holding engineers who wish to put in a detailed explanation, feel free. 
A generator produces both heat and electricity when turned. A generator is essentially a very long wire coiled together into a cylinder, with a magnet moving inside of it. The magnet induces electrons to move in the wire. As wire resists the flow of electrons, heat is generated by the electrons interacting with the wire’s atoms as the electrons try to move. You can liken this to a person trying to move across a crowed room when the people don’t want to move. Our target person will bump into other people, friction from the motion will happen and tempers will get heated (heh heh). As the magnet is inducing the electrons to flow, this resistance also resists the action of the magnet to move electrons, the magnet’s movement is also resisted. This resistance is felt by the device turning the magnet as a resistance to spin. The more power you try to push through the wire, the more resistance to movement there will be, the more heat will be generated. As you can’t make or destroy matter or energy, only convert it, this means that part of the effort expended to turn the generator will be expended overcoming the resistance to movement, and part will be lost in heat as a direct result of this resistance.
With your idea, we will be using the battery to store power that will be used to move the vehicle with an electric motor. Part of the power in the battery will be used overcoming resistance to movement of electrons in the wire. Part will be lost to heat. Part will be used to induce the motor to start moving. Part will be used to overcome inertia of the standing vehicle to become a moving vehicle. Part will be used to keep the vehicle moving against the wind.
When we add in your proposed generator, we will now have added resistance to movement, which the motor will also have to overcome. You can liken this to driving into a constant 30MPH headwind that changes direction when you do. This generator WILL produce power, but it will not produce a lot of it while still resisting motion. You have to get the motor spinning to a set RPM before it will even overcome wire resistance and produce more power than is lost in heat, so the first 30-35MPH won’t produce anything. Once you’ve reached the generator’s production speed, it will start producing power which it has to force back down the lines to the batteries, then you have to force the power into the batteries as converting electricity to electrolyte is a change that requires effort.
You can gear the motor down to spin faster than the wheels spin, but that will also increase resistance to movement. If you’ve ever ridden a 10 speed you know that from a dead stop one gear set will be easy to turn but it takes a lot of pedals to get any appreciable motion, while another, lower gearset is hard to turn but will get you moving faster from a dead stop. This same effort would be transferred to the drive motor if the generator were geared to make power at a lower speed, which would be like the car was be faced with a 60-70MPH head wind.
I hope this helps get an understanding of why running a generator off an electric motor that is run off a battery is not a good idea. Off the top of my head, a generator is anywhere from 50-80 percent efficient in converting mechanical energy to electrical energy, and a motor is anywhere from 50-95 percent efficient in converting electrical energy into mechanical energy.
There are instances where you can actually see a net gain in efficiency in using an electric motor to do work ordinarily done by a belt off an ICE, but that’s only because the device that works off the ICE must do an acceptable job across a high RPM band but a device that takes its mechanical power from an electric motor can be run at its most efficient RPM (and even made smaller) since it can run independent of the ICE. This net gain in efficiency should NOT be confused with a net gain in power though.
actually that explained a lot, thank you for taking the time to break it down for me.