Electrical Engineering & Electronics Design

I would start by doing experiments charging individual lipo battery cells. Then expand your experiments later using magnet motor generators to charge them. You can get them on ebay they are usually used for wind turbine projects. If you charge individual 4.2v cells for instance, you need to generate and regulate the maximum cell voltage 4.2v for each individual battery cell. Research individual lipo cell charging. If you charge all your cells while connected in series you need to use balance charging techniques a little more complicated, this is to balance and regulate the input to each cell individually. You can buy regulating and pre made charging circuits however It is best to build your own voltage regulating circuits for each cell so you get a good understanding of every area of your design process. Hope this helps to get you started.

thank you for your help.

Would i be able to use the magnet motor generators to power a balance charger inside of the car or would it be better to just build or get a charging circuit?

If it were me I would design and make my own 4.2v charging circuit for each Lipo cell. However if you google 4.2v regulator circuit you should find many charging circuits and pre made circuits to charge individual cells. Basic level balance charging is simply charging each cell using a voltage regulator to charge each cell however I would recommend you watch tutorials like Dave at EEV blog.

https://www.youtube.com/watch?v=A6mKd5_-abk

If i make a circuit to charge individual cells can I charge a multi cell battery?

You would need one charging circuit per cell to make sure the cells charge evenly.

There is a number of factors to consider - one is weight and the other is the extra burden of energy that is require to generate the power required to drive the charging circuit. The circuits that are used to charge such battery require 10 to 16 volts at 10 watts. You have to consider the factor of the original battery. How long does it last to run the RC model? This will determine what rate of charge is require to charge another 4.2 volt battery. Can the run time of the RC unit meet the charging time to charge a 4.2 battery. If it can't then you have to consider how to multiple the charge rate. With the knowledge of what rate the recipient battery can be charged at. This is a perpetual energy enigma.

A good start would be to head over to this web site and understand the charge rates and discharge rates of lithium batteries. https://www.powerstream.com/lithium-ion-charge-voltage.htm.

Another item to consider is using super capacitors. Check this concept out.. https://www.instructables.com/id/The-Forever-Rechargeable-VARIABLE-Super-Capacitor-/

thank you. what do you recommend using it for?

More info on super capacitors

https://www.electronicdesign.com/power/innovative-cooling-shrinks-size-100a-module-buck-regulator

This is simple ; you need to diode "wired-OR" the two voltage sources.

https://en.wikipedia.org/wiki/Schottky_diode#Switched-mode_power_supplies

If one source is off, the other source will supply current.

https://www.edn.com/design/power-management/4419835/Multiple-PSUs-share-load

The diode should be a low voltage drop schottky (simple) typically around 3.5 Vf , but may have some leakage current.

Another way is to use MOSTFETs with a voltage comparator circuit to turn on or off the batteries. This is more complex but has lower losses.

Physics says that if you use a motor to run a generator you will never be able to get anywhere near the power required to run the motor from the output of the generator, usually <70% at best.

The best way is to implement this sort of system is regenerative braking: whenever the PM motor(s), preferably outrunner type, are not actually driving the wheels a pretty simple circuit switches the motor windings to charge the batteries. Anything else will add weight and complication without much if any advantage. One aspect is when off the throttle the car will slow down much faster while charging than its freewheeling brethren. That could be an advantage when setting up for a turn, essentially a dynamic brake.

IOW, look at the whole system.