I am a mechanical engineer therefore please correct me if there is anything that I got wrong. I am working on a regenerative braking system for an IC motorcycle and I am looking for the best way to improve its performance. The current theoretical system uses a 48V, 2kW BLDC motor and a bank of ultracapacitors. The problem is quite complex since the voltage produced by the motor/generator depends on the angular velocity which drops with the speed of the vehicle. Charging the ultracapacitors up to 48V would therefore require either braking at high velocities or a DC/DC converter. There is also another issue of controlling the braking force of the motor which is beyond my understanding of electrical engineering and mechatronics.I think that breaking torque is related to current? Should I control current for smooth breaking?
Looking for a simpler solution I had an idea of having a bank of ultracapacitors which could be electronically modified by changing the number of cells connected in series. At low speeds, the electronic system would charge low number of ultracapacitors connected in series at once, monitoring the voltage and switching between different units until the bank is fully charged taking advantage of low speed braking. If more braking power is required or the velocity of the vehicle is higher, the system would connect larger number of units in series to increase the overall voltage of the bank. Once the harvested energy is used to power the vehicle, the power and torque of the motor could be controlled by modifying the number of ultracapacitors in series. Obviously, the complexity of the system means that a number of monitoring devices would be required to prevent overcharging of single cells and to switch between the ultracapacitors at the right time. Would my proposed cap switching system be more complex than a DCDC converter?How many steps would I need for reasonable smoothness?
I understand that I am quite likely over-complicating things for myself therefore if there is a simpler way please let me know.
Best Answer
While what you propose can be done in theory, it seems impractical compared to alternatives. The various switches, and controlling them, to combine the capacitors in different combinations won't be easy.
You said yourself a DC to DC converter is another option, but ignored it. That seems like a simpler system, and most likely you will want such a converter there anyway for other purposes, like charging the battery from long downhill slopes.
This also begs the question why you are using capacitors in the first place. Why not have the motor charge the battery during braking or long downhills? The energy density of capacitors is less than that of batteries. Since the batteries have to be able to produce the maximum current into the motor, they should also be able to handle about the same current in reverse charging them. What do you think the advantage of a mixed capacitor/battery system is as apposed to just a battery? I suppose they are more efficient short term storage, but at the cost of considerable bulk and weight. Of course that weight decreases overall efficiency and the extra space either makes the vehicle less usable or presents more wind drag at high speeds.
I haven't done the calculations, but I'm skeptical that adding enough capacitance to store the kinetic energy of the vehicle makes sense.