Regenerative Braking: Efficiency & EVs - An Overview

[robinfisichel]

Another EV related question from me, I would like to know a few things about regen braking...

1. For typical cars, how efficent are regen systems, I've seen 31% quoted http://engineering.wikia.com/wiki/Regenerative_braking as in how much of the kinetic energy of a vehicle is recoverable

2. Since batteries can't allways take the high amp current transfer in the time it takes to brake some EV's use supercapacitors to store the energy and then recharge the batteries slowly, but why recharge the batteries if you can just use the energy straight from the capacitor.

3. If i am recharging the battery using regen does this mean i have to somehow limit the maximum charge of a battery to prevent current being pumped into a full battery?? Cant i just send it to a capacitor? But what happens if the capacitor is full, where can i send the charge?

 

[xxChrisxx]

Another EV related question from me, I would like to know a few things about regen braking...

1. For typical cars, how efficent are regen systems, I've seen 31% quoted http://engineering.wikia.com/wiki/Regenerative_braking as in how much of the kinetic energy of a vehicle is recoverable

2. Since batteries can't allways take the high amp current transfer in the time it takes to brake some EV's use supercapacitors to store the energy and then recharge the batteries slowly, but why recharge the batteries if you can just use the energy straight from the capacitor.

3. If i am recharging the battery using regen does this mean i have to somehow limit the maximum charge of a battery to prevent current being pumped into a full battery?? Cant i just send it to a capacitor? But what happens if the capacitor is full, where can i send the charge?

I was going to mention this in the other thread, that charging and discharging batteries using a kers type system isn't as easy as people made out.

It's easy if its a paper excercise. (ie this is a Uni style project). But making one acutally work in real life is a whole different game, and ususally a much bigger PITA.

Supercapaciters are used as they can be charged very quickly and don't require charge monitoring (once they reach capacitance they simply won't charge any more). However they 'leak' charge, so over time they will lose their charge even if not used. Another factor as to they they aren't used is that they only deliver full voltage when fully charged. So for example a battery can deliver a steady voltage, the capitor won't.

So you can dump power into them and use them to trickle recharge a battery until it reaches a set amount (they usually keep betteries charged 50%-95% (I pulled this figure from memory so it may be rubbish) but they don't tend to fully charge something, just incase of a surge (the 5% gives a little leeway for problems). Control systems are key to this sort of thing, and that's all way beyond me.

 

[robinfisichel]

something, just incase of a surge (the 5% gives a little leeway for problems). Control systems are key to this sort of thing, and that's all way beyond me.

Cool, nice info, i think that's what i hear as wel. Control systems are way beyond me as well! My design is really hypothetical so I am not worrying about this too much.

One problem with the drive cycle i used for the EV is its aimed at emmissions testing (as are most) so doesn't incorporate proper braking decelerations and therefore i recover barely anything.

I will have to make my own braking routine to see what kind of values for energy recovery.

 

[robinfisichel]

I want to do a quick example of this, just to see how worthwile regen breaking would be.

If the car brakes from 40-0mph in 4 seconds (about 0.4g) the change in energy is 95925 J for a 600kg car. Assuming 31% efficiency this makes available 29736 J.
In terms of power this is producing 7.4kW and stores 8.2 Wh of energy.
Assuming i divide by the nom voltage of the battery pack (not sure about this) then that's 47 Amps which would be too great for charging some liion batteries i think.
In one of my cycles i break 14 times per cycle, and the cycle is repeated 16 times, the total power used is 15000 kWh. So the energy recovery would be 8.2 x 14 x 16 = 1905.68 Wh which is 12.7% recovery. Which i guess is ok, i was told the system doesn't add much weight, though the cost would be higher for the controller units...

 

[alphy]

I would like to provide a response to the questions regarding regenerative braking and its efficiency in electric vehicles.

1. The efficiency of regenerative braking systems can vary depending on the specific vehicle and its components. However, on average, studies have shown that regenerative braking can recover up to 70% of the kinetic energy of a vehicle. This means that for every 100 units of kinetic energy, approximately 70 units can be recovered and used to power the vehicle.

2. The use of supercapacitors in conjunction with batteries for regenerative braking is a common practice in electric vehicles. While supercapacitors can handle high amp currents and charge quickly, they do not have the same energy storage capacity as batteries. Therefore, the energy stored in the supercapacitors is used to provide a quick burst of power to the vehicle, while the batteries are used for longer term energy storage. By using both components together, the overall efficiency and performance of the regenerative braking system can be optimized.

3. In order to prevent overcharging of the batteries during regenerative braking, most electric vehicles have a battery management system in place. This system monitors the charge level of the battery and can limit the amount of energy being sent to the battery from the regenerative braking system. If the battery is full, the excess energy can be diverted to other components such as the supercapacitors or the motor. This helps to prevent any damage to the battery and ensures that the regenerative braking system is operating efficiently.

Overall, regenerative braking is a valuable technology in electric vehicles as it allows for the recovery of energy that would otherwise be lost during braking. By optimizing the use of both batteries and supercapacitors, the efficiency of regenerative braking can be maximized, making electric vehicles more sustainable and energy-efficient.