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 exercise. (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...