I've always loved the idea of capturing that energy instead of dissipating it, but this article brought a possible problem to mind:
Battery chemistry could get in the way of this working well. I've seen the capacity of Li-Ion batteries destoyed by short quick charge/discharge cycles. I'm not a chemist, so I don't know if this chemistry can be altered to reduce or remove this characteristic.
If you want to see this effect, try putting the output of your notebook computer charger on a cycling switch - charging for 10 seconds or so, then disconnecting and letting the computer discharge it for about 10 seconds, then back to charging... Chances are, after about a day of this cycling, your Li-Ion will have less than 1/10 of the capacity it had before the cycles - and you won't get it back no matter how long you charge it. (Disclaimer: Don't try this if you don't want to replace the battery.)
This is exactly what this regenerative braking will do (with varying cycle lengths, of course). This might be just fine for lead acid batteries, but I'd be shocked if this did not diminish the life span of many Li-Ion chemistries.
Regenerative braking is used to recharge the batteries to a point that is within the safe operating area of the Li+ charging curve. Once the batteries are 'full' and there is no other way to consume the resultant energy, no more regenerative braking is possible. Any additional heat developed is due to efficiency losses in the power converter modules which should be about the same when the machine is used as a motor or a generator (typically in the low to mid 90's).
Full electric braking is possible and has actually been done in Europe; very efficient and smooth - no friction brakes required!
That article states that a point of 75% efficiency is reached at the maximum point. That is a measure of the energy that is not being wasted which is why it is called "regenerative". If there is a problem with heat, it is likely to be in the batteries internal resistance.
It's effect is felt when the energy flows out on the way uphill and rather than resting on the way down, the battery then must re-assimilate 75% on the way down. This is not commonly seen in the life of a battery and so must be built into the duty cycle of the battery design.
As we all know however, that heat is the enemy of everything electrical and this will turn out to hasten the demise of the battery and when they all start to go south a few years early, and the replacement price becomes known, then it will be clear why all of this is questionable technology.
Also, since we have had the first fire in a lithium ion car battery, it is clear that it won't be the last! I lost part of my hearing in a battery explosion in an electric car and I can tell you that worring about the batteries takes the fun out of the experience very quickly.
Prius had Regenerative Braking in 2001 in Japan. My 2002 Prius had it, and I prefered their method to Volt. With the Prius, when you take your foot off the brake, you coast until you lightly press the brake to start regen braking. As you press harder, you add the mechanical disc braking. I have not driven the Volt or the latest Prius, but I think having regen braking start as soon as you lift your foot off the gas is wrong because it will be felt by you and your passengers as perhaps unnecessary braking. Just my opinion, but Prius is a proven car and Volt isn't.
You would think that the heat must go somewhere. It is sound physics that there must be a heat buildup at some point. Hopefully, there is some sort of a safety that stops allowing the aggressive regenerative breaking in the down-hill situation.
The Low setting makes the Volt's regenerative braking considerably more aggressive, like driving a manual transmission car in second or third gear. You'll slow down quickly without needing to tap the brake pedal, though you'll still need to do that to come to a complete stop. Doesn't that feel more like you're driving a sports car?
Still, the fact that you can recoup some of the energy that it took to get up to speed or up the mountain is one of those nice benefits that come with an electric vehicle.
Do we need to worry about the regen-braking system overheating when being used in this fashion?
Driving in the mountains when you’re not learned in the basics of using the ICE and it’s tranny as a brake can be very bad for your brake pads and rotors; it can take just a few good hills to get your wheels smoking if you ride the brake. That is what it sounds like is happening here.. Riding the brake.
Generally speaking… braking creates heat as all that energy has to go somewhere. I know that in regen-braking the goal is to convert all the energy into electrical and then store in the batteries… But, how much is lost as heat? And is that heat enough to create a problem for some link in the regen chain?
Regenerative braking is a great idea but it needs some rethinking. Even at 'panic stop' decelerations, the effiency is 70-odd percent; at more common coasting down rates of around 1 mph/second (which works out to approx. 0.05g) it drops dramatically.
Some more engineering needs to be done for this to be really cost-effective.
Regenerative braking seems like a great idea to boost the rechargeability (is that a word?) of the Chevy Volt, but from what you've laid out, it's somewhat questionable as to how much additional power you get, unless, of course, you put a lot of time and energy into thinking about a driving/braking strategy. While I'm sure that appeals to driving enthusiasts, what about the average driver who doesn't want to think beyond turning on the car and going?
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