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?
Some advantage is gained from regenerative braking, but not much. It is a known fact that batteries do not recharge quickly, regardless of the method. The gain is a surface charge more quickly used than the deep charge that comes with time. Granted, regenerative braking is better than having none at all but IMHO (since I am no electrical/battery engineer) it probably results in longer life for the brakes than in significantly improved efficiency. Worth the extra cost and wear and tear on the drive system? Time will tell.
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.
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?
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.
Think of how hot normal brakes get. Now if 75% of that gets converted back to stored electricity, the other 25% turns into heat. It is distributed between the electronics, the motor, and the battery. If there are any engineers left at GM they probably designed for this situation, with temp sensors on all the components.
"Normal" brakes get hot because all of the braking energy is friction generated heat. regenerative braking uses the drag created when a motor is turned into a generator. If you've ever ridden a bicycle with a tire generator on it, then switched on the lights and felt the increased drag you would know exactly what I mean.
In the VOlt, the battery and battery charging electronics are all actively liquid cooled, so yes, the engineers thought of it.
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.
The Volt battery pack has active liquid cooling. the battery fire you refer to was due to loss of coolant and it happened 3 weeks after it was crushed (and would not have occured at all had protocol been observed). I am not concerned in the LEAST about the safety of the battery. I am much more concerned about the 9 gallons of hydrocarbon incendiary fuel sitting in the tank.
I am assuming that the battery explosion you were injured in was a FLA battery? the main by products of over-charging a lead-acid battery are heat and hydrogen. not a really good combination.
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!
Full electric braking will be less effective than friction brakes, from the energy standpoint because at rthe low no-regen. speeds the computers will be forced to run the motors backward. (I know it isn't really running backward but in effect it is and consumes energy.) I am also less inclined to trust a full eletric system, things do fail occationally. Even Hydraulic friction brakes usually have a back up cable activated parking brake. We must also start NOW to develop a battery recycling system or when these cars reach end of life we will have a horrible disposal (battery) problem on our hands.
Great article! Coming from the off-highway electric machine world, regen braking has been part of my world for a long time. It's good to see it being used in the "closed system" of the new electric cars.
Bill, the Volt, like the Prius, has a dual braking system. Under most conditions, you are using regenerative braking, but in panic stops and below about 5 mph it switches to the friction brakes. With 10,300 miles on mine, the friction brake rotors still have the factory machining marks. The batteries are not a waste concern: they are expected to be able to power the cars reliably for a decade or more, and after their capacity falls below 70% of nominal they are going to be repurposed as storage devices for solar, wind, and Smart Grid systems. after they are no longer useful for these purposes (~40 years) the materials are 95% recyclable.
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.
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.
@Walt: You bring up a good point. I do not know anything about battery powered cars, but I do know I have destroyed numerous batteries in the past by improper charging.
I also know I do not want my vehicle to put the brakes on as soon as I lift my foot off of the accelerator. This article makes me think there are people making decisions based on Feel Good rather than what really improves the product.
I be interested in knowing what kind of testing and what group of people got to try this concept out. Is this something that the driver will get used to and won't mind. Or is it more of a situation that people just won't notice. I would assume it's something that most people would notice. So I guess it;'s one of those things that people learned to put up with the goal of feeling good because they are driving green.
The Volt has an incredibly sophisticated charge and climate management system: one that continually monitors the 288 cells several times a second and balances the individual charge as the battery is being recharged. In addition, the battery pack has an ACTIVE liquid climate control system to hold the battery temperature within a specific optimal temperature range.
When driving the Volt in D, the accelerator pedal "lift-off" feel is indistinguishable from driving an ICE vehicle with an automatic transmission when you lift your foot. It coasts! However, in L it feels like you downshifted to 2nd or 3rd (in a 3 speed or 4 speed tranny)
The "depends" part of the regenerative breaking is the system condition. The Volt attempts to keep the battery full until the engine runs out of fuel leaving little charge space for regenerative breaking. The exception is when the Volt is driven slowly and is on battery only.
In the mountains with long downhills, regeneration dose not happen with a full battery. Breaking is all mechanical.
Regeneration is most effective in city driving and only when the car is driven agressively. Fast and hard acceleration takes from the battery leaving room to regenerate. This is why the Prias and Volt have good MPG numbers for the city driving circuit.
For the two footed key in and drive folkes, the "feel" of the gas and break functions emulate the bulk of automatic cars. The indirect method of setting the more agressive one peddle regenerative breaking keeps the numb drives from getting into the mode by accident.
Effeciency and heat. If the motor is 90% and the controller is 90% and the battery is 90% then the total is 72.9%. At a stated 75%, there is no abnormal heat present.
Point one: You are mistaken. The Volt does NOT try to keep the battery full. It depletes to the switchover point and then maintains that level, having used approximately 60% of the 16 kWh capacity. there is plenty of headroom to absorb regenerative braking charge.
Point two: again mistaken: You said "Regeneration is most effective in city driving and only when the car is driven agressively. Fast and hard acceleration takes from the battery leaving room to regenerate. This is why the Prias and Volt have good MPG numbers for the city driving circuit."
This is incorrect. regeneration is best when NOT driven aggressively because the harder you brake the more you get into the friction brakes and bleed off the energy as heat. additionally, the Volt does not try to keep the battery full. It is not a hybrid in the same ilk as a Prius that tries to maintain a full battery. it is pure electric until the charge is dropped by 10 kWh and then that battery level is maintained.
You are off the mark on why Prius and Volt get good fuel economy in the city: they are extremely efficient at low speeds and stop-start compared to straight ICE vehicles because electric motors deliver maximum torque instantly, hence good acceleration with very low power off the line, and then recapture of electricity on braking
There does seem to be a driving technique issue here, in that the most efficient driving includes a bit of coasting, as opposed to braking. Regenerative braking when you want to brake is fine, but when I want to coast, that is different, and I DO NOT want to be braking. In many cases it works very well to take my foot off the throttle and coast up to a red light, since It was clear that it would still be red when I arrived.
As for those folks concerned about"overheating the regenerative braking mechanism", clearly they are not aware that it is all the same parts, just a different controls setting. The problem with regenerative braking is that when needed most, for hard decelleration, there is only so much energy the batttery can absorb in a short time. The solution would be to have a bank of super-capacitors, which can absorb a charge much faster, and then return it much more rapidly. The challenge is that supercapacitors are sort of expensive, and would need to be installed someplace, taking room away from the battery bank. But, they could absorb the power as fast as it was generated, and then deliver it for the next accleration. That has been demonstrated already.
Hi. I believe the best option for regen is to blend it with mechanical braking. The first portion of the brake pedal would regen as much as possible but continuing to press the pedal would engage mechanical braking. Regen should always be proportional to the brake pedal position and not an on/off function of a present regen amount. IMHO, coasting when lifting the foot off the accelerator is better than engaging regen at that point. You get much more (range) out of letting the car coast than braking it with regen in hopes to recover some of that energy. Proportional regen, blended with mechanical braking is tough to do. First, you want a certain amount of braking to always be present at a certain brake pedal position. Nothing is scarier that pushing the brake pedal to what you're used to and finding the car is not stopping as expected. Second, regen braking capacity varies depending on how much current the battery pack can sink. If you live on a hilltop you will not get much regen braking when you set off in the morning after changing your batteries all night. Regen also varies with speed which complicates things even further. Third, the transition from electrical to mechanical braking must be seamless to make it effective and elegant. I guess that's why we compromise and get regen on accelerator in most implementations.
Based on my experience test-driving a Chevy Volt -- and I realize I'm being entirely subjective here -- the regenerative braking system was easier getting used to than ABS was when I first confronted that. The Volt's brake pedal has a "squishy" feel to it, which on first use makes one think there's something wrong with it. It's simply the regen kicking in.
I found quite the opposite. I traded my 2006 Cadillac CTS for a 2012 Chevy Volt 7 months ago. I have put 10,300 miles on it since then, and in D the effect seems to be almost no drag at all compared to the Caddy with an automatic tranny. In L it definitely feels like driving a manual tranny car.
The regen braking does not deliver enough energy to the battery to overcharge it. There is a kWh used meter on the display, and I have seen the displayed number decrease in long downhill coasting, but only 1 to 3 TENTHS of a kWh. It is conceivable that you could pick up a lot more in a long drop on mountainous roads, but unless you started at the top with a full charge it is not likely that you would knock on the charge-limiter.
The Volt depletes ~10 kWh of charge before the ICE generator kicks in, and it does not REcharge the battery, it simply maintains the battery at the level it is at when switchover occurs
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