Volt vs. Leaf - What finally pushed you to the Leaf?

[surfingslovak]

I would rather be in milder climate (68F-75F) than be in colder climate (25F-68F) so I can get much more improved range at a tiny expense of capacity loss. That's why I drilled into this cold climate claim and made a big deal out of it, because to me the range advantage of milder climate far outweighs the minimal capacity advantage of much colder climate.

Agreed, but please keep in mind that it's very difficult for us to quantify and predict capacity loss accurately. Several of us collaborated on the aging model for this very reason. There will always be questions from new owners, and it's difficult to continue in a complete void. Although the manufacturers could close this gap tomorrow, and release more information, for the time being, it makes sense to draw a line in the sand, and at least put different types of capacity fade in the right ballpark.

For the average LEAF, calendar aging appears to make up the majority of capacity loss, at least initially. Charging and driving habits appear to play a much lesser role. The convenience factor aside, without wanting to discount it, there is an incentive to store the battery pack at a low temperature, and warm it up before use. This could work pretty well, except not every trip will be predictable.

The consensus based on anecdotal owner reports is that we have seen very little to no battery capacity fade with the Volt, and more noticeable and sometimes even severe loss with the LEAF. Why this is exactly, and how the different engineering and manufacturing approaches play into it, we can only guess. There is, however, a significant correlation between local climate and capacity loss with the LEAF.

That said, the Volt is and will likely remain a more convenient vehicle. It does not suffer from the range restriction, and GM went out of their way to cover a lot of use cases and scenarios. There are downsides as well, and like many here, I gravitated to the LEAF for those reasons. I have much more respect for the Volt now than I had initially, it has done spectacularly well. But in the longer-term, pure EVs have the potential of being very disruptive. Although it sounds laughable today, they could be much cheaper than ICE vehicles not too far down the road.

 

[RegGuheert]

So the claim that being in a colder climate for the majority of the year is an advantage for the battery like RegGuheert originally claimed is a double edge sword. I would rather be in milder climate (68F-72F) than be in colder climate (25F-68F) so I can get much more improved range at a tiny expense of capacity loss.

Sure, but we aren't discussing choosing where we live. The topic of this thread is why we chose the LEAF over the Volt. And battery life is one of the reasons I chose the LEAF for my application. DOD has a real impact on battery life and so does storage temperature. I believe that cycling at cooler temperatures also reduces degradation, but as surfingslovak say, there are cold-temperature effects that can come into play at very low temperatures. I don't think we get those here very often.

I will note that the heater has a much bigger impact on EV range than does the battery itself. I simply don't see ANY noticeable loss of range down to 50F and very little below that down to about 30F, unless I run the heater. Below 30F, the range drops off faster. So the fact that the Volt heats it's battery for charging and operation really will not have that much impact on range where I live, since both cars will need to heat the cabin (or not). The Volt, additionally, will need to heat the battery, which should be roughly equal to temporary battery capacity loss in a LEAF due to temperature down to about 30F. Below about 20F, however, I can see wanting an EV with a TMS to improve charging characteristics and range at those very cold temperatures, but still you will have to pay for the additional electricity to run it.

So how do we explain the observation that the Chevy Volts have not lost EV range? Certainly all Li-ion batteries lose capacity with time, and the TMS should only be helping battery life where it is actually cooling the battery, hot heating it. Perhaps the chemistry in the Volt IS much more stable than that in the LEAF. If that is the case, then the Volt batteries will certainly last longer. But if it is simply that the Volt BMS is hiding capacity loss, then this can only go on for so long before the range begins to drop. Time will tell.

The other possibility is that there is more heating within the stacks of LEAF cells than we have been led to believe. The Volt cools between the faces of each pair of cells, which is ideal. But, so far, Nissan has maintained that the cells are degrading evenly. Perhaps it is just that these types of differences will only show in areas with much more gradual degradation rates. Again, time will tell.

 

[SanDust]

Others here have posted that in the wintertime they get less than 25 miles in the Volt. That means that in cold weather the Volt would be brought down to about 21% SOC with each wintertime trip. By comparison, the LEAF would be charged to full just before the trip and would arrive home with six or seven bars, or an SOC around 60%.

Then that leaves our 50-mile weekly excursion. Again we typically charge to full just before the trip and return home with about four bars, or about 40% SOC. The Volt would be brought to about 20% during this trip.

Many people, including you, have touted the statistic that Volts drive more EV miles than LEAFs. If that is true, then it seems the Volts are typically brought to a much lower SOC than the LEAFs, particularly since the SOC in the LEAF can be brought up to a higher level before departure.

Around here in July and August, the average highs are in the mid 80Fs and the average temperatures are 73F to 74F. During those months, our LEAF battery may degrade 50% faster than a Volt's battery. During the spring and fall, it's probably a wash and for about five months in the winter, the Volt's battery likely degrades at 2X to 4X the rate of the LEAFs, at least when it is heated to temperature. But the Volt is going to a lower SOC each time it is driven and it sits at a higher SOC (86.5% versus 80% or lower) when it is not driven.

So, does the couple of months of slightly higher calendar degradation in the LEAF in the middle of the summer fully offset the higher calendar degradation in the Volt during the wintertime plus the year-round higher SOC when sitting charge plus the year-round lower SOCs achieved with virtually every trip? Maybe, but I doubt it.

The only way I can see the Volt having a better battery life in our scenario is if there is something much better in the chemistry itself. That may be the case, but I had no way to judge that at the time of purchase.

The Volt has a maximum DOD of 65%. The Leaf has a maximum DOD of 80%. If you're saying that you'll always charge the Leaf to 80% and always leave 3 or 4 bars of charge left, making the maximum DOD more like the Volt's, then yes the DODs will be similar. You keep talking about SOC but the culprit in battery degradation is DOD not SOC. They're related but they're not the same. SOC doesn't play a role. With the Volt you're prevented from taking SOC too low or too high. You're saying that you'll manually do the same thing with the Leaf, more or less making the two vehicles equivalent.

But we still have temperature, and you can't make the two equivalent here.

The first step is to distinguish between storage and operating temperatures. You seem to be conflating them. When you store batteries, colder is better than warmer. But that's not the case when they're operating. Batteries don't like to be discharged at high C rates when they're cold. They'll work best and are less stressed if they're discharged in a temperature window. This is a great advantage when you have a TMS. When you start a Volt it's going to use the TMS to get the battery into the necessary temperature window, reducing the stress on the cells. My Leaf doesn't have that capability.

Note that this does not mean -- as you seem to be thinking -- that the cells in the Volt are going to be stored at a higher temperature. The Volt, like any car with a TMS, is not going to heat the cells when the car is not operating. If you park a Volt and a Leaf for a week with the ambient temperature at 40F, at the end of the week the cells in both cars will be at the same temperature. Plugging in both cars won't change this. Because the cells will be at the same temperature when not being dischareged, contrary to what you're thinking, the cells in the Volt are not going to degrade at 2X or 3X the rate of the cells in the Leaf, at least not because of temperature.

The TMS does mean, however, that the cells in the Volt will be operated and stored at lower temperatures when ambient temperatures are high. When the Volt is operating the TMS will cool the cells so they remain inside the operating temperature window, using either the battery itself or even the generator if necessary. It's the same if the Volt is not being operated. If you park a Volt and a Leaf on the street and the ambient is 100F, the Volt will use its TMS to cool the cells, assuming it has enough juice. If you plug it in then it will definitely use grid energy to cool the cells. Again, my Leaf can't do that. If my garage is 85F then sooner or later the cells are going to be 85F.

The TMS will also reduce the stress on the cells in other ways. Batteries will be stressed when charged at high or low temperatures. A TMS helps with that. Finally, a TMS can eliminate hot spots. For example, even if the ambient is 65F, after use some of the cells may be hotter than others. A TMS can eliminate this differential even if it doesn't need to cool or heat the cells.

Basically you're postulating that there is a technical advantage in not having a TMS but, from a cell life point of view, a TMS is always advantageous and never disadvantageous. It's really just like having heat and AC. You're always more comfortable when you have these things, never less comfortable. If you'd be less comfortable using them you simply turn them off.

 

[RegGuheert]

The Volt has a maximum DOD of 65%.

When new. It apparently increases as the battery degrades.

You're saying that you'll manually do the same thing with the Leaf, more or less making the two vehicles equivalent.

They are not equivalent. I drive where I drive. If I drive in an EV with 2/3 the battery capacity, then I go to a higher DOD. Further, if i can charge my EV to a higher SOC just befor my trip, then I can further minimize the DOD that I achieve. Finally, if I then let my EV sit at 60% or 70% SOC because that is all I need in case I need to run out to the store, then it will degrade more slowly. So my battery has a lower storage SOC and much lower DODs than a Volt battery would in the same service.

They'll work best and are less stressed if they're discharged in a temperature window.

Agreed. Perhaps we disagree on the window here. It seems that 20F is not overly stressful on our batteries. Below that, I'm in full agreement.

But note that cycling losses increase as you increas temperature. I suspect that they decrease as you go down to about 40F before they start to increase again.

This is a great advantage when you have a TMS. When you start a Volt it's going to use the TMS to get the battery into the necessary temperature window, reducing the stress on the cells. My Leaf doesn't have that capability.

I will say that the climate where I live is already within that window.

Note that this does not mean -- as you seem to be thinking -- that the cells in the Volt are going to be stored at a higher temperature.

Yes, it does. Whenever you drive a Volt it heats the cells up 72F until you shut it off. Then the Volt holds this higher temperature for a much longer time because the battery is insulated to support the TMS function. I suspect the thermal time constant of the Volt battery is over eight hours. When you charge the Volt battery, it heats it then, as well. The same applies. My LEAF never heats the battery beyond normal self-heating, so I rarely ever see more than four bars in wintertime. So if you commute in cold weather in a Volt, it's battery will live its life much hotter than the battery in a LEAF would.

The TMS does mean, however, that the cells in the Volt will be operated and stored at lower temperatures when ambient temperatures are high.

No argument.

Again, my Leaf can't do that. If my garage is 85F then sooner or later the cells are going to be 85F.

Agreed. In fact, it's worse than that. If it's 90F in the garage and 100F outside, I have trouble bringing myself to drive the LEAF, since it will scoop up that hot outside air and quickly bring the battery UP to the outside air temperature. If a Volt battery is hot, then I assume you can simply turn on the charger or drive it to cool it down.

Basically you're postulating that there is a technical advantage in not having a TMS...

Not exactly. As I am postulating that having a TMS combined with a 2/3 size battery in my application is not better for battery life than having the bigger battery and no TMS. That's the difference between the Volt and the LEAF. It's not one or the other.

...but, from a cell life point of view, a TMS is always advantageous and never disadvantageous.

That's what many people seem to believe. I'm not one of them.

 

[evnow]

For the average LEAF, calendar aging appears to make up the majority of capacity loss, at least initially.
...
There is, however, a significant correlation between local climate and capacity loss with the LEAF.

Aren't these two contradictory ?

 

[TomT]

I believe that it is temperature, mileage (or battery cycles), and calendar aging, in that order.

For the average LEAF, calendar aging appears to make up the majority of capacity loss, at least initially.
...
There is, however, a significant correlation between local climate and capacity loss with the LEAF.

 

[Volusiano]

Basically you're postulating that there is a technical advantage in not having a TMS...

Not exactly. As I am postulating that having a TMS combined with a 2/3 size battery in my application is not better for battery life than having the bigger battery and no TMS. That's the difference between the Volt and the LEAF. It's not one or the other.

...but, from a cell life point of view, a TMS is always advantageous and never disadvantageous.

That's what many people seem to believe. I'm not one of them.

One observation I'd like to make in light of these points is that I've had my Volt for a couple of months now and February and March in Phoenix is very ideal and mild weather. Yet, almost every time I plug in my Volt, I can almost always hear the TMS working, circulating coolant through the battery. Not just for a short time in the beginning of the charge cycle, but for quite a while. What this is telling me is that even with mild ambient temperature, there is enough heat in the act of driving that would cause the battery to be out of the ideal temperature band enough to trigger the TMS.

If this battery had been in the LEAF, there would not have been any cooling going on while charging. You'd be charging a hot battery. The point is that even living in an ideal weather environment, not even hot weather environment, there is still value in having TMS because obviously it's being put to work not even while driving but also while charging as well. So to me, dismissing the value of having a TMS just because you have a bigger battery and your local climate is nice and mild is nonsense. If you drive and work your battery up, it'll get hot, not matter mild your local weather is or how big your battery is.

Let me rephrase your point about having a bigger battery being an advantage from the lifetime perspective (not range perspective) so I can understand better:

1. First, for simplicity sake, let's say the LEAF's battery range is 75 miles. And the Volt's range is 40 miles.
2. Now, if you need to make a 40 mile trip, the Volt's SOC at the end will be 17.5% (35% buffer). The LEAF's SOC, assuming that you charge it to 80% only, will be at 27% = ((75*.8)-40)/75.
3. So your point is that thanks to having a bigger battery, if 40 miles trips are used most of the times, the Volt ends up at 17.5% SOC, hence resulting in a deeper discharge compared to the LEAF being only discharged to 27%.

So I understand this point. However, is there any empirical data to show how much battery life is affected with this 9% SOC discharge difference? You're making an assumption of some kind of linear relationship, but maybe it's not. For all you know, maybe the data will show that battery life doesn't get reduced significantly until your SOC get below 15% (I'm just making this up, of course).

So you're just making generalization here to prove a point without empirical data to back it up. Maybe you're right, maybe not. We just don't know. But what we know is that TMS is valuable not just for hot climate weather, but even in mild climate weather, or cold climate weather, regardless of the size of the battery. That's because TMS can reduce stress to the battery at any weather. And this is overall stress at any SOC level, not the stress caused by deep discharge or full 100% charge.

I would compare the deep discharge/full charge stress RegGuheert implies as being too hungry or too full. Sure, you don't want your body to be stressed out because you're too full or too hungry. But is your body stressed out that much more if you're a little more or less hungry (17.5% vs 27%), unless you become famished (5%? 10%? 15%).

And I would compare the stress reduced by TMS, on the other hand, using Sandust's analogy of TMS as AC or heating to keep your house warm or cool depending on the outside temperature. Do I want to have AC/heating to keep my house at the ideal temperature so that my body is less stressed in any kind of weather? Heck sure, I do. That way if I work out and overheat my body, I want AC in my house to cool me down before I eat something. Also, if the outside weather is too cold, I want heat in my house to warm me up before I go jogging outside.

 

[RegGuheert]

For your example, I would use 22% to 87% for the SOC range of the Volt, which is 65%. Note that the total capacity of the 2011 LEAF is 1.5X that of the 2011 Volt, so a trip that went to the edge of the SOC range of the Volt would equate to a 44% SOC change in the LEAF assuming similar efficiencies. Since the LEAF can be charged to 94% just before departing, that will bring the LEAF SOC down to 50%. That is 28% higher than the SOC of the Volt. I'm not saying there is a linear relationship, just that deeper DOD is harder on the battery. That's pretty well accepted. Note that as the two batteries degrade, this same trip will result in the DOD getting worse and worse in both cases.

Again, I will say that the idea that the TMS reduces stress in any weather is not true. Driving the battery up to 70F gives more power and energy capability, but it is not likely to be the point of lowest capacity fade when cycling. I think the ideal temperature to cycle the battery for the least capacity fade is below 20C, likely around 10C. But 10C does not give you as much available energy or power capability as 20C. GM clearly had to pick a point which gives the best trade-off between battery life, available capacity and power capability. But they are demanding almost 40% more power from a battery with 2/3 the capacity of the LEAF. (Higher rate discharges, BTW, result in higher capacity fade.) GM likely can't get the power they need at lower temperatures, particularly as the battery degrades. Since Nissan only requires the LEAF battery to discharge at 3C instead of the 6C that GM demands from the Volt battery, they can let the temperature go a bit lower and still meet the requirement.

As far as charging goes, the LEAF charges at something like C/7 on L2, so I doubt that heating is an issue during L2 charging. NREL's testing of the LEAF indicates that 97% of the electrical energy that goes into the LEAF battery comes out as electrical energy. Only 3% is dissipated as heat. Of course some of this will be dissipated during charging and some during discharging. Discharging is not a big issue for the LEAF since it is cooled by airflow and experiences a very small temperature rise, so let's assume all the energy is dissipated during charging just to get a worst case. That's 100W spread throughout the entire 650-lb. battery for the duration of the charge. I don't know how much that would raise the temperature, but I'll say it's not a lot.

Quick charging the LEAF is another story altogether. I think we all know that massively heats the battery since it is a high rate and there is nothing to get the heat out of the battery while charging since the LEAF is not moving.

I'll certainly agree that for a vehicle that needs to cover climates all around the world like the Volt or the LEAF, a TMS makes sense to make the battery last longer. But where I live and for my application, I don't think the TMS and smaller battery combination is an improvement. I should be able to achieve lower degradation rates and have more reserve capacity with the LEAF than the Volt.

 

[surfingslovak]

I should be able to achieve lower degradation rates and have more reserve capacity with the LEAF than the Volt.

Although the driver community has amassed an impressive amount of knowledge, important details about cell composition, pack assembly, TMS and BMS are missing or unknown. I think we are still learning. And, yes, GM has been much more open with engineering information than Nissan. In fact, I think their openness and their efforts to promote their technology is something Nissan should look into, and possibly adopt. That said, it should be obvious that GM has not shown all its cards, and I guarantee you that some of the assumptions made in this thread, and elsewhere, could prove to be wrong. It wouldn't be the first time. When you go back to the beginning of this forum, a number of the predictions made then did not pan out. Others did. Same goes for the Volt forum or TMC. It's fun to talk about it, that much is sure, and I believe that we all learn from each other.

 

[JimSouCal]

When you go back to the beginning of this forum, a number of the predictions made then did not pan out. Others did.

An very interesting thread, in another topic, would point out those that came true, and predictions that did not pan out... So far, I am comfortable with outcomes. I do hope that a vibrant 3rd party upgrade market emerges for the LEAF. As a matter of fact, I'd submit that Nissan should work to foster such an environment to their own ends...

 

[P71VIC]

You can't do this in a Volt:


Untitled by RpryceM, on Flickr

 

[kubel]

I think you totally got it wrong about the Volt power train because you don't understand it well enough. It IS a series hybrid most of the times...

I understand the power train. It's not a series hybrid. It's an electric dominant parallel hybrid. The simple fact about the Volt is that there is a mechanical link between the drive wheels and the genset, and this link is why the current generation Volt will never be a series hybrid, it will always be an electric dominant parallel hybrid. You argue there are some merits to that, and that the training wheels only touch the ground in certain rare circumstances, and I don't disagree. But it still has its training wheels. I would be more impressed with the Volt if it lived up to its original promise of being a pure series hybrid.

 

[Volusiano]

I think you totally got it wrong about the Volt power train because you don't understand it well enough. It IS a series hybrid most of the times...

I understand the power train. It's not a series hybrid. It's an electric dominant parallel hybrid. The simple fact about the Volt is that there is a mechanical link between the drive wheels and the genset, and this link is why the current generation Volt will never be a series hybrid, it will always be an electric dominant parallel hybrid. You argue there are some merits to that, and that the training wheels only touch the ground in certain rare circumstances, and I don't disagree. But it still has its training wheels. I would be more impressed with the Volt if it lived up to its original promise of being a pure series hybrid.

I guess we have to agree to disagree. You can call it a training wheel or parallel hybrid or whatever you want. It's just a technical term if you like to stick to name calling. Who cares if it's pure series or part parallel or whatever? You make it sound like parallel hybrid is a dirty word or something. The fact is that the parallel hybrid in the Prius serves it very well when applied at the right time and the right place in the right situation.

The simple fact is that it allows the Volt to be more efficient at higher speed. Not just fuel efficient but also electrically more efficient, too. That's the bottom line. However they implement it or however you call it is irrelevant, as long as the end result is better efficiency in either fuel mode or electric mode without any significant trade off. Even more electrically efficient than the LEAF at high speed, that's for sure. That's what relevant. You may claim you understand the Volt power train (which I seriously doubt you do based on your comments so far), but apparently you don't understand the end result. There's nothing wrong with any kind of implementation that gives positive end result (higher efficiency in this case) with minimal trade-off.

Why don't you go ahead and name (actually don't "name" because obviously you like to "name" things, but show) one bad end result or trade-off of the Volt having this "training wheel" on (as you call it)?

 

[LEAFer]

I don't care how it does it. The Volt allows an AER (all-electric-range) of 35-40 miles, and sometimes more. We keep it within those limits whenever practical. When not practical either one of two things happens/has happened: use the BEV or we're on a looong roadtrip.

And before someone brings up the "training wheels" or "transition vehicle" argument again ... our case is "A-typical" ... the Volt is *NOT* the vehicle we bought first because we were "scared" of all-electric BEV or have range anxiety. The Volt was acquired LAST, replacing the final (third) ICE in a three person household. Thus, rather than being a "transition" vehicle ... it is our road-trip vehicle.

First year statistic of 2xBEV+Volt: 38k total miles, 2k on gas @ 40mpg = 50 gallons used. (Just for fun: that's 38,000/50 = 760 MPG ... ok I know that's not fair ... but the 36k EV miles are at 3c/mile.)

 

[SanDust]

Whenever you drive a Volt it heats the cells up 72F until you shut it off. Then the Volt holds this higher temperature for a much longer time because the battery is insulated to support the TMS function. I suspect the thermal time constant of the Volt battery is over eight hours. When you charge the Volt battery, it heats it then, as well. The same applies.

I think you're underestimating the value of a TMS and over simplifying it. The TMS on the Volt won't just heat up the battery pack to 72F every time you turn the car on, though that would be good for pack life. Here is what SAE has to say about the thermal window: The system is programmed to stay within 3.6°F (2°C) of the pack's optimal temperature, which depending on usage conditions falls in a range between 50 and 85°F (10 and 30°C).

FWIW Frank Weber, who is now the head engineer for the BMW i3 and i8, has said that keeping the Volt's battery at 72F or room temperature at all times would result in the longest life. Again, I think you're confusing the best temperature for long term storage with the temperature needed for operation.

One observation I'd like to make in light of these points is that I've had my Volt for a couple of months now and February and March in Phoenix is very ideal and mild weather. Yet, almost every time I plug in my Volt, I can almost always hear the TMS working, circulating coolant through the battery. Not just for a short time in the beginning of the charge cycle, but for quite a while.

Could also just be a pump circulating the coolant to eliminate temperature differentials in the battery pack, thereby increasing pack life. You could probably test this by putting a Killowatt on your outlet and seeing what the draw was, assuming you were using 120v and not charging.

 

[RegGuheert]

FWIW Frank Weber, who is now the head engineer for the BMW i3 and i8, has said that keeping the Volt's battery at 72F or room temperature at all times would result in the longest life. Again, I think you're confusing the best temperature for long term storage with the temperature needed for operation.

Here is Frank Weber's exact quote (with the question asked):

Can you say how low a temperature can the battery go on at?
No. A certain operating window that you have. You don’t have to always keep it at 71 degrees F. Ideally that is the temperature you would like it because that is where you have the maximum power output of the battery and you have the best life expectations.

Note that he is saying that 71F is the ideal temperature for power AND battery life. As I noted previously, a Volt can be asked to discharge at 6C or faster when it is driven and the papers I have seen indicate you need to keep a Li-ion battery pretty warm to be able to do that. So I can believe that for the Volt you need to keep it that warm. But the LEAF battery does not ever get discharged at 6C. As a result, you will likely find that the optimum temperature for operation to achieve maximum life is lower in the Nissan LEAF than for the Chevy Volt since you are never discharging as fast.

Which brings us back to the main point of our discussion: I think the battery in our application of the LEAF will last longer than a Volt battery in the same service. Again, because of the smaller battery in the Volt, all discharges will be deeper and all discharges will be at a higher rate than in the LEAF. Many studies show that discharging a Li-ion battery to higher DODs reduces battery life. Likewise, many studies show that discharging Li-ion batteries at higher rates reduces battery life.

So at 71F battery temperature where the Chevy Volt operates, which is probably about 65F ambient, it is clear that a LEAF battery will get better treatment and therefore longer life than that of the Chevy Volt in the same service since it will be cycled less and at lower rates. But the LEAF probably has an optimum point below that of the Volt, probably 50F to 60F. So there is basically a band of temperature around this optimum point in which operation of the LEAF is less stressful to its battery than operation of the Chevy Volt's battery due to DOD and cycle rate differences. At temperatures above and below this band, the Volt battery will have easier service. So what is this band of temperatures? I'm quite sure it extends further on the low side than on the high side, so I'll guess it likely extends from roughly 30F to about 75F. At around those two temperatures, operation of the LEAF and the Volt is likely quite similar in battery "damage" if you will. In the center of that range, the LEAF does less damage to its battery and the farther you move outside that range, the Volt likely does less damage to its battery.

Our LEAF lives the vast majority of its life within that range and only ever extends slightly outside of it on either end, so I expect to see a very long life and longer than that of the Chevy Volt.

We already know that the LEAF batteries do not do well at very high temperatures, so it is clear that going outside that range on the high side is harmful. Likewise, I think quick charging can be quite hard on a LEAF battery.

We do not yet have reports of shortened LEAF battery life at very low temperatures. It will be interesting to see how things play out in Norway and Colorado and Minnesota and similarly cold areas. Clearly things are not as bad as Phoenix, but we should expect to see some reduction in life in those places.

As with all things battery, time will tell.

 

[SanDust]

Note that he is saying that 71F is the ideal temperature for power AND battery life. As I noted previously, a Volt can be asked to discharge at 6C or faster when it is driven and the papers I have seen indicate you need to keep a Li-ion battery pretty warm to be able to do that. So I can believe that for the Volt you need to keep it that warm. But the LEAF battery does not ever get discharged at 6C.

I think the battery in our application of the LEAF will last longer than a Volt battery in the same service. Again, because of the smaller battery in the Volt, all discharges will be deeper and all discharges will be at a higher rate than in the LEAF. Many studies show that discharging a Li-ion battery to higher DODs reduces battery life. Likewise, many studies show that discharging Li-ion batteries at higher rates reduces battery life.

So at 71F battery temperature where the Chevy Volt operates, which is probably about 65F ambient, it is clear that a LEAF battery will get better treatment and therefore longer life than that of the Chevy Volt in the same service since it will be cycled less and at lower rates.

We do not yet have reports of shortened LEAF battery life at very low temperatures. It will be interesting to see how things play out in Norway and Colorado and Minnesota and similarly cold areas. Clearly things are not as bad as Phoenix, but we should expect to see some reduction in life in those places.

As with all things battery, time will tell.

I'm not sure where you're getting 6C from but that seems an extreme number. A Volt battery at 6C would be 100 kW and it needs less than 24 kW at the wheels when going 80 MPH. But my point here would be that a Leaf battery at lower temperature than optimal would be more stressed at a lower C rate.

The DOD points seem to assume that the problems with DOD are linear, meaning that a 40% DOD is worse than a 20% DOD which is worse than 10% DOD. That's not the case. A single 80% DOD can be 10X worse for cell life than 50X more discharges at a lower rate. The problems only occur at the deepest DOD. Differences between relatively mild DODs isn't significant (DODs less than 50% aren't usually even measured in testing). So yes a Volt might have a DOD of 30% when the Leaf has a DOD of 20% but that's not going to matter. The big deal will occur when you go 75 miles in your Leaf and have a DOD of 80%. That deep a DOD can't occur in a Volt because the generator kicks on and takes over far short of this point.

I also don't think you understand how the Volt uses its TMS. Let's use your example of what happens at 65F ambient. The battery in both cars start at 65F. You get on the freeway and start driving. My guess is that the Volt does nothing. Why? Because the chemical reactions are exothermic, heating takes energy, and the temperature will rise anyway. After ten minutes the cells are now at 68F. The Leaf still does nothing but my guess is that the Volt will start circulating the coolant through the radiator to keep the coolant and hence the cell temperature under 70F. Fifteen minutes later the Leaf cells are at 78F and the Volt cells are at 70F. You keep saying that in this scenario the Leaf cells will be at a lower temperature than the Volt's but I don't see how that would happen.

The other problem you're ignoring is hot spots. All batteries without a liquid TMS will develop hot spots. Those can be serious over the long term and the Leaf simply has no way to prevent them.

As for what will happen long term, my question is whether you've measured your gids. I did and I lost almost 10% of capacity in one year in a very mild climate. A friend who drives more miles and more on the freeway lost almost 15%. This was very surprising to me. In some sense the warranties tell us all we need to know about which battery will last longer. GM warrants capacity to stay between 10% and 30% for ten years (eight in non-CARB states). Nissan warrants capacity to stay at not more than a three bar loss over five years, which I interpret as warranting against greater than a 25% capacity loss (3/12). Those are not comparable.

 

[Reddy]

RegGuheert & SanDust: You guys are great. Keep up this civil discussion. I'm impressed, and think you're both right (if that's possible). Yes, time will tell on battery degradation. Unfortunately, it will be difficult for us mere mortals to collect data on the thousands of batteries and environmental/driving/charging conditions that would be required for a complete analysis. Only Nissan and GM will have that data and they probably won't share it with us (or especially each other). One thing that pushes me to GM's side (even though I personally own and prefer the Leaf), is the useability of the hypothetical 50% degraded product. We don't know when 50% degradation will occur, and certainly you guys have debated that better than others, but we know it will eventually happen. Most users will likely find the 35-40 mi Leaf no longer useable, whereas the 15-20 mi Volt will just be a longer-ranged PiP. Thus, Nissan may be forced to replace/refurbish batteries before GM. This may tarnish BEVs before EREVs, at least for the uneducated masses. Just my 2 cents for today. Tomorrow it may be different. :roll:

 

[Volusiano]

RegGuheert & SanDust: You guys are great. Keep up this civil discussion. I'm impressed, and think you're both right (if that's possible). Yes, time will tell on battery degradation. Unfortunately, it will be difficult for us mere mortals to collect data on the thousands of batteries and environmental/driving/charging conditions that would be required for a complete analysis. Only Nissan and GM will have that data and they probably won't share it with us (or especially each other). One thing that pushes me to GM's side (even though I personally own and prefer the Leaf), is the useability of the hypothetical 50% degraded product. We don't know when 50% degradation will occur, and certainly you guys have debated that better than others, but we know it will eventually happen. Most users will likely find the 35-40 mi Leaf no longer useable, whereas the 15-20 mi Volt will just be a longer-ranged PiP. Thus, Nissan may be forced to replace/refurbish batteries before GM. This may tarnish BEVs before EREVs, at least for the uneducated masses. Just my 2 cents for today. Tomorrow it may be different. :roll:

Reddy brings up a very good point here. With the Volt, at least when the battery reaches the end of its usefulness, it's not rendered useless because it can still run perfectly fine on the ICE until battery technology matures enough to provide a better/cheaper replacement.

On the LEAF, when the battery reaches the end of its usefulness (and this can easily be as little as 30-40% capacity loss for many people), if there's no better/cheaper battery replacement option for it yet, then one cannot afford to wait and run on ICE until an option becomes available. Hence in terms of resale value, the Volt's resale value may hold up better than the LEAF's resale value down the road because the Volt can remain to be in service while the LEAF's service may be forced to come to a more abrupt halt unless battery technology catches up with it fast enough.

One thing I notice is that on the Volt forum, there's been posters who asked about used Volt for sale and they're hard to come by. Especially because Volts are only 2 years old and many of the leases still have another year on their term before they get returned. Any used Volt found usually tend to command a decent resale price. Used LEAFs, on the other hand, are a lot more available and at much lower prices than expected.

Recently a Volt owner in Phoenix needed another car for his very short commute (15 miles RT) and went for a used LEAF because he found a great price on it ($17K for 14.8K miles and still at 12 capacity bars) and it would serve his short commute just fine hopefully for a long time even with the heat capacity concern in Phoenix.

 

[rubear]

I think you totally got it wrong about the Volt power train because you don't understand it well enough. It IS a series hybrid most of the times at most speed until you get to 70mph. And at 70mph, the decision to turn it into a parallel hybrid is not a crutch (or putting on training wheels as you call it), but it is to make the electric motor run more efficiently (10-15% efficiency improvement). It's actually a stroke of genius for GM to implement it with this approach.

To understand why GM did what it did, first you have to realize that the electric motor runs less efficiently when spinning at high speed during freeway driving (70mph range). So GM wanted to find a way to slow down the main motor at 70mph to improve efficiency. And we're just talking about the Charge Depletion mode here (running 100% on battery when it still has plenty of juice). Instead of adding more hardware to slow the main motor down at 70mph to improve efficiency, GM recognized that it already has a secondary motor right next to it (which is the generator driven by the ICE), which sits there idle during Charge Depletion mode because the ICE is not running. So why not make good use of this secondary motor at 70mph instead of letting it sit idle unused? By the way, the main motor drives the Sun gear in a planetary gear system, and the planetary gears drive the wheels (at 7:1 ratio). So GM decides to engage the secondary motor by clutching it to the ring gear at 70mph, using battery juice to drive the secondary motor. This has the effect of slowing down the main motor to a slower rpm (both motors are being driven by battery juice at this point, and both are now driving the planet gears which engages the wheels). By doing this, the main motor is slowed down enough that results in 10-15% efficiency gain at 70mph.

In the Charge Sustaining mode (when battery is out of juice and ICE is turned on), GM decided to do the same thing at 70mph as explained above, engage the secondary motor to slow down the main motor to gain the 10-15% efficiency. Except that in this case, the ICE is also driving the secondary motor as well (as a generator), so there's a mechanical coupling effect from the ICE to the secondary motor (using it as a generator to create juice to the main motor), but also a mechanical coupling between the secondary motor to the ring gear as well (to slow down the main motor). So in this situation, it's not really a pure parallel hybrid, nor is it a pure series hybrid, but it's BOTH! ICE generates juice to the main motor (via the secondary generator/motor) -> series hybrid is still going on, but ICE is also driving mechanically the wheels through the coupling of the secondary motor to the ring gear to the wheels -> parallel hybrid is going on.

As you can see, it's not because the ICE still needs "training wheels" (like you put it), but it's done that way ONLY BECAUSE GM wants to improve the main electric motor's efficiency at high rpms. So the ICE is actually induced to do 3 functions (kill 3 birds with 1 stone): 1. generate juice to the main motor, 2. provide additional mechanical power to the wheels, 3. improve the efficiency of the main motor by virtue of coupling the secondary motor to the ring gear to slow the main motor down. The mechanical link that you call the parallel hybrid is PURELY a BYPRODUCT of the idea of using the secondary motor to slow down the main motor at 70mph. The main purpose is not because GM wants to engage in parallel hybrid. The main purpose is to slow down the main motor for better efficiency. It just happens that in the Charge Sustaining mode (ICE is running), this coupling between the ICE to the second motor to the main motor causes the parallel hybrid phenomenon to happen naturally.

So what GM has implemented here is not putting a crutch or training wheels on the power train. On the contrary, it's a stroke of genius that GM makes good use of ALL motors/engine available to achieve the most efficiency at high rpms, instead of letting things sit idle and go to waste.

Excellent explanation. I am a new 2013 Leaf owner and I think the Leaf will work out best for me, for the kind of driving that I do. Considering the $7500 tax credit and in the state of Washington we get an exemption for sales tax as well. So that took me over the edge.
But the Volt is an engineering marvel and for many, it would provide the owner with more electric miles than the Leaf. As you point out, if you are going to travel 75 miles, it's not likely you'd chance it in the Leaf without a charging station. You'd take the Volt and get 40 miles.
Again, thanks for the explanation of the technology in the Volt, I think you've cleared up a lot of misinformation.

Automobiles:
2013 Leaf
2005 Prius
2001 Honda Insight (use it in my business for service)

Solar:
Two dual axis solar trackers

Home:
Heat by geothermal heat pump
Structural insulated panel walls
Heat recovering ventilator.

-Robert