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

[stephent]

There's a lot of discussion about behavior at "70mph" on the Volt on this thread. This was something that was misreported and propagated to a lot of places, but isn't actually true. I think some Volt engineer while describing operation of the drivetrain used 70 mph as an example speed where both motors were driving the wheels, and some reporter misinterpreted 70 mph as an inflection point. This was clarified on the Volt forums some years ago. There's no step-function for one vs. two-motor operation for the Volt at 70 mph, in either CD or CS modes. It's chosen by a computer depending on speed & demanded torque, it can go into 2-motor as low as ~35 mph in CS mode, and I believe higher, ~50 in CD mode. If you mash the accelerator it will switch back to 1-motor for more available torque but less efficiency, until you let up for awhile.

The explanation in previous post quoted from the earlier post is otherwise very good, other than the "70 mph" thing.

 

[RegGuheert]

Here is a spreadsheet showing every trip taken in our LEAF in the past four months, from 1DEC2012 until 31MAR2013. Unfortunately, this is all the Carwings data that I have. I have added summations at each point an excursion was completed and the car was charged to sum up the kWh used and the miles driven. These summation rows have an "S" in the column labeled "Trip ID".

I have also added two columns. One column is for the calculated DOD what would have occurred for a brand new 2011 Nissan LEAF and the other is the calculated DOD that would have occurred for a brand new 2011 Chevy Volt. The DOD on the Volt column is limited to 80% since what I read is that the Volt's ICE starts at 22% SOC and begins charging at around 20% SOC.

The DOC calculation assumes that each vehicle is charged to full just before the trip, for which I use 94% for the LEAF and 85% for the Volt. I also assume that a Volt would have used the same number of kWh as the LEAF for the trips. Battery capacities are assumed to be 24kWh for the LEAF and 16kWh for the Volt. Available battery capacities are assumed to be 21 kWh for the LEAF and 10.5 kWh for the Volt. No degradation has been used because I do not know how degraded our LEAF battery is and no one knows how degraded any Volt battery is.

In any case, this spreadsheet gives me an idea of the benefits of having a larger battery for OUR daily driving. What I see are the following:

- EVERY trip in the Volt would have resulted in a higher DOD than in the LEAF had we always charged to full.
- A Volt would have experienced 26 trips to 80% DOD within the past four months.
- The maximum DOD for the LEAF during the past four months would be about 73.5%.
- Average DOD for trips in the Volt would be 56% while average DOD for the LEAF would be about 37%.

In reality with our degraded LEAF and actual planning, there was one 90% DOD in the LEAF in March 1 due to the car not being charged for a trip. There were also no more than about four visits to LBW, but no VLBW during this period. Our LEAF has been to VLBW four times in its life but never more than one mile beyond that point.

So, can the SOC limits and TMS of the Volt make up for a higher average DOD and ~75 trips to 80% DOD each year combined with a higher peak discharge rate of over 6C and higher average discharge rate on every cycle? It certainly can in very hot climates, but I seriously doubt it can make up that deficit in OUR climate. The larger LEAF battery can degrade a long way before too many of our trips need to migrate to a different vehicle.

I will caution anyone who may conclude that we would have burned more gasoline if we had a Chevy Volt since there are trips in other ICE and hybrid vehicles which are not included here. It could even be true that we could have driven more EV miles in a Chevy Volt than we did in our LEAF, but I doubt it. There is no way to be sure either way.

 

[Volusiano]

There's a lot of discussion about behavior at "70mph" on the Volt on this thread. This was something that was misreported and propagated to a lot of places, but isn't actually true. I think some Volt engineer while describing operation of the drivetrain used 70 mph as an example speed where both motors were driving the wheels, and some reporter misinterpreted 70 mph as an inflection point. This was clarified on the Volt forums some years ago. There's no step-function for one vs. two-motor operation for the Volt at 70 mph, in either CD or CS modes. It's chosen by a computer depending on speed & demanded torque, it can go into 2-motor as low as ~35 mph in CS mode, and I believe higher, ~50 in CD mode. If you mash the accelerator it will switch back to 1-motor for more available torque but less efficiency, until you let up for awhile.

The explanation in previous post quoted from the earlier post is otherwise very good, other than the "70 mph" thing.

Great point! Yes, the 70 mph thing was just an example since that's what was used in the GM materials, too. But in case it was taken too literally as an inflection point, the clarification above hopefully puts it to bed.

 

[SanDust]

Here is a spreadsheet showing every trip taken in our LEAF in the past four months, from 1DEC2012 until 31MAR2013.

I have also added two columns. One column is for the calculated DOD what would have occurred for a brand new 2011 Nissan LEAF and the other is the calculated DOD that would have occurred for a brand new 2011 Chevy Volt. The DOD on the Volt column is limited to 80% since what I read is that the Volt's ICE starts at 22% SOC and begins charging at around 20% SOC.

The DOC calculation assumes that each vehicle is charged to full just before the trip, for which I use 94% for the LEAF and 85% for the Volt. I also assume that a Volt would have used the same number of kWh as the LEAF for the trips. Battery capacities are assumed to be 24kWh for the LEAF and 16kWh for the Volt.

- EVERY trip in the Volt would have resulted in a higher DOD than in the LEAF had we always charged to full.
- A Volt would have experienced 26 trips to 80% DOD within the past four months.
- The maximum DOD for the LEAF during the past four months would be about 73.5%.
- Average DOD for trips in the Volt would be 56% while average DOD for the LEAF would be about 37%.

In reality with our degraded LEAF and actual planning, there was one 90% DOD in the LEAF in March 1 due to the car not being charged for a trip. There were also no more than about four visits to LBW, but no VLBW during this period. Our LEAF has been to VLBW four times in its life but never more than one mile beyond that point.

I think you're obsessing with DOD. No doubt DOD is important, but more important than DOD would be the DOD and the SOC when the battery is fully charged. In short, the critical part would be the percentage of the cell that is used.

While you're right that deeply discharging a battery isn't good for cell life, you're missing the point that fully charging it can likewise be a problem. You can see that with how both the Volt and the Leaf are engineered. The Volt uses about 60% of the battery, which means the engineers have 40% of unused capacity to play with, allowing them to use it either at the top or bottom end of the SOC. If a deep DOD were more problematic than a full charge, the engineers could have designed the Volt to charge to a 90% SOC and allowed it to discharge only to a 30%. They didn't. They chose to more or less evenly apportion the unused battery space to the top and bottom of the SOC. Nissan made a similar choice. It recommends that you charge to a 70% SOC (80% of 90%), but allows you to fully discharge to 10%. It could have recommended fully charging and recommended either avoiding deep discharges or electrically preventing it. But it didn't, choosing to emphasize avoiding charging at the top end rather than discharging at the bottom.

When you take these engineering decisions together, it seems that both companies are indicating that fully charging is at least as damaging to cell life as fully discharging. And your DOD numbers only capture the latter and not the former.

Also note that the cell chemistry matters. I think A123 claimed you could discharge their cells to 100% DOD for 7000 times or something crazy like that. Different chemistries will be sensitive to different things. Some cells may have their lives extended by avoiding deep discharges; some may have their lives extended more by avoiding full charges.

 

[TomT]

It's certain that, when our lease on the Leaf is up, we'll consider the Volt again if it is then the new platform and gen-set.

 

[RegGuheert]

I think you're obsessing with DOD.

No, as I said previously, I chose the vehicle which has the lower DOD, allows me to store it at a lower SOC yet charge to a higher SOC just before a trip and discharges and charges at lower peak and average rates. I'm only addressing DOD now because you are pointing out how much worse it is to go low. I agree.

No doubt DOD is important, but more important than DOD would be the DOD and the SOC when the battery is fully charged. In short, the critical part would be the percentage of the cell that is used.

Sure, but like everything, the devil is in the details. As I have alluded to previously, the problem with a high SOC is more related to calendar life than cycle life, since storing cells with the electrolyte used in the LEAF and the Volt at a high SOC causes the cells to degrade more quickly than storing them at a moderate SOC. That is another plus for the LEAF for our application since it has twice the useable capacity of the Volt we can let it sit at an SOC of 60% most of the time and only charge higher if/when we need to go out for a longer trip. Most Volts sit around charged to 85% (or higher for the newer or degraded ones).

While you're right that deeply discharging a battery isn't good for cell life, you're missing the point that fully charging it can likewise be a problem. You can see that with how both the Volt and the Leaf are engineered. The Volt uses about 60% of the battery, which means the engineers have 40% of unused capacity to play with, allowing them to use it either at the top or bottom end of the SOC.

2011/2012 Volt uses 65% of the SOC range, 20%-85% when new. 2013 Volt uses 66.5%, 20% to 86.5%, when new.

If a deep DOD were more problematic than a full charge, the engineers could have designed the Volt to charge to a 90% SOC and allowed it to discharge only to a 30%. They didn't.

Of course they didn't! The Chevy Volt has such a small amount of energy available to the user that it must be kept charged to the max that GM allows all the time. Unfortunately it is not clear that 85% or even 80% is much better for storage than 94%. Some studies I have seen show it as being worse.

They chose to more or less evenly apportion the unused battery space to the top and bottom of the SOC. Nissan made a similar choice. It recommends that you charge to a 70% SOC (80% of 90%), but allows you to fully discharge to 10%. It could have recommended fully charging and recommended either avoiding deep discharges or electrically preventing it. But it didn't, choosing to emphasize avoiding charging at the top end rather than discharging at the bottom.

Nissan does not offer a 70% SOC charge option. When you choose 80% SOC, you get 80% SOC, or very close to that. We keep our LEAF charge to a lower SOC until the car is needed.

When you take these engineering decisions together, it seems that both companies are indicating that fully charging is at least as damaging to cell life as fully discharging. And your DOD numbers only capture the latter and not the former.

It's not true. Charging to the high SOCs is not as damaging unless you charge it up there and leave it there like you would with the Chevy Volt. We don't charge to full until just before we leave.

Also note that the cell chemistry matters. I think A123 claimed you could discharge their cells to 100% DOD for 7000 times or something crazy like that. Different chemistries will be sensitive to different things. Some cells may have their lives extended by avoiding deep discharges; some may have their lives extended more by avoiding full charges.

That's a red herring. We both know that neither the Nissan LEAF nor the Chevy Volt use the A123 chemistry. What is used in the two cars are much more similar to each other than they are to that chemistry. But I said this up front in the post that you responded to: we don't know if there are benefits to one chemistry over the other, so that is not a buyer factor at this point. It seems just as likely at this point that Nissan has a superior chemistry to the LG Chem technology that GM uses as vice-versa. In the future if we get more data on LEAF and Volt battery degradation rates, we will have a better idea. Of course by then, both companies will have moved on.

 

[tallgirl]

I own two gasoline-powered cars -- an '02 Gran Prix and a '79 Corvette. In the end, I decided I didn't need a car with a gasoline motor in the trunk when I already had two with a gasoline motor under the hood. My son is going to get one of the cars and the other will most likely be sold. When the time comes, I'll most likely get a Volt or whatever Cadillac calls their version of the Volt -- I'm an old fart and feel the need to drive a Caddy.

 

[Volusiano]

If a deep DOD were more problematic than a full charge, the engineers could have designed the Volt to charge to a 90% SOC and allowed it to discharge only to a 30%. They didn't.

Of course they didn't! The Chevy Volt has such a small amount of energy available to the user that it must be kept charged to the max that GM allows all the time. Unfortunately it is not clear that 85% or even 80% is much better for storage than 94%. Some studies I have seen show it as being worse.

This response sidestepped SanDust's point altogether that engineers didn't design the Volt to charge at higher SOC and to stop the discharge at higher SOC because they didn't deem it necessary. The Volt doesn't have "such a small amount of energy" like RegGuheert said. The Volt has plenty of energy for the majority of trips most people would make in a day. It's irrelevant that it must be kept charged to the max from the perspective of Sandust's point that GM engineers didn't need to choose a higher SOC at the top and higher SOC at the bottom. I feel like RegGuheert used a diversionary response here to not have to address SanDust's very valid point, that GM engineers didn't see the need to do what RegGuheert advertised as important, because to GM engineers, it's not as important as claimed by RegGuheert.

They chose to more or less evenly apportion the unused battery space to the top and bottom of the SOC. Nissan made a similar choice. It recommends that you charge to a 70% SOC (80% of 90%), but allows you to fully discharge to 10%. It could have recommended fully charging and recommended either avoiding deep discharges or electrically preventing it. But it didn't, choosing to emphasize avoiding charging at the top end rather than discharging at the bottom.

Nissan does not offer a 70% SOC charge option. When you choose 80% SOC, you get 80% SOC, or very close to that. We keep our LEAF charge to a lower SOC until the car is needed.

But it did. The LEAF has 24 kwh but only 21 kwh is used. So if you take (21khw/24kwh)*.8, you get 70% from 24kwh.

When you take these engineering decisions together, it seems that both companies are indicating that fully charging is at least as damaging to cell life as fully discharging. And your DOD numbers only capture the latter and not the former.

It's not true. Charging to the high SOCs is not as damaging unless you charge it up there and leave it there like you would with the Chevy Volt. We don't charge to full until just before we leave.

One can argue just the same that if one charges up right after one reaches the low SOC, then it's not as damaging either. Why wouldn't the same logic apply there? Has there been any published paper to prove otherwise? And usually by the time one reaches the low SOC, one already ends up at home in the garage charging up right away again.

What RegGuheert is implying here is that reaching a deep SOC causes irreversible damage even if you recharge it right away to take it out of that mode asap, while topping off at high SOC does not cause irreversible damage if you use it up right away. While I don't argue with the later, I don't necessarily agree with RegGuheert's point about the earlier, especially if it's not a VERY DEEP discharge. Remember that you're NOT ALLOWED to cause a very deep discharge on the Volt battery anyway, because the ICE will take over as soon as the MILD discharge point (20%) is reached. On the other hand, the LEAF is allowed to drive all the way to Turtle mode, way past LBW and VLBW, to the point where limp mode occurs. Surely this is much deeper than the 20%. So while the Volt is protected against very deep discharge, the LEAF is not.

We can beat this horse to death, but the whole point is that the Volt, because it has the ICE and TMS, has these luxuries in place to allow it to be engineered to have all possible protections in place, from too high a charge to too low a discharge, from too high a temperature to too low a temperature. Meanwhile, the LEAF, because it doesn't have an ICE and no TMS, has to be compromised in order to provide as much range as necessary to owners. The compromise allows from 100% charging to Turtle level discharging. And the lack of TMS allows as much as 7 to 8 temperature bars operation. QC, which is allowed (a necessary thing to sell the car), can exacerbate the deterioration of the battery if used in hot weather or right after heavy driving.

Sure, a careful LEAF owner can make sure to avoid all those conditions to help prolong their battery life as much as possible. But without all the protections that has been put in place on the Volt to avoid owners' abuse, the LEAF is much more vulnerable to owners' abuse. And sometimes you can't even blame the unsuspecting owners for "abusing" it because Nissan creates false marketing impression to advertise the LEAF's capability to its limits (claiming 100 mile range for example), without doing much to prevent owners from pushing the LEAF's battery to its limits. The fact that Nissan is still selling LEAFs in AZ like nothing ever happened underlines this point for sure.

 

[dgpcolorado]

But it did. The LEAF has 24 kwh but only 21 kwh is used. So if you take (21khw/24kwh)*.8, you get 70% from 24kwh.

That isn't quite true. IIRC Ingineer has measured the range from 94-95% at the top ("100% charge") to about 2% at turtle. Call it 92% or about 22 kWh for the usable part of a new LEAF battery pack. My impression is that "21 kWh" is used here as the capacity for convenience, since most people aren't going to want to go to turtle on a regular basis.

 

[Herm]

IIRC, 80% on the Leaf is actually 80%, if you go by GIDs.. that crazy GOM!

 

[RegGuheert]

Sure, a careful LEAF owner can make sure to avoid all those conditions to help prolong their battery life as much as possible. But without all the protections that has been put in place on the Volt to avoid owners' abuse, the LEAF is much more vulnerable to owners' abuse. And sometimes you can't even blame the unsuspecting owners for "abusing" it because Nissan creates false marketing impression to advertise the LEAF's capability to its limits (claiming 100 mile range for example), without doing much to prevent owners from pushing the LEAF's battery to its limits. The fact that Nissan is still selling LEAFs in AZ like nothing ever happened underlines this point for sure.

No argument from me on all these excellent points! The Volt is engineered to allow the battery to have a long life in a very wide of environments and with a wide range of user behaviors.

But I feel we can do better for our application given the 50% larger battery and more lenient restrictions. That doesn't mean I feel that will be the typical user experience. Many, like Volusiano, in Phoenix cannot possibly get as long a life from a LEAF battery as they would from a Volt. And even in more temperate climates like Seattle, most LEAF customers are not likely to achieve a very long battery life given the lack of user education provided by Nissan.

 

[Stoaty]

Interesting discussion, I don't have much time for the remainder of the week, but you might want to look at this technical paper from NEC, which was posted on the forum earlier. There were many questions about the validity of the Arrhenius equation, and its applicability to the battery pack in the LEAF. Similarly, a question was raised about the activation energy and the applicability of the 10-degree-Celsius rule.

NEC is Nissan's partner in the AESC joint venture, which manufactures the battery packs for the LEAF, and I would consider them to be a relevant source. Perhaps even more so than the Battery University.

Added to Wiki under Battery Aging Model section, as NEC used assumptions similar to the Battery Aging Model:

http://www.mynissanleaf.com/wiki/index.php?title=Battery_Capacity_Loss#Battery_Aging_Model" onclick="window.open(this.href);return false;

It is interesting to note that this study estimated the Arrhenius factor for this new battery to be 6.85 degrees C. based on a model with 66% cycling and 33% storage (that's an awful lot of cycling). The new battery purportedly has twice the life span of the previous battery (which is presumably the one used in the 2011-2012 Leaf).

I wonder if that new electrolyte made it into the 2013 Leaf batteries...

 

[Volusiano]

Here's an interesting read just recently posted on the GM-Volt web site using some real data collected in Phoenix using the DashDAQ on a Volt.

http://gm-volt.com/2013/05/03/volt-battery-thermal-management-system-in-the-hot-arizona-sun/" onclick="window.open(this.href);return false;

 

[RegGuheert]

Here's an interesting read just recently posted on the GM-Volt web site using some real data collected in Phoenix using the DashDAQ on a Volt.

http://gm-volt.com/2013/05/03/volt-battery-thermal-management-system-in-the-hot-arizona-sun/" onclick="window.open(this.href);return false;

Very cool! (Literally!) Thanks!

 

[surfingslovak]

Here's an interesting read just recently posted on the GM-Volt web site using some real data collected in Phoenix using the DashDAQ on a Volt.

http://gm-volt.com/2013/05/03/volt-battery-thermal-management-system-in-the-hot-arizona-sun/" onclick="window.open(this.href);return false;

Very cool! (Literally!) Thanks!

Fantastic! Great stuff.

 

[Deleted member 9549]

I haven't committed either way yet, but one of the biggest drawbacks for the Volt is that I've seen the gm-volt forum ban multiple people not for saying anything offensive or inflammatory, simply for stating obvious facts about the Volt that they didn't like. Things like it only seats 4, only sold 1306 in April, inventory is running high at 9300, enough to last until 2014. wopontour (the moderator) specifically stated they want rose colored glasses.

If you have the primary discussion website for a product which bans people for being less than glowing about the Volt, how can you ever hear realistic viewpoints about the vehicle. Its scary. Censorship. Only allow positive posts.

As stated throughout this thread, advantages I see of the Leaf include:
- Much roomier, seats 5, bigger trunk
- Better visibility
- Longer EV range
- Much lower cost. I'm sitting on a 2yr lease offer that is hard to resist. Have to sell my wife on it.

 

[planet4ever]

I haven't committed either way yet, but one of the biggest drawbacks for the Volt is that I've seen the gm-volt forum ban multiple people not for saying anything offensive or inflammatory

Presumably you are referring to gm-volt.com. That website is neither owned nor controlled by GM. It was started by Volt enthusiasts, just as this forum was started by LEAF enthusiasts (and is not owned or controlled by Nissan). If I may say so, it seems a bit idiotic to badmouth a corporation for the way an independent forum is run.

Ray

 

[Deleted member 9549]

If I may say so, it seems a bit idiotic to badmouth a corporation for the way an independent forum is run.

Ray

I've always felt a product should stand on its merits. If owners of a product ban anyone who makes a non-positve comment, it shows a large level of insecurity which makes me as a potential consumer concerned.
Who wants to willing listen to only positive propaganda? GM owned or not, its the leading discussion forum for the Volt and has an astonishingly low bar to determine what posts to disallow. Wouldn't it make you more nervous to buy a Leaf if discussions of reduced battery capacity where not allowed? What other problems are being hidden?

 

[Deleted member 8423]

I've seen the gm-volt forum ban multiple people not for saying anything offensive or inflammatory, simply for stating obvious facts about the Volt that they didn't like.

I've stated many things there that I'm sure they don't like. I have not been banned yet.

I've not seen any such pattern. There are several people over there that I'd classify as total wack jobs that are not staying on topic. Not sure why they'd let those people there if they're only looking for positive posts.

 

[Deleted member 9549]

I've not seen any such pattern. There are several people over there that I'd classify as total wack jobs that are not staying on topic. Not sure why they'd let those people there if they're only looking for positive posts.

Just don't be negative on the Volt or GM on that forum. Its hard to notice disappearing members. Such members magically become inactive, otherwise known as banned. Only way to tell is that you can no longer PM them. One such thread was started by 'dogma' who magically disappeared after stating too many non-positive Volt sales stats.