Volt thermal management system temperature band?

[Charles Whalen]

Does anyone know the temperature band, and the average or optimal temperature within that band, that the Volt’s thermal management system keeps the batteries at?

For instance, the Tesla Roadster’s thermal management system, which also uses a liquid-based heat exchange method, keeps the batteries between 24C and 48C and at an average temperature of around 35C (at least here in my hot climate), which is about my daytime ambient, or just slightly above.

That 35C average temperature that the Roadster’s battery pack is maintained at by its TMS goes a long way towards explaining the 4 to 5 year expected pack life in a hot climate (and 3-year battery pack warranty). The Volt, on the other hand, has an 8-year battery pack warranty, and my understanding is that expected pack life in a hot climate should come close to that, except under some extreme circumstances. (1)

Though of course the Roadster’s and Volt’s battery packs have different cathodic chemistries -- LiCoO2 and LiMn2O4, respectively, those are both at the higher end of the heat sensitivity/degradation spectrum among the various lithium cathodic chemistries.

Given the difference between the Volt battery pack’s expected 8-year calendar life and the Tesla battery pack’s expected 4 to 5 year calendar life (in a hot climate), together with the 35C average temperature that the Tesla’s TMS keeps the batteries at, Arrhenius curve profiles relating battery life to long-term temperature exposure would lead me to deduce, or guess, that the Volt’s TMS might keep the batteries at an average temperature of somewhere around 21C. But I’m just wondering if anyone knows this for certain and what that temperature actually is?

Another thing I’m wondering is ... Does anyone know how much energy, i.e. how many kWh, on average, the Volt’s TMS consumes per day to run itself, to maintain the batteries at their optimal temperature and/or within the desired temperature band?

(Of course this might vary to some extent regionally/climatically and seasonally, as well as be dependent on specific conditions and circumstances, such as amount of solar loading exposure, availability of a climate-controlled garage, etc.)

For instance, when the Tesla first came out, its TMS was consuming around 10-12 kWh per day. The company was later able to damp that down to around 3-4 kWh per day (albeit likely at the expense of a higher optimal/average temperature and/or wider temperature band/limits).

I’m assuming that under extreme conditions, such as in a hot climate, and especially if exposed to significant solar loading on a daily basis (typically 50-60C), one would want to keep the Volt plugged in at all times, whenever the car is parked and within reach of an available 120V outlet or Level 2 EVSE, so that the Volt’s TMS can run itself on offboard power, rather than having to power the TMS from the batteries themselves. Does anyone know if that is indeed the case -- that the Volt’s TMS can and does power itself from offboard power whenever the car is plugged in, whether charging or not (i.e. before, during, and after charging)?

***

*(1): The kind of extreme circumstances under which the Volt’s battery pack might be expected to have a shorter life than 8 years could, for example, be the following type of scenario:

A Volt owner lives in a hot climate and has a 40-mile commute each way, 80 miles round-trip. Running the air-conditioning on his morning commute, he arrives at work each morning with the battery pack fully depleted, at 30% SOC, having just entered charge-sustaining mode. There are no 120V outlets nor Level 2 EVSE to plug into at work. Nor is there any shaded parking, so he has to leave his Volt baking out in the hot blazing sun, subject to 50-60C solar loading, every day. Having a fully depleted battery, with no reserve capacity above 30% SOC to run the TMS, nor any offboard power available to run the TMS, the liquid-cooled/water-chilled TMS can’t operate during the day while he’s at work.

The Volt’s well-insulated, sealed battery compartment will, of course, slow the rate of heat conduction into the battery compartment from the 50-60C interior of the car. Yet battery temps could possibly reach 32-38C by 5pm. ... Whereas, in contrast to a Nissan Leaf, not having an active TMS, under the same circumstances the Leaf’s batteries might get up into the 42-48C range. (2)

The Volt’s liquid-cooled TMS will then start back up and run on the 40-mile evening commute back home in charge-sustaining mode, with the gasoline engine providing the energy both to drive the car and run ancillary systems like the TMS. But the battery pack would likely spend at least a few hours each day exposed to elevated temperatures, above the presumed desired temperature range at which the TMS would optimally like to keep the batteries. Over time this will take its toll in a faster rate of degradation, possibly resulting in the batteries having a shorter life than 8 years.

Andrew Farah and Bob Lutz have both referred to this particular extreme example as the kind of worst case scenario representing the type of application for which the Volt is not well suited and an individual in this situation would be best advised that the Volt might not be right for him.

*(2): The Leaf has a reasonable TMS within the limited scope of what it is designed to do -- which is just to dissipate operationally-generated heat. With the way the battery cell form-factor and packaging, as well as the layout of the battery compartment and materials used, are all designed, it should do that quite well. However the Leaf’s passive TMS can only dissipate and shed operationally-generated heat down to the environmental temperature. The lack of an active TMS means that it cannot reduce battery temps below the environmental temperature, which in a hot climate combined with 50-60C solar loading, may well be up in the 40-50C range during the daytime.

[hermperez]

It was revealed by a GM engineer a bit back, it was one of the main page articles.. I seem to remember the battery was kept at 75° F

[Charles Whalen]

Very interesting. Pretty close to what I had guessed, which was 70F (21C). 75F sounds about right and makes sense.

Any idea where I might be able to search for that article? Do you remember what website domain it was published on or posted at? Was it here at GM-Volt.com? A couple months ago I went through and perused, at least glanced at, almost all of the articles Lyle has posted here on the main page of GM-Volt.com over the last year, but I didn’t see that one that you’re referring to, where you said a GM engineer revealed that the Volt’s TMS keeps the batteries at 75F. Would be very interested to see that.

Thanks for the info, hermperez!

[ChrisC]

I did some advanced googling myself,and came up empty except for this tidbit. Note that this is from Jan 2009, nearly two years ago, so is probably outdated.

http://gm-volt.com/2009/01/05/chevy-...ature-control/

Lyle: Can you say how low a temperature can the battery go on at?

GM's Frank Weber: No. A certain operating window that you have. You don’t have to always keep it at 71 degrees F (21.7 C). 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.

[Charles Whalen]

Chris -- Terrific! Thanks so much for going to the effort of finding and sharing that valuable nugget! Very valuable indeed. Provides lots of additional insight. 71F -- perfect; better than 75F. I was spot-on, almost exactly where I figured it should be to provide an 8 year life (based on exponential Arrhenius relationships of calendar life with respect to temperature).

Equally interesting and significant is how the article reveals that the TMS doesn’t operate when the car is parked but not plugged in, even when there is ample reserve capacity well above 30% SOC to do so. So for instance, to change slightly the example I gave above, let’s say that rather than a 40-mile commute each way, the Volt driver has just half that, namely, a 20-mile commute each way. He arrives at work in the morning with the battery pack at 55% SOC. Here again, same conditions as in the example above ... in that there are no 120V outlets nor Level 2 EVSE, and no shaded parking, so he has to leave the car parked out baking in the hot sun all day long, with 50-60C solar loading on the battery pack. This is now revealed as somewhat of a design deficiency that GM didn’t provide for the possibility for the TMS to operate autonomously, powered by the battery pack itself, for, say, at least the first 8 hours after the car is turned off, assuming that there is sufficient battery capacity above 30% SOC to do so, ... as now we see what’s going to happen in this modified example I just gave, ... where the well-insulated, sealed battery compartment will slow the rate of -- but can’t completely stop and prevent -- heat conduction into the battery compartment, ... such that by 5pm, it’s likely that battery temps will be up over 100F, having been solar loaded at 120-140F all day long. As I mentioned in my post above, this will gradually takes its toll over time and likely result in a shorter life than 8 years. This revelation is quite significant and provides a lot more insight into comments both Andy Farah and Bob Lutz have made where they have warned about this exact situation and scenario. ... Only now, we see that the universe of possibilities and people that will fall into this extreme category is expanded and much greater than would have otherwise been the case if the TMS were capable of operating when parked and not plugged in. In fact, my wife and I fall into this now newly-revealed, much-expanded, extreme condition category, as we’ve only got 10-15 mile commutes each way but do not have anywhere to plug in at work, nor do we have covered/shaded parking, either at work or at home (we have no garage and for years now have been charging our EVs outside in the driveway). If GM had designed the TMS properly, we should have had, and would have had, plenty of reserve capacity, above 30% SOC, during the day to power the TMS, to keep the battery temps at 71F. But because of that design flaw, combined with the lack of ability to plug in at work, our Volt(s)’ (I’ve ordered one and am hunting for a second one, but so far no luck) battery pack(s) will unfortunately be exposed to daily temperature excursions creeping up above 100F for much of the year. This really highlights the importance of *always* plugging the Volt in, and leaving it plugged in all the time, whenever a 120V outlet or Level 2 EVSE is within reach, in a hot climate where the car is subject to solar loading.

I’ve been dealing with this specific issue, of advanced chemistry batteries’ thermal sensitivity/degradation profiles, on both a practical level, from personal experience with EVs and batteries in a hot climate, and on a theoretical/academic level, for the better part of a decade.

[Rusty]

Granting that it's not great to cook the battery, it's worse to drain the battery at heat (and dangerous to try and charge the battery at heat). So would it ameliorate the issue if the Volt, sensing an over temperature condition, ran the car on the ICE, using excess energy over what was needed to drive it to cool the battery to a usable range?

That would essentially be driving the car on the power created but the ICE, but I'd think they probably have to do that as well at the other end of the temperature extream. Afte a Packer's game (and a -20F battery) I'd think they'd want to run off the ICE until they could bring the battery up to temp.

[Charles Whalen]

Rusty -- Yes, I would think, and believe, that the Volt does indeed do that at both ends of the temperature spectrum -- namely, run the ICE (even above 30% SOC, in charge-depleting mode) when battery temperature conditioning is needed to that extent (e.g. with battery temps above 100F or below 32F). But that’s when the car is on and running that you’re referring to, which is completely different from what I was talking about -- which is when the car is off and not plugged in.

BTW, trying to compare the low end of the temperature spectrum (below 32F) with the high end (above 100F) is an apples-to-oranges comparison in which there are some very important differences. The most important and salient (to my discussion above) difference is that at the low end of the temperature spectrum, cold-soaking the batteries, below 32F, for extended periods of time, or any length of time for that matter, when the car is off, doesn’t do any damage whatsoever to the batteries nor result in an increased rate of degradation. In fact quite the contrary and opposite -- that actually results in a lower/slower rate of degradation and ageing and a *longer* battery life.

Not so with the high end of the temperature spectrum, however, which is the exact opposite. Heat-soaking the batteries, especially at temps above 95F, for any length of time, results in higher/faster rates of battery degradation and ageing in direct proportion to both the temperatures and the amount of time that the batteries are exposed to those high temperatures.

When you turn the Volt on and start it up, if the batts are at such extreme temps (at either end of the spectrum), yes, of course the ICE will come to power the TMS to bring the batts as quickly as possible back to within the desired/optimal temperature band. But that’s not really relevant to the point here, which is what happens to the batts when the car is turned off and not plugged in, which is at least one-third of the time in a 24-hour day and cumulatively over the batts’ lifetime.

[Rusty]

I'm not sure about your 32F low figure. I'm a cell phone guy, and I've seen dual cell batts (7.4V) run with reasonable energy at -10C, and with enough to power a phone at -20C (albeit not for long). The Volt is an N cell power plant, and they should be able to get enough to get the car to move at quite low temps without using the ICE.

Then you get in to the "if we're pulling power, we're heating up" cycle.

Storage tempurature for this type of battery is quite low, so as long as it warms up well it shouldn't be damaged by cold soak. Whether that's by self warming or ICE heating.

At -20C will it be more efficient to run the ICE to warm things up? Probably. But they may be able to operate the car in "really slow mode" without it.

I understand you're more concerned about what happens when it's hot. This class of battery tends to be only whimpy when they're cold. But they tend to engage in spontaneous rapid disassembly when you do rough things with them when they're hot.

And that's unpleasant to the driver sitting next to the thing if it happens.

[Charles Whalen]

Rusty -- Yes, my focus is really on the extreme hot end of the temperature spectrum, not the extreme cold end, as the former is where the problem is with a faster/higher rate of degradation and ageing and shorter battery life, while the latter is the opposite (i.e. slower/lower rate of degradation and ageing and longer battery life).

So I’m not going to try to get into the cold end of the spectrum, but as for your comments on that, you might want to go back and read that article with the interview of Frank Weber (which Chris provided a link to above), where he explains that the TMS wants to keep the batteries above 32F for the car to be able to drive electric (without the ICE running), and below that, the ICE will come on to provide energy both for driving the car and for running the TMS to bring the batteries up above freezing as quickly as possible.

Fundamentally, you and I are talking about two completely different things which aren’t really even related to each other, or only just barely/marginally so (only in a very brief transition period from at-rest to discharging/charging). You’re talking about a battery operating -- discharging or charging -- at various temperatures, whereas I am talking about a battery sitting at rest, not discharging or charging, at various temperatures (in my case, for what I’m discussing, particularly at the extreme high end of the temperature spectrum).

The important thing to understand here is that a car (used for daily commuting and errands) is only driven 1 to 2 hours a day and spends the rest of the time, 22 to 23 hours a day, sitting, parked. Unless you have an extreme drive cycle -- such as, for instance, a long, hard drive up a steep mountain at high speeds every day, or driving 85mph on the highway for a long stretch every day, or frequent extremely heavy, hard accelerations, like a New York City taxi driver, or taking your Volt to the drag strip every weekend, ... as long as you don’t have any of those extreme drive cycle patterns but rather have more of a normal urban/suburban commuting driving pattern (as the Volt is intended for), then the primary, dominant factor in determining the battery pack life is going to be the environmental temperature (ambient + solar loading) that the batteries are exposed to for those 22 to 23 hours a day that the car is sitting, parked, day in and day out, week in and week out, over several years. The fact that the Volt’s TMS does not operate when the car is turned off and not plugged in is a serious design flaw for a Volt that will live in a hot climate and be exposed to solar loading on a daily basis, which will likely result in a battery pack life shorter than 8 years.

[ChrisC]

I don't think we have enough information to say that the TMS doesn't run when the car is not plugged in. I think it will.

Further, there is NO WAY that they would trigger the ICE to startup for TMS purposes, or for any purpose really, when the car was parked and unmanned. One word: GARAGE. Well, three words: exhaust in garage. Absolutely no way they'd have it autostart.

Again, that article I linked to above is nearly two years old, so I'd taken anything in it with a big ole grain of salt.