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Battery Management and Cell Balancing
The pack is divided into 4 distinct management blocks each with there own group of 24 triplets to monitor. This is accomplished by four interconnected Battery Interface Control Modules (BICM). Each BICM monitors the 24 voltages and 4 temperature probes interspaced between every 6 cell groups.
Thanks for the informative post. My apologies to you and to George for having lured you off topic and gotten us scolded by George!

I have a question which is on topic.

If you examine the battery pack cutaway drawing you attached it is clearly visible that the BICM at the front is cabled to two 12 triplet modules plus one 6 triplet module. The BICM in the middle is cabled to just two 12 triplet modules. If you examine the photos of the T bar portion of the pack at this link: http://gm.wieck.com/forms/gm/*query?ws4d_nav=true&search_criteria=VoltPortTech&source=all&page=4 you can see that the BICM at the driver side is cabled to one 12 triplet module plus one 6 triplet module. The BICM on the passenger side is cabled to two 12 triplet modules.

So, are you right in saying that each of the four BICMs controls 24 triplets or is the responsibility split 30 - 24 - 18 - 24 as the photos seem to show?
 

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Battery Management and Cell Balancing
The pack is divided into 4 distinct management blocks each with there own group of 24 triplets to monitor. This is accomplished by four interconnected Battery Interface Control Modules (BICM). Each BICM monitors the 24 voltages and 4 temperature probes interspaced between every 6 cell groups. .
This is interesting. While each cell has active, liquid cooling on one surface, there is only one thermocouple (temp probe) for every 18 cells. Are you sure about that WOT??
 

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Thanks for the informative post. My apologies to you and to George for having lured you off topic and gotten us scolded by George!

I have a question which is on topic.

If you examine the battery pack cutaway drawing you attached it is clearly visible that the BICM at the front is cabled to two 12 triplet modules plus one 6 triplet module. The BICM in the middle is cabled to just two 12 triplet modules. If you examine the photos of the T bar portion of the pack at this link: http://gm.wieck.com/forms/gm/*query?ws4d_nav=true&search_criteria=VoltPortTech&source=all&page=4 you can see that the BICM at the driver side is cabled to one 12 triplet module plus one 6 triplet module. The BICM on the passenger side is cabled to two 12 triplet modules.

So, are you right in saying that each of the four BICMs controls 24 triplets or is the responsibility split 30 - 24 - 18 - 24 as the photos seem to show?
I will look at this again and confirm. There is a fair amount of confusion present as to the "sectioning" of the battery. From a erviceabilty standpoint there are THREE servicable sections of the battery. Two along the tunnel and the 3rd being the entire rear section (cross of the "T") under the rear seats. However from a BMS viewpoint, there are actually FOUR sections, each with their own BICM. (the rear section is devided into two, but I'm not certain it's an even splt!) I assumed these would be divided up evenly to monitor cell-groups (triplets) but perhaps not.

This is interesting. While each cell has active, liquid cooling on one surface, there is only one thermocouple (temp probe) for every 18 cells. Are you sure about that WOT??
Same thing Geo. The data I was stating came from a textual description, but I will take a closer look at the schematic and confrm. I guess you are assuming we should have a temp probe on each cell or at least every triplet? Keep in mind theres also temp probes within the cell cooling system as well (just coolant IN-OUT for Delta-T but still)

Lyle states my Volt Cooling System article should appear out front next week sometime


Stay Tuned ;)
WOT
 

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The battery cells are grouped together into three "serviceble" sections.
The first 90 cells (30 cell groups/triplets) make up battery section one . This section is adjacent to the cowl and contains battery groups #67 through #96 and managed by BECM 1.
Battery section two is located behind section one. It is made up of 72 cells (24 cell groups) and contains batteries #43 through #66 and managed by BECM 2 .
The transverse battery section is section number three, it is made up of the remaining 126 cells (42 cell groups) and contains batteries #1 through #42 and managed by BICM 3 and BECM 4.

BICM 1 managing 30 triplets has 5 temperature sensor probes
BICM 2 managing 24 triplets has 4 temperature sensor probes
BICM 3 manages the passenger side of the transverse "T" section (24 triplets) has 4 temperature sensor probes
BICM 4 manages the drivers side of the transverse "T" section (18 triplets) has 3 temperature sensor probes

WOT
 

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BICM 1 managing 30 triplets has 5 temperature sensor probes
BICM 2 managing 24 triplets has 4 temperature sensor probes
BICM 3 manages the passenger side of the transverse "T" section (24 triplets) has 4 temperature sensor probes
BICM 4 manages the drivers side of the transverse "T" section (18 triplets) has 3 temperature sensor probes
Thanks WOT. I'm looking forward to your upcoming front page post.
 

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Triplet balancing

Per Wot the BMS balances triplets, and these triplets are the lowest level of instrumentation in the pack. An interesting scenario develops if one of the cells goes bad within a triplet. During the charge cycle this triplet with one bad cell would presumably be charged up to the proper, max voltage earlier than most so the BMS would terminate charging of this triplet early. Once the entire pack is charged, and the Volt is on the road in EV mode, this triplet would hit it's min voltage earlier than the other healthy triplets in the pack.

Hmmmm, so what happens then??? At this point we need to stop discharging that bad triplet or we will damage the good remaining cells by discharging them to tooo low a state. Looks like the only option is to switch this triplet out of the series string. Sooooo:

Question: Is the Volt BMS able to switch out triplets from the series string during the discharge cycle.??? Must be.
 

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Hmmmm, so what happens then??? At this point we need to stop discharging that bad triplet or we will damage the good remaining cells by discharging them to tooo low a state. Looks like the only option is to switch this triplet out of the series string. .
I think it was mentioned months ago the Volt would have this ability, you would need a 2P2T relay to bypass each triplet in the series chain, not too likely.. probably what they do is is bypass one of the four groups instead, and trigger a low reduced power emergency mode, and a trip to the dealer. The BMS will attempt to balance out any triplets that dont have the same voltage as the others but eventually the computer will light up the service light in the dash.
 

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I think it was mentioned months ago the Volt would have this ability, you would need a 2P2T relay to bypass each triplet in the series chain, not too likely.. probably what they do is is bypass one of the four groups instead, and trigger a low reduced power emergency mode, and a trip to the dealer. .
The failure of one cell for sure can't knock out 1/4 of the pack and a trip to the dealer. I can't believe it.
 

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Just a hint of an incipient failure will prompt a trip to the dealer, its an expensive pack and they do catch fire on occasion. The dealer installs a refurbished pack and your packs goes back to Holland for repairs. Will the dealer keep a spare pack?.. I doubt it. How long will it take to ship a pack to the dealer?.. probably a week by truck.

Can a Volt pack be shipped thru air freight?
 

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Just a hint of an incipient failure will prompt a trip to the dealer, its an expensive pack and they do catch fire on occasion. The dealer installs a refurbished pack and your packs goes back to Holland for repairs. Will the dealer keep a spare pack?.. I doubt it. How long will it take to ship a pack to the dealer?.. probably a week by truck.

Can a Volt pack be shipped thru air freight?
I still can't believe it Herm, there is no way they are going to go to all that expense if only one cell fails. Wanna bet??
 

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From the SAE feature about the Volt, page 21:

... 'the delta between 70°F (21°C) and 90°F (32°C) can be critical to battery life,' [Frank Weber] asserted. The battery is designed to work while plugged in, at temperatures from -13°F (-25°C) to +122°F (+50°C).
Not sure if we'd heard that yet, but there you go.
 

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I read that too, and thought it said something about where they maintain the temp band. But I don't think it does, other than to say it maintains some good temperature with ambient anywhere between -25 to +50. Based on that text alone, that could be an internal temp of 15, 20, or 25. And that could be +/-1 or +/-5.
 

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Discussion Starter #113 (Edited)
I’ve got some more information about the TMS, its parameters, and modus operandus, from a long, extended conversation I had on this topic with one of GM’s Volt engineers.

1) When either a) the car is on, or b) the car is plugged in and charging: the TMS operates continuously and keeps all the battery cells in a narrow temperature range between 20C and 22C. If the car is plugged in and charging, it uses offboard grid power to run the TMS. If the car is on, it uses onboard power from the battery itself to run the TMS.

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2) When either a) the car is off and not plugged in, or b) the car is plugged in but not charging (i.e. either prior to commencing a timed/programmed charge or after having completed charging but still plugged in):

If the battery pack is at very high SOC, from 85% SOC (top of the charge and top of the Volt’s usable SOC range) down to around 75% SOC, the car’s electronics will wake up the TMS periodically, around once every half hour, to see if the TMS needs to run. If the battery pack is below about 75% SOC, the TMS never wakes up and never runs.

In the 75-85% SOC range, when the car’s electronics wake up the TMS every half hour or so, the TMS checks all the cell temps and does the following:

i) If all the cell temps are below 22C, the TMS does nothing and immediately goes back to sleep (no matter how cold the battery temps may be).

ii) If any of the cell temps are above 22C, the TMS runs for as long as it takes (unless and until the battery pack falls below 75% SOC) to cool all cells down into the 20-22C temperature band, then shuts off and goes back to sleep for another half hour.

If the car is plugged in but not charging, it uses offboard grid power to run the TMS. The TMS could easily run for several days or more in this mode, waking up every half hour to check battery temps and keeping all cells below 22C, under a scenario where the car is fully charged and left plugged in and parked for a long time.

If the car is off and not plugged in, it uses onboard power from the battery itself to run the TMS. Once the battery pack drops below 75% SOC, the TMS will no longer run and goes to sleep and won’t wake up again, no matter what happens with the battery temps, however hot they may get.

If your morning commute is more than about 6 miles (and you don’t have any place to plug in and charge at work), then the TMS won’t operate at all during the day while parked at work, no matter how hot the battery temps get. If you leave home on a full charge and your morning commute is very short, 4 miles or less, then there should be enough reserve above 75% SOC for the TMS to operate for a few-to-several hours during the day, depending on environmental conditions (ambient + solar loading).

As Frank Weber and others have noted, there is a substantial lifetime difference between 21C (70F) and 32C (90F) in the lithium-manganese batteries (which have the highest heat sensitivity/degradation profile of all lithium battery chemistries) that GM is using in the Volt. At 60% SOC, lithium-manganese batteries have a little over 8 year life at 21C (70F) but only a 5 year life at 32C (90F). At higher states of charge, the heat sensitivity and degradation rate is even greater.

What is not yet known, or been revealed, with any degree of specificity in a quantifiable way, is the degree to which the sealing and insulation of the Volt’s battery compartment slows the rate of heat conduction and radiation into the battery compartment from a 120-140 degree F solar-loaded car interior and surrounding pavement. This can only be determined, checked, and verified through the use of a MDI/GDS, if one is able to procure this tool and software, as I am going to attempt to do.

In the absence of that, unless and until one can score a MDI/GDS, for a Volt owner who lives in a hot climate, has a morning commute longer than 4 miles, no charging availability at work, and no shaded parking but rather has to park out in the blazing hot sun, the failsafe procedure to absolutely guarantee adequate cooling and thermal protection of the battery pack, to ensure long (8 year minimum) battery life, is to leave the car powered on, with the doors locked, during the day while at work, so that the TMS can operate continuously throughout the heat of the day. (This is a procedure that a number of first-generation EV owners in hot climates are well practiced in and why they carry an extra key.) This procedure (of leaving the car powered on during the day while at work) should not be done if one’s morning commute is longer than about 33 miles, as in that case the continuous operation of the TMS could possibly draw the battery pack down to 20% SOC (the bottom of the usable SOC range) before the end of the day, reaching the CS-mode threshold, thereby causing the engine to start and run unattended, which would be environmentally irresponsible as well as possibly even illegal in some locales (to leave one’s car engine running unattended).
 

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It looks like the Volt should have a user-configurable lower-bound on the battery SOC for TMS rather than having a fixed 75% level. Leaving the car turned on as Charles suggests might be an acceptable hack for an obscure low-volume vehicle but it's an invitation to theft for criminals in hot states with a hopefully higher-volume car like the Volt which they may learn to target.

Let's hope they can fix that in a software update....
 

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Sounds like GM may need to add another mode, call it "hot climate mode". Basically, it would do the same function as leaving the car powered on for thermal management, but would disengage the engine start. In this mode, the car could also operate in a pseudo "mountain mode" to ensure a higher battery SOC to maintain the TMS over an 8-10 hour period. Once the SOC drops to a min level, the TMS would go into sleep mode.
 

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Discussion Starter #116
(Apologies for the length of this post, which requires that I break it up into two separate posts.)

Well, I guess all of this probably shouldn’t come as too much of a surprise, simply because GM Volt senior engineers and managers, from Andrew Farah to Bob Lutz, have been pretty clear in their statements pointing to the fact that GM wasn’t going to be as aggressive as it could have been -- and maybe should have been, at least for hot-climate operation -- in the implementation of its TMS regime.

As far as technically-savvy early adopters having to be a bit flexible, creative, and resourceful in adapting such workarounds (or “hacks”, as you referred to them), you should understand that that just comes with the territory of where we are right now. The EV pioneers of the last decade have had to be even more creative and make even bigger adaptations, workarounds, and hacks. This one here is quite mild and tame in comparison, more in line with an evolution -- and commensurate technological refinement one would expect -- from the pioneer stage to the very early adopter incipient stages of fledgling commercialization which we are now moving into, where nevertheless there will still be some of these types of infancy “teething” issues to work through. At least I won’t have to string ice bags over the top and sides of the car, hanging down over the quarter vents, or rig up a Rube Goldberg robo-chiller on wheels with home-made ducts, as Gen 1 EV drivers have had to do (and are still doing) in hot climates. Even as a work-in-progress with a few infancy issues yet to be sorted through and design and implementation of some of its subsystems yet to be tweaked and perfected, the Volt is still light years ahead and a quantum leap improvement over the Gen 1 vehicles that many of us are still driving, with their 15-year old technology. Let the perfect not be the enemy of the good, as the old saying goes.

You kinda have to take a step back and look at the longer term perspective here and realize that we are in the very, very early developmental stage of lithium-powered EVs. By buying a first-model-year Volt (or a first-model-year Leaf, for that matter), you have to understand and accept that as a very early adopter, the bargain you are making, and the adventure that you are willingly undertaking, is to basically be an extended beta tester for the avant-garde, technologically-leading-edge automakers (principally GM, Tesla, and Nissan) in their early developmental process of their first-generation lithium-powered EVs and to assist them in their ongoing learning process in that regard. This is a very exciting time and that is an exciting role to play, but you also have to understand and be willing to accept some of the risks and responsibilities that come with that. These three leading-edge automakers are taking enormous risks by doing this and making these second-generation EVs, with commercial intent and commitment this time, on this second go-round, but without a demonstrated, proven market for them; and we early adopters have to likewise be a partner with them and be willing to take a few (much smaller in comparison) risks on our end too. It’s very much a shared-risk and shared-adventure partnership in that regard. Above all, it really properly requires a highly-educated and technically-savvy early-adopter customer to undertake this role, so that you go into it with your eyes wide open, well informed on the various infancy issues that you might face and have to deal with, and that indeed, you should have some flexibility, creativity, and resourcefulness to occasionally adapt a few workarounds (or “hacks”) when needed, to deal with such “teething” issues.

Speaking from a decade of EV industry experience, having come at this and seen it from all sides and angles of the industry -- professionally, from having been on the OEM side of the fence, to working with battery manufacturers, to the charging infrastructure side, to public advocacy, the public policy arena and writing legislation, as well as consulting and advising governments and corporations on EV, battery, and charging infrastructure technologies, and last but not least, especially from the consumer side of the fence as a longtime EV owner and driver -- what I can say with certainty (because I have seen this before) is that GM will learn a lot more about lithium battery degradation and ageing, with respect to long-term heat exposure, over the next several years from the accumulated data of the real-world experiences of its Volt-owning customers in hot climates than it ever could, and did, from trying to extrapolate: i) a few weeks of driving a small captured test fleet of production validation vehicles around Death Valley, and ii) simulated accelerated heat/degradation bench testing in the lab. Those latter two can never capture the variety, diversity, and range of experience out in the real world, as well as just the fact that battery degradation and ageing in an EV is a phenomenon that can only really be properly understood by actually observing and experiencing it through living with, using, charging, and driving EVs, and caring for their battery packs, on a daily basis, day in and day out, continuously over a period of several years.

The Volt early adopters will teach GM a lot about lithium battery degradation and ageing over the next several years. There is much that GM will have to learn about battery degradation and ageing that longtime EV drivers already know, having learned from their own experience over the last decade. So to follow up on, and second, the comments in the two previous posts ... yes, the Volt’s TMS is one area where I think we could possibly see GM make some tweaks in its implementation. The good thing, as you said, is that such tweaks should be relatively easy to make by apparently needing just to change various control module software configurations and parameters.

From my perspective of having a good deal of real-world experience (more than GM) over a number of years with hot-climate EV battery performance, degradation, and ageing issues, upon initial reflection I might be tempted to conclude, from what we now know, that the TMS implementation regime in the Volt is a bit weak and represents a design deficiency, at least with regard to hot-climate operation. However that would really be premature, so I will refrain from making that call and will withhold judgment on that for now, at least until I can specifically determine and quantify the degree to which the battery compartment’s sealing and insulation slows the rate of heat gain (from external environmental forcing) into the battery compartment. That is really the crux of the matter that this whole thing hinges upon. All we’ve got at this point is a vague, fuzzy, general assurance from GM that the battery compartment’s sealing and insulation does a “pretty good job” in that regard, but nothing specific in terms of real, hard numbers. I think it’s entirely possible that that is indeed the case, but I wouldn’t be willing to roll the dice and blindly bet a precious $10k battery pack on that without independently checking and verifying it for myself, in my own case and under my own set of local circumstances and conditions. GM actually probably can’t give any real, hard numbers to quantify this in any universal way, simply because there will be so much climatic and other variation factors from region to region, and within regions, from one location to another and one person to another, depending on one’s specific conditions and circumstances.
 

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Discussion Starter #117 (Edited)
(Follow-on continuation from previous post, immediately above)

Looking at the lithium-manganese battery life relationship to temperature, my own personal judgment and comfort level is that I would draw the line at a 2C rise above 22C, so up to 24C, for the temperature rise and level, respectively, that I would find acceptable and tolerable after 8 hours of intense 120-140F solar-loading/environmental forcing on the battery compartment exterior (with the car being turned off). If at 5-6pm the battery temps are 24C or less, that would be an acceptable heat gain and exposure level that would lead me to conclude that the workaround procedure of leaving the car on all day long is not necessary. If, however, on the other hand, battery temps exceed 24C upon initial boot-up of the car at 5-6pm, after baking all day long in the blazing hot sun (with the car turned off), then that would be unacceptable and would constitute -- in my judgment, for my particular case, conditions, and circumstances -- a clear design deficiency in the TMS implementation regime that would present a compelling case and argument for following the workaround procedure of leaving the car turned on all day long as a routine matter of standard practice.

As I mentioned in my previous post, the only way to determine and measure this is with a MDI connected to a laptop running GDS, which one would need to find and procure. The MDI is apparently a bit easier to source in the aftermarket, but I’ve been told that GDS/GDS2 is the hard part and that it is difficult to near-impossible for an individual without a GM commercial relationship to get a GM TIS online account required to access, download, and once a week renew the weekly lease on, the GDS/GDS2 software. So we’ll have to see about that.

In the absence of that, unless and until I can acquire MDI/GDS, the conservative, prudent course of action and appropriate early-adopter/owner risk management and mitigation strategy to pursue, as a practical matter, would be to follow the precautionary principle and operate on a working assumption, of a less than best-case/most-rosy scenario, that the current TMS implementation regime might be suboptimal, and not as aggressive as needed, for operation in a hot climate, and thus to adopt the failsafe workaround procedure of leaving the car turned on all day long, while parked out in the hot sun. Indeed, statements by GM Volt senior engineers and management seem to support this interpretation and augur caution along these lines. To do otherwise, and just blindly assume a best-case/most-rosy scenario, without any independent verification and actual measurement to check and see whether that holds up and is indeed the case, would be foolish.

With regard to the potential car theft concern you mentioned, I’m not really worried about that and don’t see it as a problem, simply because we have already been doing this for years now -- leaving our EVs turned on, with the doors locked, for the exact same reason (due to TMS design deficiencies, suboptimal for operation in a hot climate) -- and have never had any kind of attempted theft issues arise out of this.
 

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Apparently the Leaf manual has become available. Folks on the ev mailing list at work are commenting on it this evening. Purportedly from the manual:


The Nissan Leaf Warranty Manual says on page 9:

GRADUAL CAPACITY LOSS
The Lithium-ion battery (EV battery), like all lithium-ion batteries, will experience gradual capacity loss with time and use. Loss of battery capacity due to or resulting from gradual capacity loss is NOT covered under this warranty. See your OWNER'S MANUAL for important tips on how to maximize the life and capacity of the "Lithium-ion battery."
To which a coworker and friend replied:

The whole MyNissanLeaf forum had a field day on this.

I read the so-called "warranty" and concluded that it reads like:
"Dear Sir or Madam -- we have no idea how these batteries will perform in the real world. Please consider leasing."

I was going to lease anyway, but with a warranty like that, you'd be nuts to buy.
Charles? Good call!
 

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(3 weeks ago) Lyle states my Volt Cooling System article should appear out front next week sometime.
Gimme gimme gimme! :)

I'm afraid that if it doesn't get published on Monday, we won't see it for another week, because it seems likely that we're in for a busy week starting Tuesday ...
 

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you violate the Tesla battery warranty if ...
Update to my post in October (following the quote link) where I described generally the Tesla battery warranty fine print ... I just came across what Tesla actually says voids the warranty:

- exposing an unplugged vehicle to ambient temperatures above 120F for over 24 hours
- storing an unplugged vehicle in temperatures below -40F for over seven days
- leaving your vehicle unplugged where it discharges the battery to at or near zero SOC

Note that the first two conditions above imply continuous. Briefly going beyond either temperature threshold isn't a problem, it's soaking it there for the stated period, without being plugged in.

Just FYI. Again, this is for the Tesla Roadster, not the Chevy Volt :)
 
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