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Discussion Starter #1 (Edited)
Some Volt drivers (wrongly) assume that "kWh used" estimate can show or imply battery degradation. This is a complicated topic and has resulted in numerous threads from concerned drivers. Below is a summary of those threads.

What is the "kWh used" estimate and where is it displayed?
The Chevy Volt comes with an Energy Information display in the center console that is controlled by the leaf button. Starting MY 2012, it displays how much energy the car estimates it used during the last trip cycle as "kWh used". This display is not a measurement of battery capacity, nor should changes be used to conclude battery degradation has occurred. Did I say this number was an estimate and not a measurement? :)

What's the purpose of showing a "kWh used" estimate?
Rumor has it the kWh display was added because 2011 owners had requested a way to see how much energy they were using on a given trip.

What affects the "kWh used" estimate?

  • Powering on and off
  • Short trip usage
  • Partial charges (frequent topping-off rather than full discharge/recharge)
  • Brake regen
  • Coming in and out of mountain mode
  • Coming in and out of hold
If you need to know more, read on, but simply said, it's complicated.

My Volt's "kWh used" estimate has changed. Does this mean I have lost battery capacity?
No, it does not. The "kWh used" is not a battery capacity meter. The "kWh used "display in The Energy Usage screen is an estimate based on the total of all drive cycles since the last time the high voltage battery was fully charged. A change in this kWh number does not indicate battery degradation. Instead, like the battery range estimator, it's only giving a calculated estimate of kWh used based on a snapshot of recent conditions and the SOC (State of Charge) values necessary to extrapolate it, plus correction factors. I think of it as a highly sophisticated and well-educated (but fallible) best guestimate of energy used. Again, it is not actually measuring the battery SOC or capacity. More on that below.

Instead, it uses an algorithm (a formula) to constantly update the kWh used display based on numerous variables and provides nothing more than an estimate. Most 2011/12s start at 10.4kWh and drop down within a year to 9.7-9.9kWh on a full discharge. Yet most owners get the same battery driving range. So if "kWh used" was actually measuring battery capacity it would imply your Volt battery somehow got more efficient as it aged since it was able to cover the same distance on less energy than before. Of course, that is not the case.

Illustrating an extreme example, one gm-volt member had this to say, "I apparently drove 41.2 miles on just 6.8kWh of juice, yeah right! That tells me all I need to know about relying on this particular display." Instead, the effects of mountain mode, hold, regen and topping-off where skewing the kWh used display.

What does GM have to say?
I don't have an official GM explanation, but here is an explanation from a GM Engineer responsible for the Volt's kWh display:

"I work at the Milford Proving Grounds in Milford, MI. I am the lead calibration engineer for the Hybrid Powertrain Control Module 2 - HPCM2 Controller, the Electronic Control Unit responsible for calculating the displayed kWh number.

The way this number is calculated is based on the change of the battery state of charge and it's total usable energy. From a displays standpoint we only update the display when the vehicle is running.

Where this may be influenced to a lower than expected value is when there is a battery state of charge correction. While energy is moving into or out of the battery, we use a Battery State Estimation algorithm to determine the State of Charge. Our battery conditioning capabilities in the Volt are engineered to consistently draw the most from our battery while protecting its long term performance capability. After the vehicle rests for a while, we can more accurately determine the state of charge. If that correction causes the state of charge to drop a little bit, that drop will not be accounted for in the kWhs used because the vehicle is not running. This doesn't mean that during that drive you used less of the battery, or the battery's capacity is less, it just shows that there's been corrections in that battery state of charge estimation. Many factors can cause this to happen such as temperature changes and the amount of energy that goes in and out of the pack.. ie mountainous driving.

Personally, I find that I can cause this number to change after many short ev drives with several opportunities for State of Charge corrections."

That was "from the horses mouth" so to speak.

One way of interpreting the above is when you're driving the Volt it uses a formula (not a measurement) to estimate the electric usage. That "kWh used" estimate appears on the display, but isn't as accurate as a direct measurement—which is not possible or practical (more on that below)— and therefore can be off. An indirect state of charge (SOC) measurement will occur when you turn the car OFF and it sits for a bit but that measurement won't show up on the display, only the estimate gets displayed. The SOC of a Li-Ion cell cannot be directly measured accurately by any non-chemical means, (by some sort of “sensor” for instance) so the Volt always uses an estimation.

However, even while ON there will be times where a more accurate detection (not a direct measurement) of existing SOC can be made, and if significant enough, a correction CAN be made that would therefore be reflected in the displayed kWh readout. The displayed "kWh used" value may vary depending on the number and levels of these corrections that took place during the trip cycle (last time of full discharge/recharge).

In conclusion, the "kWh used" display does not measure or indicate battery degradation and can be affected (lowered) by things like powering on and off, short trip usage, partial charges (frequent topping-off), brake regen and coming in and out of hold or mountain mode. These can result in less accurate estimates of the kWh used.

Can I reset the module that calculates the "kWh used"
estimate?
Probably. Of the factors affecting the kWh used display, a charge routine of always recharging the car before the battery is completely drained (i.e., topping-off) seems the most common way the "kWh used" display can become distorted. To reset the display, drive the Volt until the battery is fully depleted, give it a full recharge, then repeat this process a few more times. The kWh used display will likely slowly skew back to more realistic values approximating the numbers you were originally seeing when the car was newer. This has proven to be the case in a number of cases reported here on gm-volt.com.

Check out this story on the "tale of two volts": http://gm-volt.com/2013/02/26/a-tale-of-two-volts-the-summary/

My other Volt shows a different kWh used estimate, is one car bad?
The display will vary and because of the way it is calculated (not measured) it should not be used as a reliable method of performance comparison between cars.

Why can't the battery capacity degradation be displayed?
Because a battery pack, with a complex combination of cells, is actually pretty complex and some criteria such state of charge can't be directly measured via sensors.

Every car has some weak battery health monitoring (cell voltage differentials and charging rates), but in the full pack configuration with mixed parallel/serial patterns as used in the vehicle there is no easy way to actually estimate overall health and capacity. The state of charge of a Li-Ion cell cannot really be directly “measured” by any non-chemical means, (by some sort of “sensor” for instance) it is therefore always an estimation. An estimation based on different sets of factors that are dependent on a wide range of conditions that might exist when that battery is being utilized by a plug-in electric vehicle.

An additional complication is the physical construction of the pack, the number of cells and their parallel and series connections to each other as well as the limitations of the BMS itself (most notably processing power and the precision and shear number of the necessary input channels for collecting the required data such as temperature, voltage, and current)

One of our gm-volt members observed, "When my fellow researchers do tests of different control algorithms and other design changes they have to take the battery apart and do specialized testing to properly evaluate it." That's not going to happen while you drive to the store!

The easiest way to tell if your battery capacity has significantly degraded is simply by looking at your actual battery miles. See "How can I tell if I have lost battery capacity?" below.

How can I tell if I have lost battery capacity?
If you were losing significant capacity your actual (not estimated) battery driving range would be significantly decreasing. In other words, you used to get, say 40 miles on the battery and now you get only 30 miles all other things being the same. If you have lost actual AER (All Electric Range) and believe there's something wrong take it to your dealer.

I have a Dash-DAQ and I disagree with the above.
Please submit your research to the SAE Journal for peer review and publishing. There have been hundreds of papers about this very thorny issue and GM is one of the leaders in this technology. But it's definitely possible you have an angle they did not consider.

Related posts with more discussion on the above:
http://gm-volt.com/forum/showthread.php?18845-Further-Response-From-Volt-Engineers-On-KwH-Consumption-Reading-Variation
http://gm-volt.com/2013/02/26/a-tale-of-two-volts-the-summary/
http://gm-volt.com/forum/showthread.php?60874-Battery-degradation-range-loss-(if-you-have-any-post-amount-age-miles-and-testing
http://gm-volt.com/forum/showthread.php?62425-Anyone-else-seen-changes-in-their-battery
http://gm-volt.com/forum/showthread.php?58209-Lowest-Displayed-kWh-after-full-battery-deplete
http://gm-volt.com/forum/showthread.php?49817-Unusual-Volt-behavior-followed-by-loss-of-total-usable-kWh
http://gm-volt.com/forum/showthread.php?39489-Only-getting-about-9.6kWh-from-a-charge-with-18%B0C-outside-and-battery-at-24%B0C
 

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Thanks for putting this together. Personally I've always avoided any threads discussing potential battery degradation since I was pretty sure that the kilowatt used measure was not really a good indicator... now I know exactly why :)
 

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Wait! I thought there was no memory effect on Li-Ion batteries? I top off a lot, and I frequently only re-charge 8-10 miles at a time. Is that going to hurt the longevity of my pack?
 

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Discussion Starter #5 (Edited)
Wait! I thought there was no memory effect on Li-Ion batteries? I top off a lot, and I frequently only re-charge 8-10 miles at a time. Is that going to hurt the longevity of my pack?
This is not about memory effect, Ziv. It's about how the Volt calculates an estimate of energy used. Many of us top-off and opportunity charge. That will have no memory effect on the battery. They MAY distort the "kWh used" estimate however. And from there people leap to the false conclusion that the battery has degraded. This post is about why that conclusion is unsupported and why.
 

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Very interesting, thank you.

Seems like it would be a lot simpler to use something similar to a kill-a-watt on the battery terminals to directly measure the energy consumed, and replenished, regardless of the state of the battery. That would also be a check and balance.

So, let me get this straight, Hypothetically speaking,

If, for example, my 16kW battery had degraded to 12kW, 25%, then, I should expect a drop in my usable kW likewise, to 7.5kW; Or, should I expect to see fewer miles driven on a calculated constant 10kW? If the later then it is indeed my driving style, if the former then my battery is degrading. I would have expected the former.

Moving forward, If I do believe that my battery is degrading excessively, determined by the above means, and I do take it in to be checked out, just how does who make a determination? From what you are describing there is no way to tell other than chemical testing, which I highly doubt any dealer is equipped to do, right, wrong, other? This certainly sounds like the dealer waving a magic wand and declaring if not to be the battery; end of story.
 

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Well done gathering all this together Steverino!
You are the KING at creating this sort of compendium.
It was getting tiresome repeating some of these statements to new owners ;)
WOT
 

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Discussion Starter #8 (Edited)
Very interesting, thank you.

Seems like it would be a lot simpler to use something similar to a kill-a-watt on the battery terminals to directly measure the energy consumed, and replenished, regardless of the state of the battery. That would also be a check and balance.
This idea and the related idea of measuring energy used to charge the car, both have their own inaccuracies. At least one and I think several of the links in my post above go into this. My summary: it's not so simple.

So, let me get this straight, Hypothetically speaking,

If, for example, my 16kW battery had degraded to 12kW, 25%, then, I should expect a drop in my usable kW likewise, to 7.5kW; Or, should I expect to see fewer miles driven on a calculated constant 10kW? If the later then it is indeed my driving style, if the former then my battery is degrading. I would have expected the former.

Moving forward, If I do believe that my battery is degrading excessively, determined by the above means, and I do take it in to be checked out, just how does who make a determination? From what you are describing there is no way to tell other than chemical testing, which I highly doubt any dealer is equipped to do, right, wrong, other? This certainly sounds like the dealer waving a magic wand and declaring if not to be the battery; end of story.
The only indication you as a driver will have of significant battery degradation is if your actual (not estimated) battery driving range is significantly decreasing. In other words, you used to get, say 40 miles on the battery and now you get only 30 miles all other things being the same. If you have lost actual AER (All Electric Range) and believe there's something wrong take it to your dealer.

But note the qualifier. If you are driving differently, or under different conditions, etc. all things are not equal and battery degradation is not indicated.

Also, if you are rarely fully depleting the battery and simply topping-off as your standard procedure, then you should follow the procedure in the "Can I reset the module that calculates the "kWh used" estimate?"above. It may be your battery needs a "cleansing", haha. Oh no, now people are going to use that.

Well done gathering all this together Steverino!
You are the KING at creating this sort of compendium.
It was getting tiresome repeating some of these statements to new owners ;)
WOT
I am fortunate to be able to stand on the shoulders of others, WOT. You and other gm-volt members will note many of your patient and detailed explanations were, ahem, repackaged above. :) I too have noticed there has been a LOT of repetition (both in Q&A) on this topic and it seemed time for something that pulled it all together. I think I'm happy not having this display in my 2011, LOL.
 

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Thanks, Steverino, I guess I better read the info with a bit more attention to detail!

This is not about memory effect, Ziv. It's about how the Volt calculates an estimate of energy used. Many of us top-off and opportunity charge. That will have no memory effect on the battery. They MAY distort the "kWh used" estimate however. And from there people leap to the false conclusion that the battery has degraded. This post is about why that conclusion is unsupported and why.
 

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Discussion Starter #11
Great job Steverino! Like it or not, I'm thinking you are now the area expert.
Not so fast, you're one of my references sources above DonC!
 

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OK this will probably make you slap your head and say some people never get it, but I don't get what all the fuss is about. I thought it was just a fact of life that Li-Ion batteries do degrade. It would be miracle of physics if your battery didn't, wouldn't it? I have 2 years of the most pleasurable driving I've ever had and 44k miles on my Volt, and I just assume that the battery is on its normal curve down. It should be about 80% of what it was new, shouldn't it? I haven't seen a loss in miles that I can drive, however, and I just assumed this was smart software, opening up more battery capacity just looking at my odometer. Isn't that it? Or has a miracle truly occurred!
 

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Discussion Starter #13
OK this will probably make you slap your head and say some people never get it, but I don't get what all the fuss is about. I thought it was just a fact of life that Li-Ion batteries do degrade.
The fuss is that some think the "kWh used" estimate is a measurement of whatever amount of battery degradation there is. As the above FAQ explains, it's not.
 

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Thanks for putting that together, Steverino.

The issue did bother me at first and I did have a couple posts on it and surely contributed to WOPonTour's headaches.

I think there is an aspect of this phenomenon that isn't mentioned in your very good explanation. There does seem to be some actual effect on battery range. (albeit unrelated to battery degradation.) In Tale of Two Volts, it wasn't just that the kWh used meter on one of the Volts was low; the battery range was also lower. The explanation I've come to believe on that is that as the charge estimator gets off, the SOC where the car quits accepting charge drifts lower and/or the SOC where the generator comes on drifts upward and the usable SOC window contracts a bit. So while the kWh meter is not a good indicator of battery health and only useful as an estimate of actual use, I think that consistently lower kWh used numbers for a full battery charge could also be a sign (an indirect sign related to the same issue of electric use estimates that are off) that your battery range may be down a bit from where it was in the past.

If this is the case, the good news is that it isn't permanent and those few miles of range could be recovered with the full discharge and full charge prescription. Personally, the issue bothered me previously, but I just quit worrying about it. If I actually did the full discharge and charge enough times to get the kWh meter back up along with recapturing those few miles of possible range loss, I would use more gas doing that than I would save with those few extra miles of range since I don't fully discharge the battery very often in day to day use.

This is a complicated issue, but it does seem to give some owners heartburn and GM should improve the way the car handles its estimate of electric use in the future. I am sure that will come. (It is my hypothesis that there were some improvements in the algorithm in the 2013 Volt, since I have not seen the same level of complaints on this forum from those owners compared to the 2012 owners.)
 

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I understand that the kWh used display is an estimate, but how does the car determine when to stop charging or when to go into CS mode? If it's so hard, those must be estimates as well. Are they related in any way to kWh used? They are obviously not enforcing an x kWh window, since kWh used/remaining is impossible to determine exactly. What exactly is the window then? Voltage? Some chemical measurement? How would GM prevent this window from using more of the available capacity as the battery degrades?

I found this to answer my question:

http://www.phmsociety.org/sites/phmsociety.org/files/PHMSalman__LiIon%20State%20and%20prognosis%2026Sept2011.pdf

The answer is that they form estimates of SOC and SOH (state of health). It isn't simple. The SOC estimate is based on both current and voltage. I'm not sure how effective this is at preventing the SOC window from increasing as the battery degrades, but they apparently attempt to keep it constant as WOT has said before.
 

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Discussion Starter #16
I think there is an aspect of this phenomenon that isn't mentioned in your very good explanation. There does seem to be some actual effect on battery range. (albeit unrelated to battery degradation.) In Tale of Two Volts, it wasn't just that the kWh used meter on one of the Volts was low; the battery range was also lower.

If this is the case, the good news is that it isn't permanent and those few miles of range could be recovered with the full discharge and full charge prescription.
Yes, it seems that a regimen of never fully charging/discharging can have some effect in lowering the AER, and as you point out the cure is to fuuly discharge/charge the car for a few cycles. That the "kWh used" display was off as well in Tale of Two Volts we can just as easily chalk up to coincidence. After all, many have seen their kWh used estimate numbers go down without a loss of AER. So I don't think we can go as far as saying kWh used is an indicator. That's the whole point, it's not.

This is a complicated issue, but it does seem to give some owners heartburn and GM should improve the way the car handles its estimate of electric use in the future.
Agreed on all points. I have a 2011, no kWh used display, no heartburn. Ignorance is bliss? I'm also one of those who use some gas every day, so my Volt gets a full discharge/recharge on a regular basis in addition to topping-off opportunity charging.
 

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Discussion Starter #17
I found this to answer my question:

http://www.phmsociety.org/sites/phmsociety.org/files/PHMSalman__LiIon%20State%20and%20prognosis%2026Sept2011.pdf

The answer is that they form estimates of SOC and SOH (state of health). It isn't simple. The SOC estimate is based on both current and voltage. I'm not sure how effective this is at preventing the SOC window from increasing as the battery degrades, but they apparently attempt to keep it constant as WOT has said before.
Yes, the one thing I have learned about this issue is, it's not so simple! Just the opposite, it's very complicated and this is why we can only use the "kWh used" estimate for what it is, a sophisticated approximation of energy used since the last full charge, an estimate that can be skewed by a number of factors both real and mathematical, and which should not be used as a battery health indicator.
 

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K, still getting my head wrapped around this. I drive about 8 miles a day and usually only charge twice a week, when the estimate is usually between 2 and 6 miles remaining. I was trying to pick a happy medium between the top off every day and the routine usage of the ICE.

What this is saying, if I understand it, is that it would be better for the battery, as well as the estimate, if I were to not recharge it until it has fully depleted via at least a little ICE usage? I have no problem with that as it would solve the stale gas burns.
 

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I know it's only an estimate, but my Volt is behaving very strangely at the moment and the kWh estimate is the only thing I've got to work with. Most days I get 8.8kWh from a full charge. Sometimes I get up to 10.8kWh but that usually triggers PPR mode. Today bizarrely it said I got 11.4kWh from a full charge (and no, I didn't recharge at any point during the day). No, I don't believe that estimate for one minute because I only got 39.5 miles from the charge.



(In case you're wondering, there was no recharging during the day. This is one full charge, at the time the photo was taken I had 1 mile range remaining but no green bars.)
 

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Discussion Starter #20
I drive about 8 miles a day and usually only charge twice a week.

What this is saying, if I understand it, is that it would be better for the battery, as well as the estimate, if I were to not recharge it until it has fully depleted via at least a little ICE usage? I have no problem with that as it would solve the stale gas burns.
If it was me, I'd simply continue as you are and once in a while (once a quarter? Once every 6 months?) I'd do a few full discharge/recharge cycles.
 
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