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
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?
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?
- 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
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