RegGuheert
Well-known member
Nissan has made the following claim quoted in the Canberra Times in Australia (September 4, 2012 article: http://canberratimes.drive.com.au/motor-news/nissan-to-step-up-ev-production-20120904-25b60.html" onclick="window.open(this.href);return false:
While I do not believe for one minute that the LEAF does not have a battery problem, I do think there could also be an instrumentation problem which is either adding to the battery problem or possibly even leading to some of the damage.
More specifically, if the Nissan LEAF's ability to ACCURATELY measure the DC voltage of the main traction battery is not extremely high and extremely stable over temperature and time, the available capacity of the LEAF battery could be severely affected. Please note that there is no doubt that voltage measurement inaccuracies will impact LEAF battery capacity. The question here is how much of what we are seeing is due to this effect.
I am focusing only on DC voltage measurement here, since both charging and discharging in the LEAF appear to both be terminated based upon voltage measurement. Any errors in current measurement should be almost completely inconsequential in regards to available battery capacity and actual range.
Here are a few scenarios that might be affecting the capacity in our LEAF batteries:
1) Coming from the factory: LEAF measurements of battery voltage are higher than the actual battery voltage: In this scenario, the LEAF would terminate charge and discharge at a lower battery voltage than intended, resulting in lower overall capacity available when new. However, the battery will tend to be stored at a lower SOC which may improve calendar life of the battery.
2) Coming from the factory: LEAF measurements of battery voltage are lower than the actual battery voltage: in this scenario, the LEAF would terminate charge and discharge at a higher battery voltage than intended, resulting in higher overall capacity available when new. However, the battery will tend to be stored at a higher SOC which may reduce calendar life of the battery.
3) As temperature rises: LEAF measurements of battery voltage trend higher than the actual battery voltage: In this scenario, at higher temperatures the LEAF would terminate charge and discharge at a lower battery voltage than intended, resulting in lower overall capacity available when hot. However, the battery will tend to be stored at a lower SOC which may improve calendar life of the battery.
4) As temperature rises: LEAF measurements of battery voltage trend lower than the actual battery voltage: In this scenario, at higher temperatures the LEAF would terminate charge and discharge at a higher battery voltage than intended, resulting in higher overall capacity available when hot. However, the battery will tend to be stored at a higher SOC which may reduce calendar life of the battery.
5) As the LEAF ages: LEAF measurements of battery voltage trend higher than the actual battery voltage: In this scenario, as the LEAF ages it would terminate charge and discharge at a lower battery voltage than intended, resulting in lower overall capacity available. However, the battery will tend to be stored at a lower SOC which may improve calendar life of the battery.
6) As the LEAF ages: LEAF measurements of battery voltage trend lower than the actual battery voltage: In this scenario, as the LEAF ages it would terminate charge and discharge at a higher battery voltage than intended, resulting in higher overall capacity available. However, the battery will tend to be stored at a higher SOC which may reduce calendar life of the battery.
Note: All of the above scenarios are focused around the ability of the LEAF to measure 393V.
So while I do not expect a huge variation in 1) an 2) due to the ability to calibrate at the factory, it could explain part of the range differences seen in new LEAFs.
Scenarios 3) and 4) may explain some of the seasonality we see in the LEAF. Scenario 3) may actually be intentionally implemented in the BMS to try to extend the life of the battery.
Scenarios 5) and 6) could also be an issue. If scenario 5) were occurring, we would have a difficult time distinguishing it from battery degradation. If scenario 6) we're occurring, the battery would tend to get more and more abuse from the car as time goes on and would tend to wear out more quickly.
So, how hard is it to measure battery voltage? It's not as easy as many here seem to think. Let's look at accuracy available in a few voltmeters [http://www.fluke.com]Fluke[/url] makes: Fluke 80, Fluke 77 and Fluke 113.
Fluke 80: + or - 0.1% equates to a reading of 392.6 to 393.4 V for a battery at 393 V. Frankly, I seriously doubt the LEAF has the accuracy of a Fluke 80.
Fluke 77: + or - 0.3% (best) equates to a reading of 391.8 to 394.2 V for a battery at 393 V. That level of accuracy would equate to about a 1-kWh total discrepancy range in LEAF battery capacity. I HOPE that the LEAF has at least this much accuracy.
Fluke 113: + or - 3.0% equates to a reading of 381.2 V to 404.8 V for a battery at 393 V. I Seriously doubt the LEAF is anywhere near this bad. If so, then Houston, we have a problem.
Possibly: + or - 1.0% equates to a reading of 389.1 to 396.9 V for a battery at 393 V. While I do not think the LEAFs are possibly this bad when new at room temperature, but I wonder if there could be this much error over time and temperature. I think that would be entirely reasonable to occur.
I will say that TickTock's voltage charging curves appear to me to show about a 3-V difference in voltage at which the knees occur at both ends of the charging curve. This would correspond with either scenario 3) or scenario 5), but it could simply be operation of the BMS temperature compensating the voltage that is displayed.
So, how can we go about figuring out how much impact the LEAF's voltage measurement accuracy is having on our battery capacity? It's not a simple matter to resolve, but I am confident it is a part of the battery story with the Nissan LEAF. How big of a part it plays I have no idea.
Thoughts?
Palmer above refers to Andy Palmer, executive vice president of Nissan.Canberra Times said:Palmer also dismissed recent reports of battery problems in hot weather for the Leaf. A number of owners in America complained of reduced range during summer, but Palmer says the problem is a faulty battery level display.
"We don't have a battery problem," he says.
While I do not believe for one minute that the LEAF does not have a battery problem, I do think there could also be an instrumentation problem which is either adding to the battery problem or possibly even leading to some of the damage.
More specifically, if the Nissan LEAF's ability to ACCURATELY measure the DC voltage of the main traction battery is not extremely high and extremely stable over temperature and time, the available capacity of the LEAF battery could be severely affected. Please note that there is no doubt that voltage measurement inaccuracies will impact LEAF battery capacity. The question here is how much of what we are seeing is due to this effect.
I am focusing only on DC voltage measurement here, since both charging and discharging in the LEAF appear to both be terminated based upon voltage measurement. Any errors in current measurement should be almost completely inconsequential in regards to available battery capacity and actual range.
Here are a few scenarios that might be affecting the capacity in our LEAF batteries:
1) Coming from the factory: LEAF measurements of battery voltage are higher than the actual battery voltage: In this scenario, the LEAF would terminate charge and discharge at a lower battery voltage than intended, resulting in lower overall capacity available when new. However, the battery will tend to be stored at a lower SOC which may improve calendar life of the battery.
2) Coming from the factory: LEAF measurements of battery voltage are lower than the actual battery voltage: in this scenario, the LEAF would terminate charge and discharge at a higher battery voltage than intended, resulting in higher overall capacity available when new. However, the battery will tend to be stored at a higher SOC which may reduce calendar life of the battery.
3) As temperature rises: LEAF measurements of battery voltage trend higher than the actual battery voltage: In this scenario, at higher temperatures the LEAF would terminate charge and discharge at a lower battery voltage than intended, resulting in lower overall capacity available when hot. However, the battery will tend to be stored at a lower SOC which may improve calendar life of the battery.
4) As temperature rises: LEAF measurements of battery voltage trend lower than the actual battery voltage: In this scenario, at higher temperatures the LEAF would terminate charge and discharge at a higher battery voltage than intended, resulting in higher overall capacity available when hot. However, the battery will tend to be stored at a higher SOC which may reduce calendar life of the battery.
5) As the LEAF ages: LEAF measurements of battery voltage trend higher than the actual battery voltage: In this scenario, as the LEAF ages it would terminate charge and discharge at a lower battery voltage than intended, resulting in lower overall capacity available. However, the battery will tend to be stored at a lower SOC which may improve calendar life of the battery.
6) As the LEAF ages: LEAF measurements of battery voltage trend lower than the actual battery voltage: In this scenario, as the LEAF ages it would terminate charge and discharge at a higher battery voltage than intended, resulting in higher overall capacity available. However, the battery will tend to be stored at a higher SOC which may reduce calendar life of the battery.
Note: All of the above scenarios are focused around the ability of the LEAF to measure 393V.
So while I do not expect a huge variation in 1) an 2) due to the ability to calibrate at the factory, it could explain part of the range differences seen in new LEAFs.
Scenarios 3) and 4) may explain some of the seasonality we see in the LEAF. Scenario 3) may actually be intentionally implemented in the BMS to try to extend the life of the battery.
Scenarios 5) and 6) could also be an issue. If scenario 5) were occurring, we would have a difficult time distinguishing it from battery degradation. If scenario 6) we're occurring, the battery would tend to get more and more abuse from the car as time goes on and would tend to wear out more quickly.
So, how hard is it to measure battery voltage? It's not as easy as many here seem to think. Let's look at accuracy available in a few voltmeters [http://www.fluke.com]Fluke[/url] makes: Fluke 80, Fluke 77 and Fluke 113.
Fluke 80: + or - 0.1% equates to a reading of 392.6 to 393.4 V for a battery at 393 V. Frankly, I seriously doubt the LEAF has the accuracy of a Fluke 80.
Fluke 77: + or - 0.3% (best) equates to a reading of 391.8 to 394.2 V for a battery at 393 V. That level of accuracy would equate to about a 1-kWh total discrepancy range in LEAF battery capacity. I HOPE that the LEAF has at least this much accuracy.
Fluke 113: + or - 3.0% equates to a reading of 381.2 V to 404.8 V for a battery at 393 V. I Seriously doubt the LEAF is anywhere near this bad. If so, then Houston, we have a problem.
Possibly: + or - 1.0% equates to a reading of 389.1 to 396.9 V for a battery at 393 V. While I do not think the LEAFs are possibly this bad when new at room temperature, but I wonder if there could be this much error over time and temperature. I think that would be entirely reasonable to occur.
I will say that TickTock's voltage charging curves appear to me to show about a 3-V difference in voltage at which the knees occur at both ends of the charging curve. This would correspond with either scenario 3) or scenario 5), but it could simply be operation of the BMS temperature compensating the voltage that is displayed.
So, how can we go about figuring out how much impact the LEAF's voltage measurement accuracy is having on our battery capacity? It's not a simple matter to resolve, but I am confident it is a part of the battery story with the Nissan LEAF. How big of a part it plays I have no idea.
Thoughts?