LEAF voltage measurement accuracy impact on capacity

[palmermd]

Also remember that large automakers, especially Japanese ones, are very methodical about changing things, and it takes a long time to properly implement a fix. If that fix involves software in a critical system, (the LBC for example) it will take many hours of testing before they will even consider releasing it. I believe they will fix this, but it will be done on their terms which means it will take some time before we see a solution.

-Phil

Can't they at least send out some tee-shirts and toys along with a letter saying that they are working on it. haha. Silence and the comments about there not being any problem at all, are what is hurting them in this situation.

 

[Stoaty]

The Battery Systems Engineer told me that cost was the reason they used the Hall-Effect current counter rather than a more-accurate galvanic shunt.

Any ballpark figures for the cost of each of these devices? Were they penny wise and pound foolish, or is the galvanic shunt pretty expensive?

 

[RegGuheert]

The biggest problem with the Leaf's BMS (in my opinion) is the use of the Hall-effect current sensor.

While I agree that these are the biggest source of errors in the BMS, I must say that I am greatly relieved to hear your report that the voltages are within abou 0.5V of the actual value. Frankly, the inaccurate current measurements are a source of confusion for all of us trying to determine battery capacity and make trip planning a little more challenging, but they are not nearly as important to me as the voltages. If the voltage measurements are working well and the BMS is fairly well-thought-out, then I feel that our expensive batteries are not being damaged more than they need to be to propel us around. Put another way, I can live with the inaccurate current measurments in our LEAF.

I was able to meet with the Nissan engineers from Japan last December, including the battery system engineer (I had a one-on-one with him). Their explanation for why we have no direct SoC display in the car was basically that they were afraid to show it and have these corrections occasionally make it "jump" which would "confuse the customer". The Battery Systems Engineer told me that cost was the reason they used the Hall-Effect current counter rather than a more-accurate galvanic shunt.

I wonder if they have considered using the Rogowski Coil technique discussed in this application note by Analog Devices.

Also remember that large automakers, especially Japanese ones, are very methodical about changing things, and it takes a long time to properly implement a fix. If that fix involves software in a critical system, (the LBC for example) it will take many hours of testing before they will even consider releasing it. I believe they will fix this, but it will be done on their terms which means it will take some time before we see a solution.

Given the climate here and my understanding of the current measurement inaccuracies as well as the voltage measurement accuracies in the LEAF, I think I can wait a long time for them to resolve this. Had I thought the BMS was damaging our battery somehow, I would be much more impatient.

 

[RegGuheert]

Any ballpark figures for the cost of each of these devices? Were they penny wise and pound foolish, or is the galvanic shunt pretty expensive?

The link I posted just after your post gives a full *relative* comparison of various techniques:

I wonder if they have considered using the Rogowski Coil technique discussed in this application note by Analog Devices.

 

[surfingslovak]

Also remember that large automakers, especially Japanese ones, are very methodical about changing things, and it takes a long time to properly implement a fix. If that fix involves software in a critical system, (the LBC for example) it will take many hours of testing before they will even consider releasing it. I believe they will fix this, but it will be done on their terms which means it will take some time before we see a solution.

Can't they at least send out some tee-shirts and toys along with a letter saying that they are working on it. haha. Silence and the comments about there not being any problem at all, are what is hurting them in this situation.

There will be that, and more, at Crissy Field next Sunday. Numerous Nissan representatives will be in attendance as well. Please PM me if you needed more information or help with the logistics.

 

[RegGuheert]

Numerous Nissan representatives will be in attendance as well.

...in full riot gear! :lol: :lol: :lol:

 

[Stoaty]

The link I posted just after your post gives a full *relative* comparison of various techniques:

I wonder if they have considered using the Rogowski Coil technique discussed in this application note by Analog Devices.

That reference says that Hall Effect Sensor is:
--high cost
--poor linearity over measurement range
--has saturation and hysteresis problem

Doesn't mention galvanic shunt, unless that is "low resistance current shunt" (which is listed as very low cost).

 

[gfederas]

The biggest problem with the Leaf's BMS (in my opinion) is the use of the Hall-effect current sensor. These are not very accurate for coulomb counting and subject to accuracy degrading effects, such as centerline drift, effects of the earths magnetic field, temperature, etc. The inaccuracy of this is why "some gids are more equal than others". Nissan compensates for this inaccuracy by applying corrections to the SoC by sampling voltage and using it formulas that also take into account the temperature, internal resistance, aging, etc. This is why you can gain/lose SoC suddenly sometimes after power cycling. It will apply changes all at once if the car is power cycled, but if in use, it will apply a correction in the form of a drift which appears as faster/slower SoC counting than real energy out/in.

I was able to meet with the Nissan engineers from Japan last December, including the battery system engineer (I had a one-on-one with him). Their explanation for why we have no direct SoC display in the car was basically that they were afraid to show it and have these corrections occasionally make it "jump" which would "confuse the customer". The Battery Systems Engineer told me that cost was the reason they used the Hall-Effect current counter rather than a more-accurate galvanic shunt.

It's looking like there is some degradation in these hot-climate packs, but it appears that the BMS (LBC) is not dealing with it properly, and not only indicating incorrect loss figures, but also possibly not allowing for full use (charging) of the packs real capacity.

Keep in mind, Nissan did a lot of testing, but the bulk of it is accelerated life tests, which attempt to simulate a much longer real-world use scenario. Unfortunately sometimes there is no substitute for real-world life testing, and it sounds like there are some unexpected results that the BMS software is not equipped to deal with.

Also remember that large automakers, especially Japanese ones, are very methodical about changing things, and it takes a long time to properly implement a fix. If that fix involves software in a critical system, (the LBC for example) it will take many hours of testing before they will even consider releasing it. I believe they will fix this, but it will be done on their terms which means it will take some time before we see a solution.

-Phil

Phil:
excellent explanation. Technically detailed for the astute to verify, and simply stated for the rest of us.

 

[RegGuheert]

That reference says that Hall Effect Sensor is:
--high cost
--poor linearity over measurement range
--has saturation and hysteresis problem

Doesn't mention galvanic shunt, unless that is "low resistance current shunt" (which is listed as very low cost).

Yes, those are the same thing.

I think the discrepancy here is that paper lists the "galvanic shunt" as "high power consumption". Basically, if you make the shunt low enough resistance to not negatively impact the vehicle's efficiency, then it changes from "high power consumption" to "high cost" since then you need to measure very small voltages accurately.

 

[Ingineer]

Yes, Reg is correct. An instrument-grade galvanic shunt is expensive, as each one must be calibrated. The electronics needed to read the shunt are also expensive.

A low-cost shunt is not applicable as it wouldn't have the resolution to beat the hall-effect unit. Rogowski coils cannot work on DC applications such as the Leaf. (nor can current transformers)

That paper is discussing AC current metering, so none of it can apply to DC. (as in the Leaf)

-Phil

 

[RegGuheert]

Rogowski coils cannot work on DC applications such as the Leaf. (nor can current transformers)

That paper is discussing AC current metering, so none of it can apply to DC. (as in the Leaf)

Thank you for correcting me on that point.

 

[Nekota]

In my mind, their TMS/BMS is heavily driven by machine learning and as you know... machine learning is all about training and retraining. It works great in retrospect, but struggles with fast moving training sets or in our case fast moving or unpredictable environmental swings.

I think you are dead on..

Aye and considering how the DTE / GOM is kinda quirky maybe the BMS is a wee bit nervous too.

 

[Nekota]

Yes, Reg is correct. An instrument-grade galvanic shunt is expensive, as each one must be calibrated. The electronics needed to read the shunt are also expensive.

A low-cost shunt is not applicable as it wouldn't have the resolution to beat the hall-effect unit. Rogowski coils cannot work on DC applications such as the Leaf. (nor can current transformers)

That paper is discussing AC current metering, so none of it can apply to DC. (as in the Leaf)

-Phil

I have calibrated equipment using these sensors and they work very well.

http://fwbell.com/ckfinder/userfiles/files/NT-Datasheet.pdf" onclick="window.open(this.href);return false;

These AMR sensors were used in disk drive heads in the mid 1990's and was replaced by the current GMR sensors used today. An AMR sensor is 50x more sensitive than a hall effect device and can measure at 1% accuracy and 0.25% linearity and cost $32 each (1000 pcs) - single unit cost is 2x at $65. I suppose that is considered expensive for a $37K car but after a round of bad battery press they may look more cost effective. I don't know if they would help with the BMS since Nissan is saying voltage figures into their error prone calculation of battery capacity and recent data discussions are suggesting on offset voltage issue - if so I hope some Nissan engineer is working on a chopper stabilized voltage amp??

 

[surfingslovak]

Numerous Nissan representatives will be in attendance as well.

...in full riot gear! :lol: :lol: :lol:

Although very real to us here on the forum, and particularly important to the affected owners, the battery degradation problem has not gone mainstream yet. Even though loss of autonomy might be approaching 10% in many parts of California, Leaf's largest market, many owners have likely not taken notice.

The Crissy Field event is going to be an owner celebration first and foremost; Nissan is attempting to organize the largest Leaf gathering in history. I'm told that there will be an opportunity for private conversations, and it's my honest belief that all opinions should be represented and heard.

 

[Stoaty]

Although very real to us here on the forum, and particularly important to the affected owners, the battery degradation problem has not gone mainstream yet. Even though loss of autonomy might be approaching 10% in many parts of California, Leaf's largest market, many owners have likely not taken notice.

After the range test in Phoenix and my own range test, I'm not so sure about that 10% loss in many parts of California. I do have a 10% loss of Gids at full charge, but I no longer feel confident that I have lost 10% of range--it may be much less. The fuel bars, Gids, and capacity bars seem to be much less reliable as an indicator of actual range than I thought previously. On my recent range test, I only needed 25 Gids to drive home 20 miles (usually takes a minimum of 45 Gids). Those 25 Gids were at the low end of the scale (69 Gids down to 45 Gids). This indicates an extreme non-linearity for Gids, where the low end of the scale showed Gids that are perhaps 70% more energy than the middle or high end of the scale.

 

[DaveEV]

After the range test in Phoenix and my own range test, I'm not so sure about that 10% loss in many parts of California. I do have a 10% loss of Gids at full charge, but I no longer feel confident that I have lost 10% of range--it may be much less. The fuel bars, Gids, and capacity bars seem to be much less reliable as an indicator of actual range than I thought previously. On my recent range test, I only needed 25 Gids to drive home 20 miles (usually takes a minimum of 45 Gids). Those 25 Gids were at the low end of the scale (69 Gids down to 45 Gids). This indicates an extreme non-linearity for Gids, where the low end of the scale showed Gids that are perhaps 70% more energy than the middle or high end of the scale.

Here's an interesting experiment for those of you with GID meters and capacity loss.

I wonder if the 100% GID count will recover any amount after one of these cars hits turtle. Test procedure would go something like this:

1. Charge to 100% - do something that encourages balancing (eg: leave topped off and plugged in for 4+ hours, charge to 100% multiple times in a week) and get a GID/voltage reading.
2. Drive to turtle. Record GID/pack-voltage/miles/mi/kWh at LBW, VLBW and turtle.
3. Charge to 100% - check GID/voltage reading.
4. Drive to turtle. Record GID/pack-voltage/miles/mi/kWh at LBW, VLBW and turtle. Use as similar driving as possible.
5. Charge to 100% - check GID/voltage reading.

Would be interesting to see if the LBC adjusts at all. One also might consider turning the car off for some period of time (a couple minutes to 15 minutes?) at LBW, VLBW and turtle to see if the GID count recovers at all after the car is able to get a good voltage reading with no load on the pack.

In the days of rather dumb BMS systems such as those in laptops, it's well known that the BMS can start to think that the capacity of the pack is less than what the battery is physically capable of holding. A full discharge/charge cycle is typically enough to reset it.

It seems that the LEAF's LBC takes some time to adjust - multiple cycles may be necessary to bring it back in-line.

 

[GlennD]

In the morning my Leafscan shows just over 84K when I floor it to get on the freeway. This leads me to think that the voltage and current measurements are reasonably accurate since the motor is 84K.