eNate
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Thoughts on cutting off 100% to account for lost capacity?

Mon Mar 23, 2020 7:36 pm

This is hypothetical, just interested in your response to this idea.

We talk about battery capacity, loss over time, checking Max Kappa on the hidden menu, etc.

What would you think about EVs (not specifically the i3) adjusting their displayed SOC downward to account for this degradation in capacity?

Rather than the owner doing mental tricks to figure out what their battery health is, there would simply be a tick mark on the charge bar.

Car is 3 years old and the battery has lost 10% of its capacity? SOC only displays 90% when fully charged. Battery approaching 70%? Then so is the max SOC.

On my 94Ah battery, I know I can drive roughly 1.1-1.2 miles per percent SOC. Using this method I'm suggesting, as the battery ages, I would never have to make an adjustment to that miles per percent figure.

It might be a downer to no longer have your older EV hit 100%, but it feels more honest and transparent.
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jadnashuanh
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Re: Thoughts on cutting off 100% to account for lost capacity?

Mon Mar 23, 2020 9:11 pm

It's useful to know when the batteries are fully charged. From that point, the estimated range value gives you a decent idea of how far you can go, at least once you understand the car (if ever, for some people).

BMW could display a value indicating the percentage of the original capacity, but that's harder to do. Keep in mind, when they do the determination on what the 'real' capacity is, the whole procedure takes about a day at the dealership...if it could be determined by the car itself, then your suggestion would make sense. I'm not saying that it wouldn't be useful, but actually measuring it accurately doesn't seem to be an easy thing to do in the vehicle itself.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

eNate
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Re: Thoughts on cutting off 100% to account for lost capacity?

Mon Mar 23, 2020 9:24 pm

Ah, but can a degraded battery ever truly be at 100%?

The car would still need to indicate "fully charged" and make it clear that it can't go any higher.

I think, with use, the system can accurately self-diagnose the battery's capacity. It wouldn't need to be perfect, just present an honest representation of what's available.
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MKH
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Re: Thoughts on cutting off 100% to account for lost capacity?

Tue Mar 24, 2020 4:02 am

the whole procedure takes about a day at the dealership...if it could be determined by the car itself,
Not sure that the car could keep a 'running' accurate capacity determination. My understanding of how the dealerships do the battery capacity test, is the battery is drained/discharged of charge, then recharged while measuring the amount of electricity the battery is able to take in/store to full charge. Like emptying a water tank, then measuring how much water the tank holds when full, by refilling it while counting how many gallons it takes.
Mark H.
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eNate
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Re: Thoughts on cutting off 100% to account for lost capacity?

Tue Mar 24, 2020 7:54 am

Let's not get lost in the weeds with technical details here, I'm interested in big picture. Not the i3 we're driving today -- the EVs we'll be buying in the future.

Yes, the car can do it, it does it ever day.

Many owners with older i3s report regularly that their range is no longer what it used to be. This is the car "recognizing" the degradation its battery has experienced, and adjusting on the fly.

Bart Kapa Max Is good enough to do this. It's not perfectly accurate, but it's a calculation that a BMS could refine over time.

And because I'm generous, I'll grant you a dealer visit to tweak it, but I'd bet a similar process can be a routine built into the car that any owner could run at home when they had a day to spare.

So skip past the technicalities for the moment and share with me the reasons you wouldn't want to know this / see this as a regular up-front reminder of your battery's health? If this became the standard among all EVs?
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alohart
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Re: Thoughts on cutting off 100% to account for lost capacity?

Tue Mar 24, 2020 12:29 pm

I would prefer a battery pack charge level gauge in which 0% was actually fully discharged and 100% was the actual full charge, neither of which is allowed by the BMS. Two other limits on this gauge would be the lowest (e.g., 9%) and highest (e.g., 95%) charge levels allowed by the BMS. Apparently, ISTA+ shows these BMS charge level limits, so a gauge could display them. This gauge would give the driver an indication of how the BMS is adjusting the lower and upper charge level buffers over time to account for battery cell degradation or to minimize the cell degradation rate.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

eNate
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Re: Thoughts on cutting off 100% to account for lost capacity?

Tue Mar 24, 2020 1:58 pm

Art, in your scenario with a true 100% displayed at delivery, would that 100% always be 100%, or would the max decrease with degradation? That's a theoretical question I suppose -- the battery never gets to true 100%.

But OK, 100% is not achievable, and your new car shows 94% driving off the lot. After a year or so your pack has lost 5% of total capacity, the max charge you can attain, as displayed, has decreased now to 90%. For you, Go or No-Go?

My issue with your "absolute" approach is that I wouldn't want 0% to be a moving target. I believe it benefits the drive to know exactly what they're dealing with until the car can't move another inch, even if it's a virtual floor.
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alohart
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Re: Thoughts on cutting off 100% to account for lost capacity?

Tue Mar 24, 2020 11:12 pm

eNate wrote:
Tue Mar 24, 2020 1:58 pm
Art, in your scenario with a true 100% displayed at delivery, would that 100% always be 100%, or would the max decrease with degradation? That's a theoretical question I suppose -- the battery never gets to true 100%.
Apparently, ISTA+ can display the current charge level as well as the lower and upper charge level limits (e.g., 9% and 95%). I don't know how true 0% and 100% charge levels are defined and whether they should be adjusted with degradation. With 96 new series-connected 60 Ah cells, apparently Samsung SDI specifies the 0% - 100% charge level range as 259 V - 396 V. However, I don't know whether the cell voltage range varies with degradation. I prefer a gauge that displayed what ISTA+ displays.

I would like to know the lower and upper charge level limits imposed by the BMS in absolute charge level percentages in addition to the current charge level.
eNate wrote:
Tue Mar 24, 2020 1:58 pm
But OK, 100% is not achievable, and your new car shows 94% driving off the lot. After a year or so your pack has lost 5% of total capacity, the max charge you can attain, as displayed, has decreased now to 90%. For you, Go or No-Go?
Depends on why 5% of usable capacity were no longer available. If true cell degradation were responsible for the entire 5% loss, I'd accept that because the Idaho National Laboratory measured a 7.5% usable capacity loss on 3 new 2014 i3 BEV's after only 15 months and 12k miles. However, if the BMS is reducing the usable capacity in addition to cell degradation, I would be unhappy with BMW.
eNate wrote:
Tue Mar 24, 2020 1:58 pm
My issue with your "absolute" approach is that I wouldn't want 0% to be a moving target. I believe it benefits the drive to know exactly what they're dealing with until the car can't move another inch, even if it's a virtual floor.
I agree that an i3 driver must know how much energy remains before the BMS prevents any further discharge.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

ksnax
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Re: Thoughts on cutting off 100% to account for lost capacity?

Sun May 17, 2020 11:15 pm

I recently learned of the Deep OBD app for Android. It shows real time battery voltage and charge level.

Fully charged, mine was displaying right around 390V and 80% charge via the OBD data, but showed full capacity on my dash. Last time I checked kappa, it was something like 18.1kW, which is just about 80% of nominal 22kW rated capacity.

That said, I think the i3 adjusts usable displayed capacity based on a formula and recent performance history. There really isn't any way to measure a battery directly for capacity. Voltage values are interpolated into range, but even a mostly cooked battery can still display full voltage or any value in between, so it is always just an estimate.

jadnashuanh
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Re: Thoughts on cutting off 100% to account for lost capacity?

Mon May 18, 2020 2:26 pm

The i3 just will not let you use the entire battery capacity. On the original ones, around 19.x is the maximum you can use, so of the original max capacity usable, you've got more than 80%
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

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