Winter Range Loss in Mach-E -- a more detailed look with some measurements

[Mach-Lee]

It's not a dumb question.

The answer is no, there's no immediate damage to the pack.

The way the pack's capacity decreases in the cold is best understood by an analogy to a bucket of water that partially freezes in the cold. The size of the bucket is the same as always, but you can only pour out the water that remains liquid.

What is actually happening is that low temperatures reduce the rate of the chemical reactions that provide the electrochemical potential energy. But there's no damage done to the battery if the temperatures are low for a short period of time and if there's no massive current into or out of the battery while it's cold.

This lowered chemical reaction rate (lower capacity) is separate from the increased internal resistance that comes from lower temperatures. And also separate from the increased energy used to heat the cabin.

Frozen bucket analogy is good.

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[SpaceEVDriver]

A related question: if my battery SoH is 92% and I charge to 100%, isn't that like charging to 92%? I don't know the answer.

It can be answered if you have an OBD2 device and something like CarScanner. When your battery is warm (~20 C), and you charge to 100% SOCD, what does the parameter, "HVB Energy to Empty (kWh)" say?

If the vehicle is still preserving the top end of the capacity, then you might have 92% of 91.7 kWh = 84.4 kWh. If it's not preserving that full top end, then you might have anywhere from 91.7 kWh to 84.4 kWh, depending on the software approach to "aging gracefully."

 

[RedDragon]

@Mach-Lee and @SpaceEVDriver - thank you. The frozen water in a bucket visualization is a great layman's analogy.

 

[mkhuffman]

It can be answered if you have an OBD2 device and something like CarScanner. When your battery is warm (~20 C), and you charge to 100% SOCD, what does the parameter, "HVB Energy to Empty (kWh)" say?

If the vehicle is still preserving the top end of the capacity, then you might have 92% of 91.7 kWh = 84.4 kWh. If it's not preserving that full top end, then you might have anywhere from 91.7 kWh to 84.4 kWh, depending on the software approach to "aging gracefully."

Thanks for thinking through a response. But I am using both those tools - that is how I know the battery SoH is 92%. So my warm battery full charge is 84 kWh. If I charge to 84 kWh, is that as bad as charging to 91 kWh? I can't charge to 91 kWh any more, so it isn't relevant really. Just a curiosity.

 

[SpaceEVDriver]

Thanks for thinking through a response. But I am using both those tools - that is how I know the battery SoH is 92%. So my warm battery full charge is 84 kWh. If I charge to 84 kWh, is that as bad as charging to 91 kWh? I can't charge to 91 kWh any more, so it isn't relevant really. Just a curiosity.

If its 100% SOCD is 84 kWh, and it claims there's a 92% battery health remaining, then it looks like it's preserving the top end buffer and charging to 100% SOCD is probably not doing any more harm than it would if you still had 100% health. However, your battery health is lower than I would expect for the age and miles (though I have no knowledge of rapidly how you've been discharging). There's a reason the battery is reporting a lower health, but that can't easily be diagnosed online. There may be a misbehaving cell or two or there may be a misbehaving sensor that's measuring an extra bit of resistance somewhere unexpected. I don't know if a dealer would investigate this under warranty.

If it were my car, I would do two things: 1) obsessively run analyses on every bit of data I could get; 2) charge to 100% daily like I do now.

 

[ArthurDOB]

Has anyone done anything like this in Canada or the Upper Midwest states where I live? I'd really like to see data like this from the dead of winter in those places. I would do it myself next winter but there's a reason I'm not a math teacher!

 

[mkhuffman]

If its 100% SOCD is 84 kWh, and it claims there's a 92% battery health remaining, then it looks like it's preserving the top end buffer and charging to 100% SOCD is probably not doing any more harm than it would if you still had 100% health. However, your battery health is lower than I would expect for the age and miles (though I have no knowledge of rapidly how you've been discharging). There's a reason the battery is reporting a lower health, but that can't easily be diagnosed online. There may be a misbehaving cell or two or there may be a misbehaving sensor that's measuring an extra bit of resistance somewhere unexpected. I don't know if a dealer would investigate this under warranty.

If it were my car, I would do two things: 1) obsessively run analyses on every bit of data I could get; 2) charge to 100% daily like I do now.

My GT was built in August of 2021. I have to check CarScanner again to see the actual battery age, but it is about 1.5 years. A year ago my battery SoH was 100%. It dropped rapidly last summer, and is still dropping. I think it is probably normal.

I have recorded a lot of data using Car Scanner but I am not sure it is helpful to understand why the SoH is 92%. As we move into the really hot temperatures this summer, I plan to take a lot more data to see if maybe the SoH improves in warm weather. But really I don't think it will. The battery will degrade over time. This is what we can expect.

 

[JimmyMachE]

There may be a misbehaving cell or two

Cell voltage delta can be monitored in Car Scanner:
HVB Voltage Variation Battery modules
[edit] there is another interesting parameter:
HVB SOC Module variation %

 

[mkhuffman]

Cell voltage delta can be monitored in Car Scanner:
HVB Voltage Variation Battery modules

What is the spec and how can I tell if there is something off?

 

[SpaceEVDriver]

What is the spec and how can I tell if there is something off?

That's the question--We don't know if a few % variation in module voltage is a problem. I have only found partial spec sheets for the specific cells used by the Mustang (LG Chem E71A) . The nominal cell voltage is usually 3.6v, but the E71A is 3.7v. Each HVB configuration has different tolerances and I haven't found those tolerances for the Mustang pack. A typical maximum voltage at full charge would be about 4.2v, and a minimum would be about 2.8v, but could be as high as 3.0v. We would need to find a workshop manual that specifies the testing of the battery and I'm willing to be those are only available to the battery engineers and technicians, not even the dealer techs.

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