Effect of temperature on battery capacity

[Michal]

We have all witnessed a drop in range as the temperatures drop. I usually monitor my battery capacity to estimate degradation. Interestingly, the battery capacity reported by BMS changes with temperature. I started in my garage at about 65 degrees. And about 129kW capacity. When I got to my destination the outside temperature overnight was 10F.The next morning reported capacity dropped to about 122kW. I drove home starting with 0.8mi/kw efficiency. Once I plugged into L1 charger in a heated garage the reported capacity came back
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[BCondrey]

Interested in what you are using to measure that.

 

[KidThunder]

I drove home starting with 0.8mi/kw efficiency.

I just had a similar experience where I charged the vehicle up overnight, but outside with temps in the single digits. On a full charge I wasn't able to make a 190 mile trip without stopping. And even with preconditioning, we weren't able to pull charging speeds faster than 22kW and we tried two different locations and multiple stalls at each one. It was not a great experience.

 

[VSG]

Interestingly, the battery capacity reported by BMS changes with temperature

Isn't this obvious and expected?

Google "battery capacity as a function of temperature"

This is extremely well-known, for something like two hundred years. The details depend on the battery chemistry because these are chemical reactions where, like most chemical reactions, the reaction rate is sensitive to temperature.

 

[Michal]

Thank your for your insightful response

Isn't this obvious and expected?

Google "battery capacity as a function of temperature"

This is extremely well-known, for something like two hundred years. The details depend on the battery chemistry because these are chemical reactions where, like most chemical reactions, the reaction rate is sensitive to temperature.

 

[KidThunder]

Isn't this obvious and expected

If it’s so obvious and expected, then I would expect the range on the vehicle computer would say 150 instead of 300

 

[VSG]

Shrug. I don't know why people act surprised about things like this. It's all well-known. Same thing happens to the 12V on an ICE car for example. Did you do the Google search I suggested? Those results have nothing to do with EVs - this is just how batteries of all kinds work.

I would expect the range on the vehicle computer would say 150 instead of 300

Umm, the driver's screen is just a fuel gauge. It shows you percent capacity times the (fixed) EPA mileage. When the temperature drops, the percent capacity doesn't change, so the fuel gauge doesn't change.

 

[VSG]

Oh, and also, you know a similar thing happens with ICE vehicles, right? Gasoline, like most liquids, expands with temperature, so in really hot weather you actually have less energy available per gallon, because gallon is a volume. So your range on a full tank goes down.

 

[MidnightM00N]

Shrug. I don't know why people act surprised about things like this. It's all well-known. Same thing happens to the 12V on an ICE car for example. Did you do the Google search I suggested? Those results have nothing to do with EVs - this is just how batteries of all kinds work.


Umm, the driver's screen is just a fuel gauge. It shows you percent capacity times the (fixed) EPA mileage. When the temperature drops, the percent capacity doesn't change, so the fuel gauge doesn't change.

The odd thing is rivians method in the cold (per my electrafi data) is to reduce the total kWh capacity and not the available capacity based on temp. This increases percentage charge to maintain a steady range estimate and is misleading.

 

[Michal]

@VSG I dont think anyone is surprised. As much as I appreciate your explanation, I am not talking about the effect of temperature on the battery chemistry I am referring to the "perceived" battery capacity as reported by the BMS.

The odd thing is rivians method in the cold (per my electrafi data) is to reduce the total kWh capacity and not the available capacity based on temp. This increases percentage charge to maintain a steady range estimate and is misleading.

 

[Michal]

The odd thing is rivians method in the cold (per my electrafi data) is to reduce the total kWh capacity and not the available capacity based on temp. This increases percentage charge to maintain a steady range estimate and is misleading.

exactly

 

[Michal]

Interested in what you are using to measure that.

I am using Home Assistant with Rivian API integration to collect the data
https://github.com/bretterer/home-assistant-rivian
Let me know if you have any questions...

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