Charging at home "complete" before reaching set percentage by a large margin

[cappo3]

Hi guys!
I know, I know, the question has been asked repeatedly on the forum.
I looked around and found many instances of people wanting to charge to, let's say, 80% and finding their car stopped at 79%. Or 49% instead of 50%.
My car is usually charged at 11 kW on 400V (public charging), or at the office where I get 230V 32A. At home I can only get up to 230V 10A, which immediately sags to 220V 10A. Not much power indeed, so I tend not to charge there.
Yesterday night I tried plugging in my car at home for a change. Given the reduced charging speeds, I settled for a 52% target, up from a SoC of 32%.
During the charging the ambient temperature was around -3C/27F, but the car was driven before. After an hour the snow flake started appearing intermittently, a behaviour that persisted throughout the charge.
In the middle of the night my phone woke up and told me the charging process was complete. At 48%.
So I tried restarting it by upping the target percentage to 53%. Bacons came up in the app, the SoC fell to 47%, I went back to sleep.
After maybe half an hour came the new notification. Charging complete, 48%.
I won't need more than that today, so that's no problem. Just curious about the reasoning behind this happening twice!
Any opinions/experiences?

 

[GtiMart]

First, the battery SOC is just an estimate, the BMS cannot be extremely precise. It's pretty good but not perfect.
Second, and this one is a bit more complicated... Say your battery is charged to a perfect 50% at the perfect temperature, let's say 30C. If you let the battery cool down, say to 0C, it will still be at the same voltage (representing 50% SOC) but you will be able to extract less energy out of it than if it was at 30C. The car likes to show that by showing a lower SOC. I believe this is your situation. The battery is charged to 52% (if the bms looks at the voltage of the cells) but it reports that you would only be able to extract the equivalent of 48% when driving.

 

[cappo3]

First, the battery SOC is just an estimate, the BMS cannot be extremely precise. It's pretty good but not perfect.
Second, and this one is a bit more complicated... Say your battery is charged to a perfect 50% at the perfect temperature, let's say 30C. If you let the battery cool down, say to 0C, it will still be at the same voltage (representing 50% SOC) but you will be able to extract less energy out of it than if it was at 30C. The car likes to show that by showing a lower SOC. I believe this is your situation. The battery is charged to 52% (if the bms looks at the voltage of the cells) but it reports that you would only be able to extract the equivalent of 48% when driving.

Thank you for your answer!
I understand that there is a discrepancy between the actual SoC of a battery and what the BMS shows to the end user, and that this discrepancy is dependent on various criteria.
My question pertains more to the extent of this discrepancy. If the battery is actually charged up to 53%, but only 48% is available, that is a "potential loss" of nearly 10% of the SoC. At an ambient temperature around the freezing point, it does seem a lot to me! I parked the car in an underground parking lot for the day, it is around 15C/60F. The percentage did not budge, still at 47% as when I left it cold soaked. Where did that energy go?

 

[jjrandorin]

Any opinions/experiences?

That the battery is cold soaked so some of the capacity is locked out by the BMS and that it will return when the battery warms. "Warming the battery" takes longer than most people think.

I can drive from my home to my workplace, which is 38-40 miles 1 way, with a large portion of the drive being a freeway that has average speeds around 80MPH (not kph) when I drive, In Southern California which is one of the more temperate climates in the US, and still have regen dots on my screen the entire drive.

 

[KenC]

Hi guys!
I know, I know, the question has been asked repeatedly on the forum.
I looked around and found many instances of people wanting to charge to, let's say, 80% and finding their car stopped at 79%. Or 49% instead of 50%.
My car is usually charged at 11 kW on 400V (public charging), or at the office where I get 230V 32A. At home I can only get up to 230V 10A, which immediately sags to 220V 10A. Not much power indeed, so I tend not to charge there.
Yesterday night I tried plugging in my car at home for a change. Given the reduced charging speeds, I settled for a 52% target, up from a SoC of 32%.
During the charging the ambient temperature was around -3C/27F, but the car was driven before. After an hour the snow flake started appearing intermittently, a behaviour that persisted throughout the charge.
In the middle of the night my phone woke up and told me the charging process was complete. At 48%.
So I tried restarting it by upping the target percentage to 53%. Bacons came up in the app, the SoC fell to 47%, I went back to sleep.
After maybe half an hour came the new notification. Charging complete, 48%.
I won't need more than that today, so that's no problem. Just curious about the reasoning behind this happening twice!
Any opinions/experiences?

As others have motioned, your battery was cold, and may have shown a blue snowflake.

The power is still there. It's just not accessible until the battery warms up. Here, I'm showing my battery right now. It charged to 60%, when I added 3kWh, but it shows 54% and a blue snowflake, cause it is cold. When I drive, the battery will warm up and the amount of energy usable will return.

I suppose it should say "temporary", since the energy will become available.

 

[GtiMart]

At its worst I've seen 7% less available energy on a very cold day (-20C). It's just physics (or chemistry ), you cannot extract as much energy from a cold battery. Some of it will come back as it heats.

The general answer to your original question is: the car has charged to the correct percentage, the one you asked, but you have a bit less than that available. And it's all just estimates, not precise things. Carry on, there's nothing to see

 

[davewill]

There's no way around the fact that these cars take a significant range hit during cold weather. Causes are: cold battery, cold tires, snow tires, heater use, and rain or snow on the road, among others.

 

[cappo3]

Thanks so much to all of you for chiming in with your thoughts and experiences.
I thought I'd chime in after having given this whole home-charging thing another go.

At its worst I've seen 7% less available energy on a very cold day (-20C). It's just physics (or chemistry ), you cannot extract as much energy from a cold battery. Some of it will come back as it heats.

The general answer to your original question is: the car has charged to the correct percentage, the one you asked, but you have a bit less than that available. And it's all just estimates, not precise things. Carry on, there's nothing to see

So, last night I tried again charging at home.
The main difference between my home-charging scenario and the other possibilities I tend to adopt is that, when charging at home, the car sleeps outside and is exposed to ambient temps.

I set the car to charge to 60% up from 40%, it stopped at 59%. That's fine by me, it finished at 3 in the morning and it was 33F/1C outside.

I'm about to head out now, 7 hours later. I opened the app to check the car wasn't actively charging in order to disconnect the charger from the mains.

Outside temp 35F/2C, cabin temp 34F/1C, snowflake, battery at 52%.

You say you've seen a max of 7% in -20C/-4F weather. Personally I'd call that pretty extreme weather. A loss of 7-8% within 7 hours in temps constantly over freezing seem wrong to me. I totally understand there is an underlying chemical process behind the principle, but its extent simply worries me.
Am I mistaken?

 

[Regaj]

I think you're fine.

Like you, my car sits out in the weather. And my charger only delivers 16 amps (it's a 20-amp circuit) at around 236 volts. That's pretty low for AC charging. At 10 amps yours is even lower.

If the pack temp drops into the 30's (Fahrenheit), the car will attempt to raise pack temp if you have climate turned on or you are charging. A dual motor Tesla will absorb around 7 kW trying to heat the pack. So your already marginal charging setup becomes even more challenged when it gets cold.

Most of us don't think of 30-some (Fahrenheit) temps as all that extreme. And yet it is a not-very-good place from a Lithium battery chemistry perspective. Your car's BMS knows this and is simply trying to get the pack to a healthier state.

Anytime you get the blue snowflake or bacon strips your pack is too cold.

Driving will raise pack temp. But the ability of your car to raise (or lower) pack temps is quite limited. It doesn't happen quickly. So the fact that you just got back from a drive doesn't mean your pack is at a nice, warm state. It's not like an ICE car where 15-20 minutes will bring the engine up to temp. In an EV, if it's a short drive, your pack is probably not much warmer than when you left.

Most of us will plug into the charger some time during the day, after returning back home. Ambient temps are typically at their highest. And the car has just been driven. The pack is at its warmest. All good.

But over the next few hours, even as the battery is charging, its temp is declining in step with the rapidly diminishing ambient temps. At whatever point it hits our desired SOC, the charging ends and whatever little heat was being generated by the charging itself goes away. The pack cold-soaks overnight.

When you get up the next morning, there's frost on the ground outside... and you've got a very cold pack. As soon as you wake the car, the BMS sees this cold pack temp and pulls a few percent from the displayed SOC. What you don't see is that later, as you're driving the car and the pack slowly warms, those few percent are returned.

If you're worried, you can install SMT and get a much more refined view of your car's behavior at various pack temps. But I think you're fine.