Battery heating while charging with Wall Charger

[Crim]

It occurs to me that when charging with my wall charger at home, the battery often preheats up before and during charging. I only have a few percent to be charged most of the times and then it seems to me that preheating is a waste of energy, the costs of heating the battery maybe almost as high as the charging itself.

I thought it might be wise not to plug the car in when a little power is used but the Tesla manual says: "It is important for a long battery life that the car is PLUGGED IN when you are not using the car".

I have tried to prevent preheating by reducing the charging speed but unfortunately this does not work. Now I keep lowering the charging limit so the car doesn't really start charging after plugging in but this is kind of inconvenient.

Does anyone know why the battery heats up for these short charges? I don't tink it's really necessary when charging slowly? Or is this better for the battery?

The graph below shows that the battery charges more slowly during the first 30 minutes of charging because energy is used to heat the battery. Most of my charges only last 30-60 minutes.

 

[justsomeguy]

I found this exact same thing. My solution was to charge in big chunks, especially now that I never drive due to social distancing, etc.

I will charge my car up to 70% and then immediately lower it to 50% (you could probably automate this via mosquitto tie in with Teslamate, I've been too lazy to figure it out), and rinse and repeat.

As for why it does this, probably battery health. I'm going to assume it's in the same logic as the DC charging battery heating, which appears to be to get to the target heating temperature regardless of whether or not it's required to hit the max output of the power source.

 

[Rocky_H]

It occurs to me that when charging with my wall charger at home, the battery often preheats up before and during charging.

Because it needs to be somewhat warm to charge to not damage the battery. I just looked up the temperature in the Netherlands (I picked Arnhelm as an example) and it shows 35 degrees Fahrenheit. That is barely over freezing. The car won't charge if it's below freezing, and it is a gradual scale above that of how many amps it can take with how much warmer it is.

I thought it might be wise not to plug the car in when a little power is used but the Tesla manual says: "It is important for a long battery life that the car is PLUGGED IN when you are not using the car".

Well, that is a bit of a marketing statement. There seemed to be a lot of old outdated "knowledge" that people thought they needed to run batteries down really empty first before refilling them or the opposite--that when they finished charging, people had to immediately unplug to avoid overcharging and damaging. So this recommendation is just to give one simple rule that everyone can follow, and it will be fine. Keep it plugged in as often as you want when you are home.

But if you go for a day or two without charging, it's not going to make any difference if it is still staying mostly in the middle of the state of charge and not getting very low.

Most of my charges only last 30-60 minutes.

So yes, if you want to save some energy from that unnecessary preheating, you could wait and plug in every 2 or 3 days to reduce that since your charges are so short. We don't drive a lot of miles, so that's what we do too.

 

[camalaio]

Most of us assume it's to protect the battery health in some way. I don't get this for low charge power though, since it allows much more regen power and for an extended period of time too.

It'll keep the battery above 10C or so when charging, usually around 12-18C. However, your graph implies it is blending heating and charging which is not what my car does on version 2020.4.x.x. Mine heats fully (using all available power to do so), then begins charging. What version are you on? And is it a LR AWD? (mine is)

You may know this already, but you can minimize active heating energy by charging only after driving it for a bit (which will have warmed the battery, not by a lot but some). Any other measures (e.g. using preconditioning) are actively heating the battery anyways, so you're not gaining anything except control over when it heats it.

Out of curiosity, why are your charge sessions normally so short? Necessity?

 

[rrolsbe]

When charging my LR RWD at 240V @ 24A (5.76KW) and using the Scan My Tesla App, follows are my experiences.

With the garage and traction battery are cold soaked to around 49F, the charge rate will be 7 to 8 MPH instead of the normal 22 MPH when the battery temp is 54F or higher. The available power is split between 3.5KW being sent to the rear motor stator windings to heat the battery, charging overhead and the remaining for actually charging the battery. The battery will heat about 1 deg F every two minutes so will takes about 10 minutes to reach 54F to charge at the full rate of 22MPH.

If I charged the car daily with the battery at 49F, the power used to add charge to the battery in the first 10 minutes would be about 0.32KWH which would add about 1 mile of range at the 7-8MPH rate; likewise, the power used to warm the battery over the first 10 minutes would be about 0.64KW for battery heating. If the added daily charge was only 10 miles, the first ten minutes would use around 1KWH to add around 1 mile of range adding the remaining 9 miles would use around 2.4KWH and take about an additional 25 minutes. So the total power usage would be about 3.4KWH with 0.64KWH used to warm the battery or about 20%. If the traction battery was colder, that 20% overhead would increase. So as you lower the daily charge miles added and the temps lower the overhead can be significant due to the required battery heating.

Given the same conditions as in the previous paragraph, I lowered the charging current to 5A thinking the battery would not require heating and would charge at a rate of 3-4MPH; however, that was not the case, all the available power 1.2KW was sent to the motor stator windings. Since I have viewed an online video of a charging session that started with the battery at 26F and the battery started to take a charge at around 34F, this does not compute.

 

[camalaio]

When charging my LR RWD at 240V @ 24A (5.76KW) and using the Scan My Tesla App, follows are my experiences.

With the garage and traction battery are cold soaked to around 49F, the charge rate will be 7 to 8 MPH instead of the normal 22 MPH when the battery temp is 54F or higher. The available power is split between 3.5KW being sent to the rear motor stator windings to heat the battery, charging overhead and the remaining for actually charging the battery. The battery will heat about 1 deg F every two minutes so will takes about 10 minutes to reach 54F to charge at the full rate of 22MPH.

If I charged the car daily with the battery at 49F, the power used to add charge to the battery in the first 10 minutes would be about 0.32KWH which would add about 1 mile of range at the 7-8MPH rate; likewise, the power used to warm the battery over the first 10 minutes would be about 0.64KW for battery heating. If the added daily charge was only 10 miles, the first ten minutes would use around 1KWH to add around 1 mile of range adding the remaining 9 miles would use around 2.4KWH and take about an additional 25 minutes. So the total power usage would be about 3.4KWH with 0.64KWH used to warm the battery or about 20%. If the traction battery was colder, that 20% overhead would increase. So as you lower the daily charge miles added and the temps lower the overhead can be significant due to the required battery heating.

Given the same conditions as in the previous paragraph, I lowered the charging current to 5A thinking the battery would not require heating and would charge at a rate of 3-4MPH; however, that was not the case, all the available power 1.2KW was sent to the motor stator windings. Since I have viewed an online video of a charging session that started with the battery at 26F and the battery started to take a charge at around 34F, this does not compute.

Note that it stops heating and starts charging based on the battery inlet temperature, not the battery temperature. Or at least that seems to be the case. It gets the inlet to 21C or so, which can explain charging starting at a colder battery temp (also, potentially different behaviour on different software versions).

I'm now guessing it sends all available power up to a limit to the motors (3.5kW/motor), and any excess is just sent to the battery. Interesting.

 

[Crim]

It'll keep the battery above 10C or so when charging, usually around 12-18C. However, your graph implies it is blending heating and charging which is not what my car does on version 2020.4.x.x. Mine heats fully (using all available power to do so), then begins charging. What version are you on? And is it a LR AWD? (mine is)

What kind of charger do you have and what is the power output?
I have an AWD Performance on 2020.8.1

You may know this already, but you can minimize active heating energy by charging only after driving it for a bit (which will have warmed the battery, not by a lot but some).

That's a good tip, I mostly plan charging at night now because the power is cheaper (but it's also colder). I will check if I can prevent the heating when I charge it directly after driving.

Out of curiosity, why are your charge sessions normally so short? Necessity?

Usually I only drive 20-40 km a day