Busted Charging/Precondition Myths

[camalaio]

Hey folks,

After using Scan My Tesla for a bit, I've made a number of interesting observations that I don't think have been outlined before. These behaviours might be new as well. Reporting as of firmware 2020.4.x.x and in relation to public chargers (via J1772 adapter).

Myth: Being plugged in will only use power from the wall for various functions.
Reality: Even when "charging", the net power on the battery can actually result in losing charge. There is no way via the built-in display to see how much you're losing either.

Myth: Preheating the cabin won't draw energy from the battery when plugged in, thus not putting wear on it.
Reality: Extremely false, unfortunately. This is twofold: the net power drawn can exceed the amount given by the charger, and the power runs through the battery first anyways. This does not "save" your battery or limit climate control power.

Myth: After charging is completed, climate control power will come from the charger.
Reality: This is... partly true? Sometimes it re-engages charger power, sometimes it disconnects, and this can change every minute or so. As above, it's all running through the battery anyways.

Myth: The battery is not heated for Level 2 charging (i.e. not Supercharging, regular public chargers).
Reality: I was surprised to see it does actually heat the battery. It's not by a lot and the temperature varies a bit, but it will be kept above 10C (50F). If you use percentage for displayed capacity, you can't see this. If you use distance, it will report as 0mi/hr or 0km/hr (or some lower than expected number) while the battery is being heated.

Myth: The battery does not heat itself when parked in the cold.
Reality: It does! This is very hard to catch and I have almost no data on it, and that data may be confounded (e.g. I was close to the car with my phone key, car seemed awake). More data needed. Basically, I need to be in a much colder area to test this.

Recommendations/Tips:

1. Use "Distance" for energy display in order to tell what's going on. "Energy" will show something like 7kW even if all of that is going to battery heating, while "Distance" will appropriately adjust the number of distance units being added. However, for Level 3 charging (Supercharger/CHAdeMO) the kW rate is actually correct and represents net power being fed to the battery.
2. Be aware that energy used for climate control is still using/wearing your battery. There is no way around this.
3. All types of charging need to warm the battery. In the case of at-home or public Level 2 charging, it won't charge at all until the temperature threshold is met. Keep this in mind if you're tight on time.
4. Your battery is colder than you think if it sat outside. You can be needing to run the AC because the cabin is too warm from direct sun, while your battery is nearly freezing in the shade of the rest of the car.

Non-Recommendations:

1. You don't need to turn off climate control at an L2 station, if your goal was to allow more power for heating the battery. It will simply draw more power from the battery for this rather than capping power draw to the charge rate.
2. Don't drive around trying to preheat your battery prior to an L2 stop, just to avoid the preheat period when plugged in. This is a larger waste of time and energy unless you're getting something else out of the drive.
3. Regen limit has nothing to do with whether or not your battery will be heated at an L2 station, only temperature.

Bonus observations:

1. The front motor is used for regen below about 14 mph (iff you were accelerating prior), otherwise it's always the rear motor.
2. Drive mostly uses the rear motor. Reverse mostly uses the front motor.

Hopefully this was helpful to someone.

 

[justsomeguy]

Myth: Preheating the cabin won't draw energy from the battery when plugged in, thus not putting wear on it.
Reality: Extremely false, unfortunately. This is twofold: the net power drawn can exceed the amount given by the charger, and the power runs through the battery first anyways. This does not "save" your battery or limit climate control power.

Is this actually possible? IIRC the battery merely provides the excess that the wall power can not provide. How would the wall power going through the battery work anyway? It can't exactly charge and discharge at the same time.

 

[camalaio]

Is this actually possible? IIRC the battery merely provides the excess that the wall power can not provide. How would the wall power going through the battery work anyway? It can't exactly charge and discharge at the same time.

I don't think what I said would apply to DC fast charging. I'll explain my thoughts.

I mean, I could be wrong, I'm no electrical engineer. The car/BMS certainly counts it as energy used, which would be an odd choice if it didn't use the battery.

If I were to guess, for AC charging, it might not be internally boosting it to ~400V or so and applying that across the battery, but perhaps using a lower voltage across a small number of smaller modules. This way the charge current would need to enter the cells first before the series voltage of the pack is used for the high voltage climate control components.

I'm pretty sure that would require isolation though...

Maybe I'm wrong! I'm happy to be wrong. Learn something that way. But then I don't know why the car would be counting that as energy used over its lifetime.

 

[GtiMart]

My experience in cold Quebec City Canada matches what you are saying for the parts that I've been paying attention to.
Regen might not quite be what you're saying. I think regen might use both motors when it's slippery for example, but I don't have time to look at my cell phone when that happens to confirm

Nice information!

 

[qdeathstar]

i thought someone posted a while ago that the battery heater in model three is always running...

 

[w0ss]

The statement that when connected to a charger the current still goes through the battery to be used for cabin conditioning(or any purpose) is not true. As long as the charger can supply more current than is needed then it will go direct to the heater/load. This is nothing special Tesla is doing just basic electric principals. While I am not an EE I do sit next to one at work

 

[Candleflame]

Hey folks,

After using Scan My Tesla for a bit, I've made a number of interesting observations that I don't think have been outlined before. These behaviours might be new as well. Reporting as of firmware 2020.4.x.x and in relation to public chargers (via J1772 adapter).

Myth: Being plugged in will only use power from the wall for various functions.
Reality: Even when "charging", the net power on the battery can actually result in losing charge. There is no way via the built-in display to see how much you're losing either.

Being plugged into the wall will use wallpower to run the AC and heater and battery warmer. Everything else will use the 12V battery which can only be charged from the battery.

Myth: Preheating the cabin won't draw energy from the battery when plugged in, thus not putting wear on it.
Reality: Extremely false, unfortunately. This is twofold: the net power drawn can exceed the amount given by the charger, and the power runs through the battery first anyways. This does not "save" your battery or limit climate control power.

This is not a myth and indeed correct. As long as you get sufficient power from the wall (lets say 3kw for AC and maybe 6-7kw for heating it will not use the battery). Even if the power gets taken from the HV battery, that's not an issue really, as long as the battery gets immediately charged by the wall it doesnt put any wear cycles on the battery.

Myth: After charging is completed, climate control power will come from the charger.
Reality: This is... partly true? Sometimes it re-engages charger power, sometimes it disconnects, and this can change every minute or so. As above, it's all running through the battery anyways.

as above

Myth: The battery is not heated for Level 2 charging (i.e. not Supercharging, regular public chargers).
Reality: I was surprised to see it does actually heat the battery. It's not by a lot and the temperature varies a bit, but it will be kept above 10C (50F). If you use percentage for displayed capacity, you can't see this. If you use distance, it will report as 0mi/hr or 0km/hr (or some lower than expected number) while the battery is being heated.

Myth: The battery does not heat itself when parked in the cold.
Reality: It does! This is very hard to catch and I have almost no data on it, and that data may be confounded (e.g. I was close to the car with my phone key, car seemed awake). More data needed. Basically, I need to be in a much colder area to test this.

Doesn't the manual say that the battery does get heated if it drops below a certain temp? It's pretty low though and the advice is to not leave the car longer than 24h in temperatures below.... whatever it was.


Hopefully this was helpful to someone.

 

[Candleflame]

i thought someone posted a while ago that the battery heater in model three is always running...

not possible, the car uses way too little energy when its sleepy time. I mean for me it sometimes uses less than 200watt/day.

 

[TimothyHW3]

Furthermore there is no "battery heater" in the Model 3. It uses the rear and if available front motors to heat up the battery. This is the noise you hear if you preheat the car or on a Supercharger/ HPC.

This is actually a myth - when the car is idle it will NEVER heat the battery. This is evident by the snowflake symboo and also the fact that the car says waking up when you turn it on.

There is a feature called prevent overheating in cabin, but this one works through the HVAC and it cools the cabin, not battery.
So this one is actually a myth, at least on Model 3.
The only time the car will heat the battery is when the car is locked and you select preheat cabin OR if you navigate to Supercharger with a cold battery.
And of course while you charge and the battery is cold. And it uses the motors to do so.

 

[TimothyHW3]

I also have a video on the part regarding the HVAC consumption from the grid or battery.
When you change to energy you can see the amount used without Scan My Tesla up until the power of the charger attached.
So if you find 11kW Charger you can actually see most of the usage in the car with this method.

While the car is preheating it can jump to about 15-17kW on an AWD or P. 7-8kW for the motors to heat the car and about 7-8kW for the HVAC on max.

 

[nayr14]

This is actually a myth - when the car is idle it will NEVER heat the battery. This is evident by the snowflake symboo and also the fact that the car says waking up when you turn it on.

The car will come out "sleep" and heat the battery once it falls below the active heat temperature (as seen in ScanMyTesla), at slightly below freezing. This is to prevent damage to the batteries. You'll still see the snow flake and the blue unusable section of the battery when it's near that temperature, but at that set level it will kick on the motor(s) to maintain a safe temperature.

 

[TimothyHW3]

The car will come out "sleep" and heat the battery once it falls below the active heat temperature (as seen in ScanMyTesla), at slightly below freezing. This is to prevent damage to the batteries. You'll still see the snow flake and the blue unusable section of the battery when it's near that temperature, but at that set level it will kick on the motor(s) to maintain a safe temperature.

I have never seen that and the car has been hovering around 0° at some point. Have you actually tested this - you said you never did. I remember Bjorn Nyland doing some tests with the battery at -20 and the car didn't take any charge, an indication it was very cold, so it seems that theory is not correct.

 

[derotam]

Myth: The battery does not heat itself when parked in the cold.
Reality: It does! This is very hard to catch and I have almost no data on it, and that data may be confounded (e.g. I was close to the car with my phone key, car seemed awake). More data needed. Basically, I need to be in a much colder area to test this.

This is actually a myth - when the car is idle it will NEVER heat the battery. This is evident by the snowflake symboo and also the fact that the car says waking up when you turn it on.

There is a message in the CANbus for target temperatures on the battery and powertrain. One of them is TargetBattActiveHeat. When Idle, this is set at -7C, so presumably if the BATTERY temperature goes below -7C, the car will start to actively heat the battery(via motor waste heat/"preconditioning") to keep it no lower than -7C. This was a warm winter for me so I was never able to get the battery down to below -7C to see what would happen.

When charging at Level 2 in cold temps, this value is from what I remember, set to +10C. When preconditioning in route to a supercharger, it gets set to somewhere around +30C.

 

[KG M3]

Hey folks,

After using Scan My Tesla for a bit, I've made a number of interesting observations that I don't think have been outlined before. These behaviours might be new as well. Reporting as of firmware 2020.4.x.x and in relation to public chargers (via J1772 adapter).

Myth: Preheating the cabin won't draw energy from the battery when plugged in, thus not putting wear on it.
Reality: Extremely false, unfortunately. This is twofold: the net power drawn can exceed the amount given by the charger, and the power runs through the battery first anyways. This does not "save" your battery or limit climate control power.

Hopefully this was helpful to someone.

While there was some good information in the original post, this part has a key technical falsehood. The vehicle is set up with a high voltage DC buss that interconnects the inverters for powering the car, the auxiliary loads like cabin heating and air conditioning, and either the output of the onboard charger or the output of the Supercharger/Public DC charger.

Power from whichever charger is being used is input to the high voltage DC buss. If the power demand for heating and other auxiliary loads is less than the amount input from the charger, the "excess" power then charges the battery pack. The power used for heating and auxiliary loads does NOT go "through" the battery. If the power demand for heating and other auxiliary loads is more than the amount input from the charger, then the battery is discharged by the amount of the difference and that power is supplied to the heating and accessories.

So preheating while plugged in will reduce the amount of energy that is stored in and then drawn from the battery pack. The statement "the power runs through the battery first anyways. This does not "save" your battery" is incorrect.

 

[TimothyHW3]

There is a message in the CANbus for target temperatures on the battery and powertrain. One of them is TargetBattActiveHeat. When Idle, this is set at -7C, so presumably if the BATTERY temperature goes below -7C, the car will start to actively heat the battery(via motor waste heat/"preconditioning") to keep it no lower than -7C. This was a warm winter for me so I was never able to get the battery down to below -7C to see what would happen.

If it is set to -7C ok maybe. But I am yet to see a value of -7C of the battery. The lowest I have seen it was at -20C outside temp in a Bjorn Nyland Video and it was at 0C +1C. I doubt anyone will ever experience this...

 

[derotam]

If it is set to -7C ok maybe. But I am yet to see a value of -7C of the battery. The lowest I have seen it was at -20C outside temp in a Bjorn Nyland Video and it was at 0C +1C. I doubt anyone will ever experience this...

-7C is 19.4F. That is far from being excessively low for anyone to get the battery there. Bjorn never actually looked at this, and he has definitely had the required temperatures to look at this but he as of yet has not documented this.

If this was last year, I would have had a close to 0F night where I am positive the battery would have gotten close to that overnight.

 

[TimothyHW3]

-7C is 19.4F. That is far from being excessively low for anyone to get the battery there.

That is the battery temp, not the outside temp. There is still some housing around the battery and the coolant. From my experience the battery is at least 5C-10C warmer than the outside temp and I guess below 0C the difference is even higher.
Whatever, if it is -7C, 95% of the drivers will never experience it. Maybe you can adjust the text to reflect that and type in that it only does that below -7C battery (not outside temp)

 

[camalaio]

There is a message in the CANbus for target temperatures on the battery and powertrain. One of them is TargetBattActiveHeat. When Idle, this is set at -7C, so presumably if the BATTERY temperature goes below -7C, the car will start to actively heat the battery(via motor waste heat/"preconditioning") to keep it no lower than -7C. This was a warm winter for me so I was never able to get the battery down to below -7C to see what would happen.

When charging at Level 2 in cold temps, this value is from what I remember, set to +10C. When preconditioning in route to a supercharger, it gets set to somewhere around +30C.

That value is actually in reference/reaction to the "battery inlet" temperature rather than the actual average battery temp, which I found interesting.

And the value for it en route to a Supercharger varies by how close you are and what your SoC is too! It doesn't warm nearly at all if you have a low SoC, but absolutely bakes it at higher SoC.

While there was some good information in the original post, this part has a key technical falsehood. The vehicle is set up with a high voltage DC buss that interconnects the inverters for powering the car, the auxiliary loads like cabin heating and air conditioning, and either the output of the onboard charger or the output of the Supercharger/Public DC charger.

Power from whichever charger is being used is input to the high voltage DC buss. If the power demand for heating and other auxiliary loads is less than the amount input from the charger, the "excess" power then charges the battery pack. The power used for heating and auxiliary loads does NOT go "through" the battery. If the power demand for heating and other auxiliary loads is more than the amount input from the charger, then the battery is discharged by the amount of the difference and that power is supplied to the heating and accessories.

So preheating while plugged in will reduce the amount of energy that is stored in and then drawn from the battery pack. The statement "the power runs through the battery first anyways. This does not "save" your battery" is incorrect.

Thank you. Like I said, I could be wrong and I'm willing to learn.

What's odd is everything behaves like what I said is true. It's counted by the BMS for lifetime energy usage, and it even impacts charging speed. Say you were pulling 70kW at a Supercharger with battery heating active, but then it turns off. It'll then read something like 78kW. The reading when Supercharging is the net battery power, so I find this weird. Thoughts?

If it is set to -7C ok maybe. But I am yet to see a value of -7C of the battery. The lowest I have seen it was at -20C outside temp in a Bjorn Nyland Video and it was at 0C +1C. I doubt anyone will ever experience this...

Again, most of us keep them plugged in. When plugged in you probably won't see it go below 9°C based on what I've experienced. 1000lbs of battery is a lot of thermal mass as well, it would take a while to drop that temp just by ambient air.

That's also the coolant inlet target temp, not the battery temp btw. Battery will usually be a bit warmer than that, especially if it is being actively heated (it heats up to a higher temperature and then waits until the coolant needs heating again).

Once people start leaving EVs outside without charging more (i.e. as adoption increases), this will be experienced by many more people than it is today.

 

[KG M3]

What's odd is everything behaves like what I said is true. It's counted by the BMS for lifetime energy usage, and it even impacts charging speed. Say you were pulling 70kW at a Supercharger with battery heating active, but then it turns off. It'll then read something like 78kW. The reading when Supercharging is the net battery power, so I find this weird. Thoughts?

I don't have BMS data on lifetime energy usage, so I cannot comment there. However, the number shown on the screen when at a supercharger is the net amount going into the battery. So if the supercharger is supplying 78kW total and 8kW is going to heating, you would see a net of 70kW charging. Once the heating is complete and it turns off, the full 78kW can go into the battery pack. So you would see 78kW. The car is telling you what is most critical - namely how much energy the SC is supplying that goes to recharge your battery. When I am sitting at an SC, I am less interested in how much energy is going through the cable and more interested in how much energy is going into the battery pack to add range.

 

[derotam]

That value is actually in reference/reaction to the "battery inlet" temperature rather than the actual average battery temp, which I found interesting.

That is fine, but it doesn't change my point that it is not unreasonable to expect many people to be in an area that could actually see -7C whether it is battery temp or coolant temperature....now clearly the number of people with CANbus access AND those cold temperatures is limited, or they just aren't specifically looking at this specific metric.