Hidden Energy Consumption

[WattsappMTL]

I've had my 2021 MY since November and have been following the recommended procedure of pre-conditioning prior to departure. Warming the battery helps to increase the amount of regen braking available, but is this really essential? If acceleration is adequate and I can adjust my driving style so that the car will regen all the way to a stop without touching the brakes, does the pre-conditioning really help save energy while driving? It certainly consumes energy up front, and I don't have a way to determine how much.

Does the vehicle keep track of energy consumed during pre-conditioning or cabin warming cycle when the wall connector is plugged in? it seems that all of the onboard energy usage data is captured only when the car is moving. My Tesla wall connector doesn't keep track of how much power actually passes through it, so I don't have the full picture of how much energy the car is really consuming without adding a dedicated meter to track this. Am I missing something?

 

[Rocky_H]

and have been following the recommended procedure of pre-conditioning prior to departure.

Meh--"recommended" by who? That's just if you don't like the car being a little cold inside for the first few minutes you're driving while it's running the heat to warm up the cabin. Some people are very partial to having the car be really warm and toasty the second they get in.

Warming the battery helps to increase the amount of regen braking available, but is this really essential?

And that--some people are really indignant at seeing the regen being limited because of the battery being kind of cold. But really, to try to get that regen limitation to go away takes wasting huge amounts of energy preheating, so it's definitely not worth it.

If acceleration is adequate and I can adjust my driving style so that the car will regen all the way to a stop without touching the brakes, does the pre-conditioning really help save energy while driving? It certainly consumes energy up front, and I don't have a way to determine how much.

No, definitely not. It wastes a ton more energy. It's winter. The battery is going to be colder, and regen just is going to be less. It's better, I think, to just accept that and be aware of that that's the way it is.

Does the vehicle keep track of energy consumed during pre-conditioning or cabin warming cycle when the wall connector is plugged in?

Well, no, not very specifically in the displays. It's just a gradual loss of the "rated miles" from the display while the car is sitting. But that's a really general number from all energy usage while the car is sitting: idle computer use, preheating, etc.

it seems that all of the onboard energy usage data is captured only when the car is moving.

If you're looking at that information card of like "Since last charged" or "Since 6:37 PM" when you started driving, those do only count up the energy usage while the car is in Drive, not the preheating before that.

Am I missing something?

I would say you are not missing something. You are rather picking up on something, which is that preheating is mostly just wasting a lot of energy, and people do it for their own feeling or preference, rather than for efficiency.

There is a circumstance where it does have a practical use though. If it is really cold outside, and you are about to leave on a long drive, where you are trying to extend your range, it can make a lot of sense to warm up both the cabin and battery from the wall first, before you disconnect, rather than the car sucking down huge energy draw from the battery for the first 20-30 minutes of your driving, which will be cutting into your range.

 

[WattsappMTL]

@Rocky_H, thanks for confirming every one of my suspicions. The recommendation for pre-conditioning is part of Tesla's Winter Driving Tips and is also parroted frequently on relevant forums. I've driven with 25 regen dots and it wasn't that terrible. I'm also realizing that pre-heating the cabin artificially improves the displayed efficiency in winter because the only energy use while driving is to maintain the set temperature, benefiting from the interior thermal mass that is already warmed up. I display the energy bar in % because that's more meaningful to me than rated range, particularly in winter. I can always call up the Energy display for a more accurate range estimate but I seldom care about range capability. BTW the car has been parked (disconnected) for the past 2 days and I lost ~4%, which is a bit surprising. If I had left it plugged in, the SOC would have remained constant and I would have been unaware of the energy usage for battery top-up. As for pre-conditioning/warming on "shore" power, I fully agree it's appropriate in preparation for a long trip.

Although I would like to minimize energy consumption, I will continue to preheat the cabin before I board most of the time. It's one of those nice comforts that the EV offers and it's very convenient to do from a distance at the end of a physically tiring day.

 

[WattsappMTL]

Having experienced a bit colder weather and consequent poor heat pump performance, I have discovered that there's another legitimate reason for pre-conditioning prior to departure. Below -10C (14F), the HVAC heat pump struggles to extract heat from outside air, and by design it starts to draw heat from the main battery. However if the battery is cold, it doesn't have much to give and the cabin won't warm up very well while driving. Pre-conditioning in these conditions stores more heat in the battery which allows the heat pump to work more effectively on the road. This also explains why the number of regen dots stays constant or even increases while driving in cold temperatures - the battery is being chilled by the heat pump when it would normally warm up due to current draw. And yes, I know seat heaters can help to fill the gap but I prefer to feel comfortable on the front as well as the back!

 

[user212_nr]

Having experienced a bit colder weather and consequent poor heat pump performance, I have discovered that there's another legitimate reason for pre-conditioning prior to departure. Below -10C (14F), the HVAC heat pump struggles to extract heat from outside air, and by design it starts to draw heat from the main battery. However if the battery is cold, it doesn't have much to give and the cabin won't warm up very well while driving. Pre-conditioning in these conditions stores more heat in the battery which allows the heat pump to work more effectively on the road. This also explains why the number of regen dots stays constant or even increases while driving in cold temperatures - the battery is being chilled by the heat pump when it would normally warm up due to current draw. And yes, I know seat heaters can help to fill the gap but I prefer to feel comfortable on the front as well as the back!

This is the same as to say that pre-conditioning warms the battery and provides for more efficient operation.

by design it starts to draw heat from the main battery.

It takes heat from the main battery only if the battery has heat to provide.

the battery is being chilled by the heat pump when it would normally warm up due to current draw.

I think that the "system" is overall cooler and less efficient, but that the battery is still being heated.

Below -10C (14F), the HVAC heat pump struggles to extract heat from outside air

The heat pump struggles the colder it is, but that does not mean that it is unable to heat, just that it uses more energy.

The length of time that it will take to warm the battery can exceed the time of your drive, so you are operating at a lower efficiency.

 

[WattsappMTL]

This is the same as to say that pre-conditioning warms the battery and provides for more efficient operation.



It takes heat from the main battery only if the battery has heat to provide.




I think that the "system" is overall cooler and less efficient, but that the battery is still being heated.



The heat pump struggles the colder it is, but that does not mean that it is unable to heat, just that it uses more energy.

The length of time that it will take to warm the battery can exceed the time of your drive, so you are operating at a lower efficiency.

From earlier discussion in this post, I had concluded that "more efficient operation" with a preconditioned battery comes about from improved regen braking power and therefore less energy loss to manual braking. However, if it's still possible to drive the car without resorting to manual braking, is there really an improvement in operational efficiency from preconditioning? There's definitely an energy cost associated with the preconditioning cycle.

What I'm also trying to understand is how I can leave my above-freezing garage with a pre-conditioned battery and then have the number of regen dots continue to increase as I drive. I interpret this as an indication that the battery is cooling off - is this not the case? So, if it starts off with a certain level of thermal energy stored in the battery& coolant mass, where is that heat going? Is it actually radiating heat to the cold ambient exterior world, or is the heat being used elsewhere by the car?

 

[jcanoe]

From earlier discussion in this post, I had concluded that "more efficient operation" with a preconditioned battery comes about from improved regen braking power and therefore less energy loss to manual braking. However, if it's still possible to drive the car without resorting to manual braking, is there really an improvement in operational efficiency from preconditioning? There's definitely an energy cost associated with the preconditioning cycle.

What I'm also trying to understand is how I can leave my above-freezing garage with a pre-conditioned battery and then have the number of regen dots continue to increase as I drive. I interpret this as an indication that the battery is cooling off - is this not the case? So, if it starts off with a certain level of thermal energy stored in the battery& coolant mass, where is that heat going? Is it actually radiating heat to the cold ambient exterior world, or is the heat being used elsewhere by the car?

When you precondition the coolant entering battery pack is being warmed by the motor stators. The heat pump will pull heat from the battery pack to warm the cabin. As you start driving the heat pump continues to pull heat from the battery pack, that is why the maximum available regenerative braking may decrease while driving in cold weather.

 

[WattsappMTL]

When you precondition the coolant entering battery pack is being warmed by the motor stators. The heat pump will pull heat from the battery pack to warm the cabin. As you start driving the heat pump continues to pull heat from the battery pack, that is why the maximum available regenerative braking may decrease while driving in cold weather.

@jcanoe thanks for confirming. That is what I suspected, and I have found that once the battery really gets cooled off it does become impossible to stop without manual braking so there's definitely an efficiency loss in that situation. Once the pre-conditioned heat is used up, I guess I can always force the battery to condition while driving by selecting (then ignoring) navigation to a Supercharger destination. I do wish that Tesla would provide a more technically-detailed description of how the vehicle systems work, for those of us who want to understand how their operating recommendations actually play out in real life.

 

[jcanoe]

For around town driving regenerative braking makes sense and enables one-pedal driving. For a road trip the energy recovery from regenerative braking is minimal as coasting is more efficient than going through the process of converting mechanical energy into electrical energy and then storing the electrical energy.

You would use more energy to keep the battery warm just to have more available regenerative braking than if you just let the battery cool back down from the preconditioning temperature as you drive. If the battery becomes too cold while driving, heating the cabin the Tesla battery management system can always warm the battery as needed via stator heating. On a long road trip the battery will be warmed each time you navigate to a Supercarger station, while charging.

 

[WattsappMTL]

For around town driving regenerative braking makes sense and enables one-pedal driving. For a road trip the energy recovery from regenerative braking is minimal as coasting is more efficient than going through the process of converting mechanical energy into electrical energy and then storing the electrical energy.

You would use more energy to keep the battery warm just to have more available regenerative braking than if you just let the battery cool back down from the preconditioning temperature as you drive. If the battery becomes too cold while driving, heating the cabin the Tesla battery management system can always warm the battery as needed via stator heating. On a long road trip the battery will be warmed each time you navigate to a Supercarger station, while charging.

Thanks for the assessment, makes sense to me. My issues with battery cooling have been on short (25 km) drives with a mix of highway and city speeds but I suspect that longer highway drives with more drag would generate enough battery heating to offset this. I started this thread with the suspicion that preconditioning may not always be as beneficial as Tesla suggests, and might actually waste more energy than it saves in some situations. That's why it would be good to have a deeper understanding of all the factors at play, in order to make informed decisions on how to tactically manage total energy consumption.

 

[jcanoe]

Thanks for the assessment, makes sense to me. My issues with battery cooling have been on short (25 km) drives with a mix of highway and city speeds but I suspect that longer highway drives with more drag would generate enough battery heating to offset this. I started this thread with the suspicion that preconditioning may not always be as beneficial as Tesla suggests, and might actually waste more energy than it saves in some situations. That's why it would be good to have a deeper understanding of all the factors at play, in order to make informed decisions on how to tactically manage total energy consumption.

I've tried not preconditioning in colder weather, just getting into the Model Y and driving. While this saves some energy it is not as comfortable, in the short term, as getting into a warm cabin. When regenerative braking is unavailable it requires me to make an immediate adjustment to my driving. I don't mind when regenerative braking is slightly reduces (4 or 5 dots showing) as I barely notice this reduction while driving.

I precondition every chance I get. In the morning I typically will precondition for 20 to 30 minutes even if not plugged in. I will also leave the HVAC running when picking up coffee, etc. I estimate that this winter half of my total kWh usage is due to preconditioning and running the HVAC while parked. Still, my total cost for the electricity I use is no more than buying gas at current prices. During spring, summer and fall my fuel costs are about half of what gas would cost, or less.

 

[mswlogo]

I've had my 2021 MY since November and have been following the recommended procedure of pre-conditioning prior to departure. Warming the battery helps to increase the amount of regen braking available, but is this really essential? If acceleration is adequate and I can adjust my driving style so that the car will regen all the way to a stop without touching the brakes, does the pre-conditioning really help save energy while driving? It certainly consumes energy up front, and I don't have a way to determine how much.

Does the vehicle keep track of energy consumed during pre-conditioning or cabin warming cycle when the wall connector is plugged in? it seems that all of the onboard energy usage data is captured only when the car is moving. My Tesla wall connector doesn't keep track of how much power actually passes through it, so I don't have the full picture of how much energy the car is really consuming without adding a dedicated meter to track this. Am I missing something?

I was gonna say “Duh”.

But I forget that most people don’t have home energy consumption monitors. So I can see why you would have no idea.

Don’t waste energy on preconditioning. It’s a huge waste.

Consider investing in a home energy consumption monitor like “Sense”. One of my best “gadget” investments. Good chance it will save you money in the long run in teaching you what is wasteful. Including how ridiculously wasteful pre-conditioning battery or cabin is.

I had Solar before EV. And was breaking even. When I bought an EV I was going to be way over my capacity. After installed sense and made several big changes on energy use. My Solar now covers all usage including EV.

Changed Pool to a DC variable speed pump. Change HVAC to DC variable speed compressor. Added smart switches to HomeTheater (I was using 100 watts in stand by). 100 watts 24x7 was 10% of my entire (8 kW array) Solar production !!!

Only precondition if you need the range for the first leg of a trip, need to melt snow off. Or in extreme cold weather stretches like below -10F (or lower) (lots of “snow flake”, locked up battery). A little bit of snow flake on battery should clear itself driving. But if it’s significant and extremely cold, I might pre condition. But that’s only when it’s really cold.

 

[pt19713]

Keep in mind the people that post "don't waste energy by pre-conditioning " are Tesla owners with PTC heaters.

 

[user212_nr]

Thanks for the assessment, makes sense to me. My issues with battery cooling have been on short (25 km) drives with a mix of highway and city speeds but I suspect that longer highway drives with more drag would generate enough battery heating to offset this. I started this thread with the suspicion that preconditioning may not always be as beneficial as Tesla suggests, and might actually waste more energy than it saves in some situations. That's why it would be good to have a deeper understanding of all the factors at play, in order to make informed decisions on how to tactically manage total energy consumption.

I think what you are missing is that the "length of time" that you precondition affects how much energy it uses. You are "preconditioning" when you get in the car before you shift gears and press the gas pedal. If you precondition 20 minutes in advance, that will waste energy, but if 5 minutes in advance, most of that energy would be used anyways when you get in the car and turn on the heat.

Also, its not the regen that warming the battery is after, it is inefficient operation of the battery at cold temperatures.

 

[WattsappMTL]

Also, its not the regen that warming the battery is after, it is inefficient operation of the battery at cold temperatures.

So what exactly does "inefficient operation of the battery" really mean? A battery is an energy storage device that can accept or release electrical energy for the vehicle's use. We learned in high school that energy cannot be created or destroyed. The battery itself, or the car's systems, might limit the current that can be drawn from it and the voltage might be a bit lower when it's cold but I don't see how efficiency comes into the equation. Surely it's the vehicle systems that are more likely to have different efficiencies depending on temperature. The loss of regen braking does reduce the overall operating efficiency of the car because kinetic energy is lost to friction braking instead of being redirected to the battery (however inefficient that process may be). Also the battery will be cooled by the heat pump in cold ambient conditions, again reducing the available regen power if it hasn't been pre-warmed enough. I can't see a 5-minute pre-condition being enough when it's -15C outside.

 

[jcanoe]

When a battery is cold it takes longer to charge the battery, using more energy for the same amount of charge. A cold battery is also less efficient at releasing stored energy, you don't get as much energy out of the battery that you would if the battery was warmer. Peak power output is also reduced when the battery is cold. A battery is a chemical energy storage device. All chemical processes have an optimal temperature and are reduced at low temperatures.

 

[iustin]

So what exactly does "inefficient operation of the battery" really mean? A battery is an energy storage device that can accept or release electrical energy for the vehicle's use. We learned in high school that energy cannot be created or destroyed. The battery itself, or the car's systems, might limit the current that can be drawn from it and the voltage might be a bit lower when it's cold but I don't see how efficiency comes into the equation.

You are correct that in the grand scheme of things, energy is conserved, but the battery and charger are not a Closed system - Wikipedia. In my understanding, the problem with the temperature is that the internal resistance of the battery increases as the temperature decreases, so for a given input power, higher and higher percentages are used to move the electrons around in the battery - which likely is manifested as the battery warming a bit, so the energy is not "destroyed". After all, a LiIon battery is powered by chemistry, and temperature affects chemical reaction speeds.

Very non-technical explanation, sorry.