Understanding the battery heater

[Andyw2100]

Maybe the pack heater still kicks in even in range mode when the battery is too cold.

Adding more information that I had not mentioned before. On Monday I started using TeslaLog. My wife had completed her trip to work by the time I did this, so Monday morning's trip--the one when the pack was coldest and the battery heater was most likely to be engaged--is not logged via TeslaLog. But both Tuesday's and today's trips were logged, and the battery heater was not used, (assuming TeslaLog is correctly displaying the battery heating, which I understand it is.) Tuesday's trip and today's trip were in almost identical conditions, with the cabin temperature set two degrees higher today, and my wife completing the trip in less time (so travelling faster) today. The only significant difference was yesterday's departure was about 90 minutes after charging completed, and today it was 30 minutes after charging completed. Today was more efficient. How would you explain that?

I'm not trying to be difficult. I'm really trying to help us all understand all this better.

If it would help, I can provide the graphs from TeslaLog for yesterday's and today's trips.

Distance

RM

Total

Average

Cabin

Outside

Weather

Duration

Date

(Miles)

Used

Energy (kWh)

Energy (Wh/mi)

Temperature

Temperature

Wind

Conditions

(Minutes)

1/5/16

53.9

62

18.2

338

68

10

5HW

Clear

64

1/6/16

53.9

59

17.1

318

70

10

5HW

Clear

60

 

[FlasherZ]

One thing that's odd here is that I'm seeing battery_heater_on in my logs even with reasonable outside temperatures. For example, the latest record on 3/29/2015, roughly 10 am shows outside temp measured by car was 8.5 degC, which would be about 45 deg F. battery_heater_on was on for about 3 minutes while I was driving, battery_level was 98+ (range charge).

 

[Rocky_H]

Agreed. But we're not talking geothermal heat here.

- - - Updated - - -


Did you? You appear to have some fundamental misunderstandings.

Please examine this link: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html, especially the section on refrigerators and heat pumps.

It's dumbed down sufficiently.

You seem to have missed the principle within your own link you posted. On that page about the Second Law of thermodynamics, look at the section that says “Second law: Refrigerator”. There is link in there that says “heat pumps”. Click that. Now on that page, look at the second box down that says “Air Conditioners and Heat Pumps”. In the description there, it says:

“Air conditioners and heat pumps are heat engines like the refrigerator. They make good use of the high quality and flexibility of electric energy in that they can use one unit of electric energy to transfer more than one unit of energy from a cold area to a hot area. For example, an electric resistance heater using one kilowatt-hour of electric energy can transfer only 1 kWh of energy to heat your house at 100% efficiency. But 1 kWh of energy used in an electric heat pump could "pump" 3 kWh of energy from the cooler outside environment into your house for heating. The ratio of the energy transferred to the electric energy used in the process is called its coefficient of performance (CP). A typical CP for a commercial heat pump is between 3 and 4 units transferred per unit of electric energy supplied.”

So that is how heat pumps can get an effectiveness that appears to be more than 100% efficiency from the perspective of how much heat energy you get in the location you want it. They are using the electrical input to mechanically move heat energy from an outside source. You are thinking of how conversion of energy cannot exceed 100%.

 

[apacheguy]

@Flasher - Depends on the active heating target. This value has oscillated between 7 C - 10 C in the firmwares. It is quite possible that with 8 C ambient the condition to trigger the heater would activate.

 

[FlasherZ]

@Flasher - Depends on the active heating target. This value has oscillated between 7 C - 10 C in the firmwares. It is quite possible that with 8 C ambient the condition to trigger the heater would activate.

Ahh, gotcha. I would figure it would be higher than that, myself. Thanks.

 

[ArtInCT]

I was under the impression that the HVAC system, pumps, fans, radiant heating, seat heaters, steering wheel heaters etc are all on the 12 Volt side of the Model S.
Is that correct?

 

[apacheguy]

Yes, I believe the coolant pumps and fans are 12 V. The condenser and battery heater are HV, however.

 

[green1]

I was under the impression that the HVAC system, pumps, fans, radiant heating, seat heaters, steering wheel heaters etc are all on the 12 Volt side of the Model S.
Is that correct?

I beleive that the A/C and heater are on the HV, but that the rest is 12V (including the fans and such) I think of it as all the 12V bits in an ICE are still 12V, but the parts that are engine mounted are HV (ie the A/C compressor, and the heater core)
(unless someone wants to correct me??)

 

[ArtInCT]

Ok, I do recall some really early posts from the 2013 era that did discuss the battery coolant and heater pump.
This fluid pump is 12 volt.
I would imagine there is a compressor for cooling with 12 volt fans.... you can sometimes hear these running at a SuperCharger or in your garage on hot days.
But the heating element for the coolant... is there a typical 12 volt fuse for that or not?

So we have a compressor and heating element which are High Voltage?

If so can the battery heater element and compressor ever run when the Model S is turned off?
I have read that the contactors are kept in the off position when the S is off. This is done for safety reasons.
And there is a special pre-ON check of all systems prior to the contactors turning on.

 

[Andyw2100]

I have an additional bit of information for people to chew on, for whatever that may be worth.

I was leaving with my wife this morning, on a slightly different trip that I also track, so the numbers would not be directly comparable to the other numbers I posted. Also I was driving instead of her, and that alone probably changes things significantly, not to mention extra weight in the car, etc. So I won't bother posting numbers. But here's what's really interesting:

I timed the charge to end just when we left, and we wound up leaving 11 minutes after the charge completed. The temperature outside was 14F. I'm guessing the temperature inside my unheated garage was in the mid-30s. I had no regen limit!

I had preheated the cabin a bit, with range mode off, but my experience has been that in conditions like what I experienced this morning, the best I could have hoped for from that preheating would have been a reduction in the regen limit, not an elimination of it. I believe the reason I did not have a regen limit was because the pack heated enough during charging to avoid it.

 

[green1]

I timed the charge to end just when we left, and we wound up leaving 11 minutes after the charge completed. The temperature outside was 14F. I'm guessing the temperature inside my unheated garage was in the mid-30s. I had no regen limit!

How long did you charge before leaving, and at what charge rate? how long did you pre-heat for? and how long had the vehicle been cold-soaked for?

 

[ArtInCT]

Andy:
The information you related above is pretty much in line with the information that Cottonwood has stated in some related cold weather threads. He likens the pre heating of the cabin as an energy storage and savings technique, so that you do not have to use power to get the internals of the S warm. He also stated that the charge just prior to the departure was the best and most energy efficient way to warm the high voltage battery pack and minimize the amount of time in limited regen. As you may know Cottonwood lives in the mountains and would like to hyper-mile with every decline he encounters. (I on the other hand am in fairly flat ground).

It seems to me that cabin pre-heating is a 12 volt exercise and only gets the traction battery warm via conduction or its role in recharging the 12 volt battery.

I am wondering if the Ludicrous Max Battery function is supported when on shore power and simultaneously usable when the traction battery pack is being charged by shore power? This would be a great question for the Tesla California techs who will be training the VA technicians in the P85D -> Ludicrous upgrade.

In any case one has to wonder how green this "preheating" is....? Burning grid kWh's to get quick regen? My grid kWh's are not inexpensive at all.... about 25 cents each delivered and taxed.

 

[RiverBrick]

...


Again, once the battery has good enough temperature, the normal losses in the drive train seem to be enough to keep the battery at reasonable temperature.

I had to monitor energy use closely last Winter due to no Superchargers in the province, record-breaking cold, and frequent visits to off-grid parks. There appears to be a sharp uptick in consumption for trips of any decent length once the outside temperature drops below approximately 5F, even if you start with a battery temp of 60F or higher. I've always suspected it was due to the battery warmer kicking in and am somewhat skeptical that recuperation of losses from a relatively efficient drive-train are enough to keep the battery warm in severe cold.

 

[AWDtsla]

It seems to me that cabin pre-heating is a 12 volt exercise and only gets the traction battery warm via conduction or its role in recharging the 12 volt battery.

It heats at ~6kw from HV. It is not from the 12V

I am wondering if the Ludicrous Max Battery function is supported when on shore power and simultaneously usable when the traction battery pack is being charged by shore power? This would be a great question for the Tesla California techs who will be training the VA technicians in the P85D -> Ludicrous upgrade.

You can if you have dual chargers. With a single charger you will go from charge to draw from the pack for several minutes then settle on a non-relevant charge rate (plus or minus depending on external temperature.)

In any case one has to wonder how green this "preheating" is....? Burning grid kWh's to get quick regen? My grid kWh's are not inexpensive at all.... about 25 cents each delivered and taxed.

There's a break-even point which I have yet to calculate, where the return from additional regen is greater than the cost of shore power. It's probably not much.

 

[Andyw2100]

How long did you charge before leaving, and at what charge rate? how long did you pre-heat for? and how long had the vehicle been cold-soaked for?

I charged 3:28 at 56 amps. The car had sat for just under seven hours before the charge started. I preheated for less than 30 minutes. I don't know exactly how long, as the car had reached the set temperature of 70 before we left, and had stopped preheating. I would guess it probably preheated about 20 minutes, give or take.

Andy:
The information you related above is pretty much in line with the information that Cottonwood has stated in some related cold weather threads.

It's actually not, Art.

I'm pretty familiar with what both Cottonwood and Jerry33 have written about this. Unless I am mistaken, both agree with what I've observed myself previously and that is that the preheating can reduce but not eliminate the regen braking limit. Also I don't believe either Jerry or Cottonwood typically experience temperatures as cold as the ones I was talking about seeing this morning. If memory serves, Cottonwood has a heated garage, and Jerry lives where it just doesn't get that cold.

 

[RiverBrick]

...

In any case one has to wonder how green this "preheating" is....? Burning grid kWh's to get quick regen? My grid kWh's are not inexpensive at all.... about 25 cents each delivered and taxed.

For last Winter when it was literally -4F or lower at some point everyday during daylight, it was more than just a quickregen issue, but let's leave that aside.

The green question is very valid, but of course depends on each owner's situation. I'm lucky enough to pay 4X less than you for kWh's that are over 99% from renewables. So it's a bit of a perfect storm, living on a slope in a hilly city where it gets extremely cold with no Superchargers available for most trips, yet electricity is clean and cheap. So, yes, max battery pre-warming would be a very desirable feature for me. However, I understand that overall it's not a priority for now. There will come a point when there will be enough customers in my situation to warrant TM addressing this, though.

 

[Andyw2100]

I'm pretty familiar with what both Cottonwood and Jerry33 have written about this. Unless I am mistaken, both agree with what I've observed myself previously and that is that the preheating can reduce but not eliminate the regen braking limit. Also I don't believe either Jerry or Cottonwood typically experience temperatures as cold as the ones I was talking about seeing this morning. If memory serves, Cottonwood has a heated garage, and Jerry lives where it just doesn't get that cold.

I was partially right and partially wrong:

@Andyw2100, I agree with your points. I was merely trying to remind folks of a work-around for now; a direct function would be much easier and simpler. Also, I have not had regen limits after my method, but then again, I pre-condition in a 45˚ F. garage.

So Cottonwood has seen a complete lack of regen limits, though that was before Version 7 firmware, and Version 7 of the firmware seems more conservative with respect to the regen limits (they kick in sooner, at higher temperatures than they did previously.) Also it's not clear whether or not Cottonwood typically experienced temperatures in the low teens or not.

In any case, I really think this is a pretty interesting find for version 7, and very cold conditions.

 

[David99]

I had to monitor energy use closely last Winter due to no Superchargers in the province, record-breaking cold, and frequent visits to off-grid parks. There appears to be a sharp uptick in consumption for trips of any decent length once the outside temperature drops below approximately 5F, even if you start with a battery temp of 60F or higher. I've always suspected it was due to the battery warmer kicking in and am somewhat skeptical that recuperation of losses from a relatively efficient drive-train are enough to keep the battery warm in severe cold.

Makes sense. There is only so much heat produced by the drive train. If you are driving at 25 kW you might see 2.5 kW lost in heat which of course can't be all captured. When it gets extremely cold, additional heating will definitely be needed.

- - - Updated - - -

There's a break-even point which I have yet to calculate, where the return from additional regen is greater than the cost of shore power. It's probably not much.

It does take quite some energy to heat up the massive battery and unless you go downhill quite a bit, you aren't going to see much energy come back from regen. So I think just from an energy point of view, it doesn't make sense to heat up the battery to where you get full regen vs lets say just half. Again, I believe Tesla did think about all of this when programming the battery temperature management and made it to energy efficient. I mean after all, they spent a huge amount of effort in making every aspect of the car very efficient. It only makes sense to follow the same idea when it comes to battery temperature.

 

[green1]

I charged 3:28 at 56 amps. The car had sat for just under seven hours before the charge started. I preheated for less than 30 minutes. I don't know exactly how long, as the car had reached the set temperature of 70 before we left, and had stopped preheating. I would guess it probably preheated about 20 minutes, give or take.

Thanks, I have not had the same success in similar situations, however I can only charge at a max of 32A (NEMA 14-50 with the crippled Canadian UMC adapter) so perhaps the higher current is what did it for you?

 

[FlasherZ]

It seems to me that cabin pre-heating is a 12 volt exercise and only gets the traction battery warm via conduction or its role in recharging the 12 volt battery.

It isn't. 6 kW @ 12V is 500A. That heater would have some monster cables attaching it at that current level. Conversely, 6 kW at HV is roughly 15-17A, which can be done on a much, much thinner wire.