Understanding the battery heater

[David99]

I have seen many discussions about the battery heater and most people seem to want it to heat the battery more and faster. Many want this to be a separate setting in the app so they can heat the battery manually (for various reasons).

So why doesn't it warm the battery more aggressively? Why is there still a regen limit after preconditioning?

Because it is a waste of energy and doesn't help the battery. The battery works just fine at cold temperatures. Only at temps below freezing will it start to show limit at driving and charging. So as long as the battery is just above those temps, there is very little benefit in keeping it warmer but a big hit in energy used. You don't get much extra range out of the battery by heating it up from 10 Celsius (50 F) to 25 Celsius (77 F). But it would require about 8.5 kWh to heat the battery from 10 to 25 degree Celsius. That's almost one full hour of full power charging with a single charger. Since pre-heating only goes on for 20-30 min and a good amount of energy is needed to heat the cabin, you get an idea how much is left to heat the battery. The battery heater itself has a max power of 6 kW. So even if it would run at full power for one hour, it would not be able to heat the battery to a level where it has zero regen limit.

So Tesla dropped the ball on their battery heating? No! It's not a design flaw, it's just a matter of the battery not needing to be cozy warm to work. You can drive just fine and the regen limit is only there for the beginning of the drive. Additional heating is free from the losses of the motor and inverter and inside the battery itself.

Here is an example: Let's say the battery is at freezing (0 C / 32 F) in the morning. You want to heat it up to be perfectly warm to have no regen limitation (25 C / 77 F). It would require 14 kWh. That's the equivalent of 47 miles of energy. That's more than most people's daily commute! It would be wasted every morning, every day! So Tesla decided to only heat the battery to a point where it's fine to drive with a reasonable limit on regen and save a lot of energy. Using the unavoidable losses in the drive train to further heat the battery as you drive is a much smarter and more efficient way to use energy. Using these losses is free energy to heat the battery.

Also keep in mind, the cooler the battery, the better for the longevity. As Elon once said, he expects the battery to last 'forever' in Alaska.

 

[FlasherZ]

I agree with you on those points, but I would add one:

Charging - especially supercharging - wants the pack to be at a temperature that would not cause premature battery failure. Lithium-ion wants the cells above freezing to charge without causing problems. You'll note that until the battery pack reaches sufficient temperature, the supercharger will refuse to charge at full current. On a trip, you can't expect to leave your hotel on a cold morning, drive half a block to the supercharger, and expect 120 kW of current to rush into the battery.

(That's also the reason that all my cordless power tools are still NiCd based 18V DeWalt. The chargers will refuse to charge their new LiIon cells at < 40 degrees or so.)

 

[arg]

Because it is a waste of energy and doesn't help the battery.

While you are quite right that in general it's not worth it, there are circumstances where it would be legitimately useful to further pre-heat the battery. Tesla are probably justified in not providing the feature, but only on the grounds of not over-complicating the controls for a little-needed function.

The case of need is when setting out on a trip to the limits of range (perhaps needing to rely on slow public charging mid-journey), where you want to depart with maximum energy on board even if that isn't very efficient in terms of overall energy used.

There's some benefit to range in having full regen available near the start, and the other instance is where temperatures are so cold that the battery heater is likely to be used on the journey and so the hotter you can get the battery at the start, the less the heater will be needed later.

Certainly a rare requirement, but not entirely useless.

 

[David99]

The case of need is when setting out on a trip to the limits of range (perhaps needing to rely on slow public charging mid-journey), where you want to depart with maximum energy on board even if that isn't very efficient in terms of overall energy used.

There's some benefit to range in having full regen available near the start, and the other instance is where temperatures are so cold that the battery heater is likely to be used on the journey and so the hotter you can get the battery at the start, the less the heater will be needed later.

If you are on a road trip and need maximum range, you would charge to 100% and then you get almost zero regen. So a warm battery would not give you any regen. The added range due to a warmer temperature battery (meaning warmer than what Tesla thinks is enough) is very small.

On trips in very cold temperatures, based on my experience, the heat produced by the motor/inverter/battery is enough to keep the battery at good enough temperature without having to worry about loss of range due to a cold battery. I actually just did a 2300 mile road trip in cold weather. Temperatures were at single digits on 200 mile legs (more than 3 hours of driving without charging) and I was able to Supercharge at full power without any delays at the end. So unless we are talking about Arctic conditions, the car seems to be perfectly capable keeping the battery warm enough just from 'exhaust' heat from the drive train.

I guess my point is that Tesla's temperature management is just fine. We tend to think the battery temperature should be higher, but when it comes to battery performance, range and being energy efficient, the car does a great job.

I think people with ludicrous mode have the 'max battery power' setting which basically heats up the battery to be toasty which gives it a few more amps. That setting could probably be used in the winter to heat the battery up much more. But again, it would be very wasteful and have little effect on range.

 

[sorka]

I left the mountains last week after skiing. It was 25F. I left with 41% and got home with 16%. Regen was 100% disabled for the entire 6000 foot decent. I would have normally netted 11.3 KW of regen on the same decent trip from that location. Just the day before, I used the battery heater before leaving for a short drive under the same temperature conditions and it only took 2% to get the battery warm enough for full regen.

I realize that's a bit of a corner case.

The other scenario is in the morning using shore power to heat the battery so I have full regen (90% charge) when taking off. The benefit is that I use less brake pad and have more power. I don't use to the point where it says "ready" for max battery power, but it's enough that I'll make 430KW rather than 395KW when taking off in the morning. In this case, 10 minutes of battery heater prior to leaving while on shore power does it.

 

[Soolim]

Battery preheating while on shore power will be a good thing to have. I think currently Tesla only have cabin preheating on shore power implemented.

 

[FlasherZ]

Battery preheating while on shore power will be a good thing to have. I think currently Tesla only have cabin preheating on shore power implemented.

I looked in my logs; I cannot find a case where battery_heater_on is true and shore power is connected. That said, there is a coolant loop that is used for the heat pump between the battery, drivetrain, and cabin systems.

When I preheat the cabin, I notice that the regen limit seems to be higher - instead of regen completely disabled, it might be at 15-30 kW or so.

 

[David99]

I looked in my logs; I cannot find a case where battery_heater_on is true and shore power is connected. That said, there is a coolant loop that is used for the heat pump between the battery, drivetrain, and cabin systems.

We know there is a battery heater in the Model S for sure. I believe it only comes on when the battery temperature drops below a level where charging would be harmful and you try to charge it. I don't think it comes on just being parked and plugged in. It probably takes some time before the battery cells actually get that cold. Between coming home in the evening and charging at night, there might not be enough time for the battery to actually cool down that much. Maybe that's why you haven't seen it on your car yet? Just guessing. I mean, we do know there is a battery heating unit.

 

[FlasherZ]

We know there is a battery heater in the Model S for sure. I believe it only comes on when the battery temperature drops below a level where charging would be harmful and you try to charge it. I don't think it comes on just being parked and plugged in. It probably takes some time before the battery cells actually get that cold. Between coming home in the evening and charging at night, there might not be enough time for the battery to actually cool down that much. Maybe that's why you haven't seen it on your car yet? Just guessing. I mean, we do know there is a battery heating unit.

There is no case in my logs where the battery heater is reported as being on while any power is connected to the car.

The only cases seen are where the car is being driven. I have 172 records in my database where battery_heater_on is 'true'. All are while the car is in motion (battery_current is generally negative).

 

[AWDtsla]

There is no case in my logs where the battery heater is reported as being on while any power is connected to the car.

The only cases seen are where the car is being driven. I have 172 records in my database where battery_heater_on is 'true'. All are while the car is in motion (battery_current is generally negative).

Do you leave range mode on? And how cold is it when you're plugged in?

 

[FlasherZ]

Do you leave range mode on? And how cold is it when you're plugged in?

Range mode is off, always.

I have an unheated garage. While rare, in any given year you're going to find at least a couple of days where the temperature in there is well (over 10 degF) below freezing. If I don't use cabin pre-heating, there are times I will unplug and be faced with regen disabled due to temp.

 

[sorka]

Battery preheating while on shore power will be a good thing to have. I think currently Tesla only have cabin preheating on shore power implemented.

When I enable max battery it uses shore power when plugged in.

 

[Cottonwood]

I do the winter hypermile from Silverton to Pagosa Springs and the other way a few times each year. See Realistic Range Expectations in Crummy Winter Weather - Hypermiling in the Winter...Brrr for one description.

A few comments:

• Even if you do a 100% charge, the ability to get full regen happens much sooner than the battery will get warm enough to allow that, especially if you have range mode on. Full regen happens somewhere above 90% SoC, 25 rated miles into the trip.
• Heating the cabin and the battery on shore power is another form of energy storage before departure. As soon as the charge taper starts, I turn on cabin heat at something like 79˚ F with range mode off. Usually, I can drive the first 15-30 minutes with HVAC off.
• Regen limits happen as warm as 10˚ C, 50˚ F. I have a heated garage that I keep at 45-50˚ F. and know this well.
• I have seen reduced charging rates, even with 24A/240V charging with a cold soaked battery. This was while the car was pulling a full 24 Amps and the cabin heater was off. I can only assume this was caused by the battery heater being on. This was with an ancient version of firmware, three years ago.
• The best way to warm the battery is to do the last 10-20% of charging just before departure with range mode off.

 

[apacheguy]

Agree that in most instances it's fine to leave the pack cold and gradually warm it up. But that doesn't work well at all for SpC. As I recently learned, after the car sat for a night at mid-thirties (F) it would only accept 40 kW even after prolonged pre-conditioning. Ouch! And mid-thirties ain't that cold. Glad I live in a warm climate, but any time I road trip in the winter this will definitely be something to consider.

 

[David99]

... after the car sat for a night at mid-thirties (F) it would only accept 40 kW even after prolonged pre-conditioning. Ouch! And mid-thirties ain't that cold. Glad I live in a warm climate, but any time I road trip in the winter this will definitely be something to consider.

40 kW is awesome When I was stuck in the recent ice storm and finally got to a Supercharger my battery was so cold that when I plugged in at the Supercharger it started at 2.5 kW and 5 min later it was still only at 6 kW. It went up very slow as the battery warmed up.

 

[FlasherZ]

40 kW is awesome When I was stuck in the recent ice storm and finally got to a Supercharger my battery was so cold that when I plugged in at the Supercharger it started at 2.5 kW and 5 min later it was still only at 6 kW. It went up very slow as the battery warmed up.

It's probably faster to drive it around for 10 minutes, then plug in, if you have the miles left to do so. My cold-soaked 15 mile trip from hotel to supercharger in high 20's weather (degF) was enough to start me out at 90 kW when plugging in. I imagine that's much faster than 15 miles at 2.5 - 6 - 15 kW.

 

[AWDtsla]

Range mode is off, always.

I have an unheated garage. While rare, in any given year you're going to find at least a couple of days where the temperature in there is well (over 10 degF) below freezing. If I don't use cabin pre-heating, there are times I will unplug and be faced with regen disabled due to temp.

More than a couple days here....

It's just a shame, the "problem" could easily be solved by doubling the size of the cabin heater and quadrupling the size of the battery heater., neither of which are either big or heavy. How about 32kW (resistive) draw while under regen to heat the battery and cabin? Regen reduction would be kept at a minimum with a pack that warms quickly.

 

[David99]

It's probably faster to drive it around for 10 minutes, then plug in, if you have the miles left to do so. My cold-soaked 15 mile trip from hotel to supercharger in high 20's weather (degF) was enough to start me out at 90 kW when plugging in. I imagine that's much faster than 15 miles at 2.5 - 6 - 15 kW.

Very true! Driving around with some high power acceleration will heat up the battery nicely right from the inside.

 

[brec]

...

• The best way to warm the battery is to do the last 10-20% of charging just before departure with range mode off.

So range mode prevents battery heating even while charging? Is that a bug or a feature?

Is the same true during cabin heating on shore power?

 

[AWDtsla]

So range mode prevents battery heating even while charging? Is that a bug or a feature?

Is the same true during cabin heating on shore power?

Bug, imho.