Cold (cool?) weather performance

[Boatguy]

I've read Tom's post regarding winter performance, but I'm curious about a finer point related to cold weather (or really just "cool" here in Northern California) performance.

The conclusion is that in colder weather the range is reduced. Let's take that as a given. Now is the reason due to higher consumption, or lower real capacity, or a combination of both? And how is this reflected in the vehicle reporting of performance? Let me pick this apart into the component questions and I hope that the group can shed some light on the answers.

1) Is the battery effectively smaller or not? For simplicity let's say the battery is 19kWh at 70F. Is it essentially unable to deliver 19kWh at 50F? My theory is that since it the battery is conditioned to be at its optimal operating temperature, the battery's effective capacity is constant at 19kWh.

2) If #1 is correct then the reduced range must be due to increased consumption for battery conditioning and cabin heating. At least in California this doesn't quite ring true for me because in the summer there is consumption to cool the battery and cool the cabin; the AC is running every day. Does heating consume more power than cooling?

3) If the reduced range is due to consumption, then how is this reported? Is everyone seeing a lower mi/kWh (sorry Euro readers - appreciate your patience in translating the units).

In the i-app that my average consumption has been steady at 4.1 mi / kWh as the temperature has fallen, but the "community" has dropped to 4.0. Are most owners seeing a drop in mi/kWh?

The bottom line question is whether the drop is due to increased consumption or a smaller battery.

What are you seeing?

 

[jadnashuanh]

Let's say that it's 90-degrees outside, and you want to cool the cabin to 70. Now, say it's 40-degrees outside, and you want the cabin again at 70. In the first case, you've got to move 20-degrees of heat out of the vehicle...in the second, you've got to put 30-degrees of heat INTO the cabin. You tend to have the fan run faster when cooling than heating since just the air movement tends to make you cool, whether the air is hot or cold. If you have a heat pump, verses an a/c unit, they're fairly equivalent until it gets much colder than that, so heating is likely more of a hit. You can get more out of a battery when it is warm...let it get too warm and you can damage it, so you have to cool it some for maximum life. While you can damage a battery by drawing it down when it is cold, you can't recharge it well until it is warmed up at least some (pretty much all battery products with Li-On batteries limit or stop charging when super cold or hot). You need to warm things up. THe act of charging does this to a degree, and if you start out slowly, you may not need auxiliary heat...depends on how cold. The BEV in the USA has a heat pump standard, none of the REx units do, so resistance heating is lots more power hungry than a heat pump, often by a factor of 2-4. Someone said the i3's is about 2.3, but I've not seen anything official on this. Note, this changes (gets less efficient) as it gets colder, and could rise as it gets warmer - that may be an average.

OF the various battery tech out there, LiOn is the least expensive one with decent capacity that doesn't lose all that much as it gets cooler, but every known battery type does. The exact slope depends on the battery tech, size, and probably a few other things.

There isn't a battery tech item out there that doesn't take a hit when it is cold out, the i3 is no exception. Ideally, you get it all conditioned while on external power, and then, keep going without having to do it over and over again before you can get back on the EVSE. Even if you don't need much of a charge to get home, if you have an EVSE at the other end of your trip, you may want to use it so you can precondition while still hooked up.

 

[alohart]

1) Is the battery effectively smaller or not? For simplicity let's say the battery is 19kWh at 70F. Is it essentially unable to deliver 19kWh at 50F? My theory is that since it the battery is conditioned to be at its optimal operating temperature, the battery's effective capacity is constant at 19kWh.

A battery produces and consumers power by a reversible electrochemical reaction. This reaction produces or consumes electrons (i.e., an electrical current) based on the quantity of reactant and the number of electrons in the reaction. Lower temperatures would result in slower reaction rates which would reduce the battery pack's maximum current output or intake, but that wouldn't reduce the range.

The propensity for the reaction to proceed is its electrical potential which is directly proportional to the temperature, so a lower temperature results in a lower electrical potential. The power that a battery produces (watts) is a product of its electrical potential (volts) and current (amps), so with the electrical potential lower at lower temperatures, the battery's power would be reduced. A battery's capacity (watt-hours) is a product of the power it produces (watts) and the time period over which this power is produced (hours), so at lower temperatures, the electrical capacity of a battery is lower.

While battery conditioning can eliminate this effect, when one is driving, the power used to keep the battery pack warm reduces the range. I don't know whether the battery pack is conditioned when driving, but if the battery pack's temperature can't be kept at an ideal temperature in cold weather without using a lot of energy, the battery pack's capacity would be reduced as its temperature drops.

2) If #1 is correct then the reduced range must be due to increased consumption for battery conditioning and cabin heating.

That's part of the problem. More viscous lubricants, denser air, stiffer tires, and, if winter tires are installed, higher rolling resistance all reduce range just as they do with ICE vehicles.

3) If the reduced range is due to consumption, then how is this reported? Is everyone seeing a lower mi/kWh (sorry Euro readers - appreciate your patience in translating the units).

A lower mi/kWh should be reported.

The bottom line question is whether the drop is due to increased consumption or a smaller battery.

Some of both.

What are you seeing?

No change (Honolulu

 

[mindmachine]

I've read Tom's post regarding winter performance, but I'm curious about a finer point related to cold weather (or really just "cool" here in Northern California) performance.

3) If the reduced range is due to consumption, then how is this reported? Is everyone seeing a lower mi/kWh (sorry Euro readers - appreciate your patience in translating the units).

What are you seeing?

In the i connected drive you can view where the energy flow is going/being used. If you use ecopro mode you can watch how the car is using energy with your driving style and accessory usage, this is shown in the i connected drive too. The latter tool will show where you can improve upon you driving style and what you may be doing that is contributing to your driving efficiency. I have only used this feature once, so I am sorry I can't explain it any better for you. Others on this forum though have commented on it so you might do a search for more details.

I live in Ohio and while I see reduced range reported by the car, if I pre condition my efficiency is still around 4.4 miles/kwh. In the colder weather though I am using the ecopro + mode and I find I am exceeding the predicted range which the car has downgraded due to the temperature. During the warmer months I was using comfort mode all the time and used the AC as necessary.

 

[Stevo]

I received my BEV i3 last week and while I managed to achieve 4.2 mi/kwh during an extended test in June I have only managed 3.2 mi/kwh this past week where the temperature is averaging 4c.

I'm preconditioning before I leave in the morning while plugged in but I have to say I'm disappointed with the drastic range reduction from the summer. Some experienced EV owners had an expectation of a 60 mile winter range but this was never explained by BMW at the time of sale. 80 miles has always been the figure quoted.

 

[Goldcj7]

I agree, I'm getting 60 miles now on a BEV without heat pump, which is 10 less than the summer, I only got 80 miles when I was ambling around....

Was getting 4.0 mi/kwh summer, now about 3.5.

 

[kentheteaman]

I use to get 120km(74miles)/charge in June.
Now it is 90km(55miles)/charge in December.

I use Ecopro mode all the time with heaters always on in auto mode at 28c.
I drive mostly in motorways(Highways) at around 70miles/H speed.

so far I have done 28,000km(17,000miles).

Loss of range is really annoying. :evil:
I have to fill up the Rex tank every other day. I did not fill up at all for 4 months in summer.

However, still this is the best car I have ever driven - its like being in a flight with no vibrations or noise with sumptuous music on board by Kardoman speakers.

So kind of put up with it, I guess.

regards,
ken

 

[jadnashuanh]

Do you really need the cabin temperature at 28C? That's over 84F...do you keep your house that temp? WIth a REx, every watt going into heating the cabin comes out of max range. An a/c has a multiplier effect - one watt in equals more than that watts out in cooling. But, the REx has to use resistance heating to warm things up, so it's one to one. Lower that, and then tell us what effect it has. I think you'll be surprised, unless that was a typo...

 

[Anonymous]

I've seen similar behaviour in my Smart ED, namely, reduced range depending on temperature. Ref:
http://mysmartelectricdrive.blogspot.ca/2014/03/range-experiences.html

In my case, my constant use of maximum power after every stop sign and heating the cabin to tropical temperatures has a lot to do with the low amount of estimated range shown on the dash.

The battery pack is heated (and cooled) in my car, and I've seen cases where I need to wait before driving so the pack can be heated to a minimum operating temperature. Ref:
http://mysmartelectricdrive.blogspot.ca/2014/01/crazy-cold-and-battery-heater.html

My expectation is at least 20% reduced range in winter, and up to 40% reduced range on severely cold days when taking multiple small trips and leaving the car parked for hours at a time, giving the battery pack time to cool down too far.

 

[fdl1409]

While battery conditioning can eliminate this effect, when one is driving, the power used to keep the battery pack warm reduces the range.

The battery pack never ever gets heated while driving. I have watched this closely and there has been official confirmation from BMW somewhere in the German forum. The battery only ever gets heated if you have seat heating and set a departure time for preconditioning. Any rise in battery temperature while driving is due to internal resistance losses.

Drove 69 miles yesterday with 22% SOC remaining, so I could have done some 88 miles and over 80 bevor the REx would have kicked in. 3.9 miles per kWh. That was after a cold night with a minimum of 22°F. Car is parked in an open garage. In the afternoon temperature rose to 34°F, roads wet with slight drizzle. Up and down the hills, must have climbed over 2.000m. Had the seat heating on all the time, but no other heating, as I mostly do in winter. Average speed was only slightly above 30 mph. Country roads with 40-60mph and the rest urban traffic. Battery temp was 2°C when I started ( after charging at night ) and reached 8°C during the day.

Charged 17.7 kWh last night measured from the meter.
There really is not that much capacity loss due to temperature. In the summer my maximum capacity was around 19.7 kWh. Recently this has gone down to an average of 19.3, so far I have never seen less than 19.2 kWh. Just now after charging it was shown to be 19.6 kWh again, which I have not seen in a while. Battery temperature after charging is 7°C, air temperature 2°C ( 36F ).

Frank

 

[MikeS]

Frank - your range seem much better than most, is that in Eco Pro? Was the 69miles done in a single journey?

Im seeing about 65 miles range in temp around 2 deg C driving in Eco Pro with top speed less than 56mph and driving in the local urban area for a full charge. This is a combination of about 4 different journeys. Just charged overnight and with pre conditioning the app shows a range of 67 miles with the last journey achieving 4.6 miles/kwh with 80% efficiency ( short journey with lots of hills).

Not sure I understand your figures as 3.9 miles/kwh with 18.8kwh capacity (BMW figure) gives a total range of about 73 miles.

 

[fdl1409]

Hello Mike,

no, did 9 shorter trips between 10:00 and 18:00 with a total of 69 miles.
3.9 miles / kWh is based on consumption from the grid, which is 25% higher than BC. I charged 17.7 kWh last night. The battery capacity went up from 3.9 to 19.5 kWh, which is 15.6 kWh. The difference is a charging loss which heats the air ( charger ) and battery ( internal losses ). Consumption according to board computer was 14.3 kWh - 110.2 km driven x 13.0 kWh/100km.

So mileage / kWh is:
- 3.9 miles / kWh ( power metered from grid )
- 4.4 miles / kWh ( actual battery capacity according to service menu )
- 4.8 miles / kWh ( consumption according to BC )

Litte bit confusing.

Frank