Parasitic battery charge loss from cold temperatures?

[bwilson4web]

Has anyone tried to map the battery Ahr loss vs temperature?

We've had exceptionally low temperatures in Dixie, 15F (-10C) and noticed the battery SOC % has significantly gone down overnight. We have a new driver and one hypothesis is failure to turn car off with a second power button press. However, I remember reading the battery will try to maintain a temperature in extreme cold. So I'm trying to figure out if anyone has done a study on this loss of Ahr.

Thanks,
Bob Wilson

 

[alohart]

The i3's high-voltage system is automatically turned off within ~30 minutes of inactivity (e.g., no opening, closing, locking, or unlocking of doors, the frunk, or the hatch, the completion of charging or preconditioning). The i3 doesn't automatically maintain its battery pack's temperature when the high-voltage system is off, so the battery pack of an i3 parked on a very hot day over hot pavement could overheat or could become very cold when parked at low temperature. Battery cell damage is reduced by limiting the battery pack's power output under these conditions as indicated by the gray power chiclets at the high power end of the power meter turning black.

So there's really no high-voltage load that could discharge an i3's battery pack when an i3 is parked and off. Any energy loss overnight is almost certainly due to a drop in the battery pack's temperature. A cold battery cell is unable to provide as much power at low temperature. This energy loss would be recovered when the battery pack's temperature increases.

However, if cabin or battery pack preconditioning has been configured, battery pack energy could be consumed if an EVSE isn't plugged in. Some who have parked their i3's at an airport while traveling have noticed a drop in charge level because they forgot to turn off preconditioning.

Note that this behavior is quite different from your Model 3 whose charge level can drop noticeably while parked due to high-voltage vampire loads that don't exist on an i3.

 

[3pete]

I've tracked a decent amount of data on my car in the 2+ years I've had it and it definitely seems like battery capacity declines faster in the cold (thread here: https://www.mybmwi3.com/forum/viewtopic.php?f=11&t=17163).

That's looking at capacity though... I haven't looked into state of charge because I usually just charge to 100% at home, which is where my car usually is on winter nights when it's coldest out.

one hypothesis is failure to turn car off with a second power button press

30 minutes of running the climate control--even on it's lowest setting--could definitely be a noticeable drop in SOC in those kind of temps. What kind of % drop are you seeing? Is it a percent or two here or there, or more like 10%+ swings?

 

[eNate]

Cold temps alone shouldn't result in a state of charge decrease. The inverse would be that the battery could be charged to 100% when cold, then rise to greater than 100% when brought back to room temperature, which we know isn't the case.

The low temps mean the battery won't operate as efficiently, ie fewer miles per kilowatt. But 100% SOC is 100%, and 50% is 50%.

The methods the car uses to estimate state of charge could be playing in to what you're seeing. The displayed SOC isn't an absolute, but a very "educated guess" and is updated after the battery is at rest for a while. In other words, it's possible the SOC was recalculated to a more precise estimate.

Perhaps the 12v was getting a top-off at the end of the drive, as well?

In regards to the i3 being "left on," I'm doubtful. If I step out with the car powered up and radio playing, everything shuts down once I close and lock the doors.

The exception I believe is if I have a destination set in nav, but have stopped off before reaching it. I believe the climate system stays active just as if I had selected Activate Comfort Climate Control from the shutdown menu.

 

[bwilson4web]

I've tracked a decent amount of data on my car in the 2+ years I've had it and it definitely seems like battery capacity declines faster in the cold (thread here: https://www.mybmwi3.com/forum/viewtopic.php?f=11&t=17163).

That's looking at capacity though... I haven't looked into state of charge because I usually just charge to 100% at home, which is where my car usually is on winter nights when it's coldest out.

one hypothesis is failure to turn car off with a second power button press

30 minutes of running the climate control--even on it's lowest setting--could definitely be a noticeable drop in SOC in those kind of temps. What kind of % drop are you seeing? Is it a percent or two here or there, or more like 10%+ swings?

Excellent posting and data. Now I'm wondering to what extent we might automate data tracking. I have an "iCar II" OBD device and BimmerCode and BimmerLink.

Bob Wilson