symonray
Posts: 38
Joined: Thu Mar 07, 2019 8:52 am

Occasional Vampire Battery Drain While Parked

Sat Mar 21, 2020 7:59 am

Over the past year, I have maintained a daily log keeping track of my battery usage and electricity consumption rate during my 60 mile round-trip daily commute to work. During each normal work day, my i3 sits locked in a parking garage for about 10 hours before I unlock the car and head home. I have noticed that approximately two to three times a month when I start home in the evening, the battery percentage display shows a drop of between 2% and 4% from the battery percentage I recorded when I locked the car in the morning. The sporadic loss has occurred throughout the year and seems to have no connection with the weather and/or ambient air temperature. There is no battery percentage loss on the other days. I am trying to theorize what might be causing the sporadic loss of charge while the vehicle is sitting idle and locked. I do not use the ConnectedDrive app to climatize the cabin during the day, but I occasionally check the status of the vehicle on the app. Any thoughts?
Timothy Simonds
2016 BMW i3 Rex

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Re: Occasional Vampire Battery Drain While Parked

Sat Mar 21, 2020 8:59 am

My theory on the battery and SOC calculation is that regularly, the system has to recalibrate.

You know how a ferry boat deck hand might use a clicker to count passengers off, passengers on, or how an airline scans tickets to know their seat count.

But occasionally those numbers go askew and a manual headcount is necessary.

Our batteries are similar. Instant SOC is produced by measuring current in / current out (Coulomb counting), but that's not infallible. Other methods can't be used while the car is in use. Once the car is parked and the temp stabilizes, that's potentially one chance to reset.

I wouldn't sweat it too much. And yeah, there could be other explanations, I'm just supposing. But it's hard to imagine there's that much "vampire" draw without something big using it.
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Re: Occasional Vampire Battery Drain While Parked

Sat Mar 21, 2020 9:34 pm

Also thinking that it would be helpful to drop in on the car to track your SOC in one or two hour increments.

I'm guessing this isn't practical since you don't know what day the charge will drop.

But if you land on the right day, if my guess is correct, it'll be a once-and-done adjustment. If it's truly a phantom load, it'll show up as a gradual decrease over a period of time.

Lastly, you're not accidentally leaving Comfort Climate active when you leave the car? What about nav? I notice when I plug a second destination in, Comfort Climate automatically activates when I get out at my first stop, so the AC/ heat is blowing when I get back in to drive to my second stop.
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symonray
Posts: 38
Joined: Thu Mar 07, 2019 8:52 am

Re: Occasional Vampire Battery Drain While Parked

Sun Mar 22, 2020 8:50 am

All systems are shut off during the day. The only potential system activation is when I remotely connect to the ConnectedDrive app to check on the vehicle’s status, but I have also accessed that app on days when there is no battery drain. I am thinking the best possible explanation is that the reduction is merely a recalibration of the battery state of charge reading by the system when I put the car in ready mode for the evening commute. But when the battery charge discrepancy occurs, it is always results in a lower battery charge reading; I have never had a higher reading.
Timothy Simonds
2016 BMW i3 Rex

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Re: Occasional Vampire Battery Drain While Parked

Sun Mar 22, 2020 10:48 am

I notice the same when I'm paying attention to it -- always lower, never higher.

Using the ledger method during operation to calculate real-time SOC can only count the Amps the system "sees" passing through, but some inefficiencies will always go unaccounted for. Even during regen, it can monitor charge going back towards the battery, but can't accurately know how much was stored.

I can't think of a scenario where it measures charge that wasn't actually taken out, which would be necessary to make a net positive adjustment.
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