Battery discharging while Stationary

[alohart]

My question is, given that it can top up small amounts of normal discharge by using the HV, why can it not do a more complete recharge if the 12V becomes significantly (but not completely) discharged?

Good question! I have noticed that the high-voltage system remains on for at least 30 minutes after I unlock the doors of our 2014 i3. During that time, the 12 V battery is almost always being charged because the 12 V system voltage is ~14.3 V. Especially after having driven recently, I have seen the DC-DC converter turn off before the auxiliary power port's 12 V power turns off. In these situations, the 12 V system voltage drops from ~14.3 V to 12.x V which might indicate that the 12 V battery is fully-charged.

I have no evidence that the high-voltage system remains on until the 12 V battery is fully-charged. In fact, the voltage of our 12 V battery seems to be lower than normal, probably because we drive so little. If the high-voltage system kept the 12 V battery fully-charged, the voltage of our 12 V battery wouldn't be lower than normal. This has been true for years, so it's not due to our 12 V battery being weak.

I suspect that gt6k's explanation has merit.

[EvanstonI3]

I have noticed that the high-voltage system remains on for at least 30 minutes after I unlock the doors of our 2014 i3. During that time, the 12 V battery is almost always being charged because the 12 V system voltage is ~14.3 V. Especially after having driven recently, I have seen the DC-DC converter turn off before the auxiliary power port's 12 V power turns off. In these situations, the 12 V system voltage drops from ~14.3 V to 12.x V which might indicate that the 12 V battery is fully-charged.

That's interesting. Perhaps there is a difference between the 2014 and the 2017 then?
I've not been able to get any interior lights/power ports to stay on longer than 13 minutes on my car after ceasing activity.
I've also not ever noticed the voltage reading higher than 12.5v when the interior lights are active. It pops back up to 12.9 once the cycle times out.
These are readings from the battery terminals.

[MarkH]

Thanks for the replies.

I had the 12V on charge overnight for 18 hours without isolating it.
When I opened up this morning I no longer got the message about "discharging while stationary".
I'll check tomorrow morning to see if there is a SOC drop in the HV overnight.

[alohart]

That's interesting. Perhaps there is a difference between the 2014 and the 2017 then?

Our 2014 i3 has November, 2018, system software. I would think that differences in charging behavior would be implemented in software, although the DC-DC converters could differ, I suppose. The apparent differences between the 12 V battery charging behaviors are substantial.

I've not been able to get any interior lights/power ports to stay on longer than 13 minutes on my car after ceasing activity.

The entertainment system in our i3 turns off several minutes after our i3 exits drive ready state. However, the auxiliary power port remains on for a longer period powering the voltmeter display showing that the 12 V battery is almost always being charged.

I've also not ever noticed the voltage reading higher than 12.5v when the interior lights are active. It pops back up to 12.9 once the cycle times out.
These are readings from the battery terminals.

Voltage readings at the battery terminals agree with those at the auxiliary power port in my experience.

I'm guessing that the charging behavior differences are due to the voltage of our 12 V battery being lower than average due to our infrequent driving (less than 12k miles since November, 2014). As a result, the DC-DC converter almost always begins charging our significantly discharged 12 V battery whenever the high-voltage system turns on. I have seen 12 V battery charging cease before the auxiliary power port turns off, but that's not common.

When I unlock the doors, the auxiliary power port turns on before the DC-DC converter begins charging the 12 V battery. The 12 V system voltage drops as low as 11.8 V before the DC-DC converter begins charging the 12 V battery (within ~5 seconds of unlocking the doors). I have short videos of a voltmeter connected to the battery terminals and a voltmeter in the auxiliary power port below the center of the dashboard, both showing the voltage drop rather rapidly when the doors are unlocked. When I first noticed this behavior, I was concerned that the 12 V battery was failing and made one of the videos to show the shop manager of the local BMW dealer. He told me that this behavior was normal for i3's and that my 12 V battery tested normal.

[MarkH]

Took too long getting a replacement.
Battery now completely dead - cannot drive.
What are the specifications for a replacement 12V battery, please?
Will have to fit it myself.

[alohart]

What are the specifications for a replacement 12V battery, please?

20 Ah AGM battery with a vent valve on each cell with a common outlet tube that can be connected to the i3's vent tube (i.e., not a power sports battery)
8.13" (207 mm) L x 3.56" (90 mm) W x 6.38" (162 mm) H
The battery should be an auxiliary rather than a starter battery.
The terminals need to be automotive terminals or a screw terminals to which the screw-on automotive terminals on the OEM battery can be attached.
The positive terminal should be in the front right corner.

In U.S. i3's, the OEM battery is a relabeled East Penn Manufacturing AUX18L. This can be ordered from a local East Penn Manufacturing retailer or from an online store. This battery is sold under several brand names including Deka and Remy.

[MarkH]

Thanks Art.
Will see what I can find here in NZ.
Frunk is in the dark end of the garage: any way of turning the car around (pushing) that you can think of?

[jadnashuanh]

FWIW, you can't maintain a battery trickle charge in most any BMW vehicle when using a 12v socket (i.e., cigarette socket), as those get disconnected when the vehicle goes to sleep. You need to connect the trickle charger directly to the battery or the provided terminals, on those that have them, and except for some of the earlier software configurations, it's safe to do that, but earlier software needed to have the HV safety disconnect pulled to then safely recharge the 12vdc battery.

[MarkH]

You need to connect the trickle charger directly to the battery or the provided terminals

Yeah. I did that. Don't think that was the cause of the battery failure. Just that it's 5 years old

[alohart]

Frunk is in the dark end of the garage: any way of turning the car around (pushing) that you can think of?

Probably easier to do what's necessary to get better lighting into the frunk. Without 12 V power, I doubt that it's possible to shift out of Park or to release the parking brake.