How to prevent a low 12V battery alarm when away for a long period

[scrambler]

Hi,
We are away 2 or 3 months at a time, and I am worried that one day the 12V battery could die or discharge to the point where the alarm would go on and drive the neighbors nuts.
So I have the following questions.

1. If the low 12V battery alarm goes on, how long will its stay on?
2. Is there a way to deactivate the low 12V battery alarm, may be with something like bimmercode?
3. If there is no way to deactivate that alarm, would the best way to proceed, be to disconnect the 12V battery and its High voltage connector while we are away?
4. If so would the prolonged 12V battery disconnection cause any issues with the car?

Thank you for any insights

 

[EvanstonI3]

If the alarm goes off, it means your 12v battery is failed. It is just one of many symptoms when the 12v fails.

If you do not want to simply replace the battery now BEFORE it fails so you can have at least 3 years of driving/use of the car without worrying about the 12v battery, then you should disconnect the negative terminal of the battery before you let it sit.

If the battery is in good condition, it can sit for months while disconnected without losing more than 1-2 volts, which means you can hook it back up when you return and everything will be fine.
If the battery is NOT in good condition, or fails while sitting disconnected, the car will not function when you return but replacing the 12v battery will bring everything back to normal with minimal fuss.

If the car sits for an extended period of time while connected to a failing/failed 12v battery it will cause all sorts of issues and error codes with the car because the car does try to keep it charged and "alive" but at some point it will not be able to keep up and the car gets bricked.

 

[alohart]

There is no 12 V battery alarm. As @EvanstonI3 wrote, the burglar alarm can be triggered when a 12 V battery is failing. However, this would occur only if the 12 V battery's voltage decreases significantly. This feature prevents a burglar from disabling the burglar alarm by disconnecting the 12 V battery because the alarm includes its own internal battery which powers the alarm. The 12 V batteries on our i3's have failed twice without triggering the burglar alarm.

I have stored our i3's 8 times for periods of 3 to 9 months each time. I have always disconnected the HV disconnect followed by disconnecting the 12 V battery's negative cable as @EvanstonI3 suggested. I have always been able to start our i3's after ending their storage by merely reconnecting the 12 V battery's negative cable followed by connecting the HV disconnect. To be safe, I have always charged the 12V battery with a battery charger after disconnecting it and again before reconnecting it. This reduces the possibility of a weak 12 V battery being reconnected at such a low voltage that spurious and occasionally troublesome diagnostic trouble codes (DTC's) might be stored. This isn't necessary for 3 months of storage assuming that you've been driving your i3 regularly prior to storage so that the charge level of its 12 V battery is high.

Some tips:

• Don't disconnect the 12 V battery with the burglar alarm armed (i.e., the doors locked) to avoid triggering the alarm;
• Close but don't latch the hood so that it can be opened when you return without using the emergency unlatch cable;
• With the 12 V battery disconnected, the front windows won't drop when the doors are opened, so be careful opening and closing the doors to prevent damage;
• Lock the passenger door from the inside and use the physical key in the fob to lock the driver door;
• Be sure to reinstall the driver door lock cover securely so that it doesn't fall off while driving.

 

[scrambler]

Thanks for these.
You mention that the alarm that sounds if the 12V battery fails is the burglar alarm.
Does it mean that if I deactivate the burglar alarm (double click on remote), if the 12V battery fails it will not sound an alarm?
Or is the 12V battery failure alarm a different one than the one that can be deactivated by the remote?

And from what you said, it is safe to disconnect the 12V battery (and HV connection) for a few month
Thanks Again!

 

[alohart]

You mention that the alarm that sounds if the 12V battery fails is the burglar alarm.

Yes

Does it mean that if I deactivate the burglar alarm (double click on remote), if the 12V battery fails it will not sound an alarm?

No. You would need to leave the doors unlocked to deactivate the burglar alarm.

Or is the 12V battery failure alarm a different one than the one that can be deactivated by the remote?

Clicking the fob lock button twice deactivates only the motion sensor in the cabin.

And from what you said, it is safe to disconnect the 12V battery (and HV connection) for a few month

Totally safe. I'll be disconnecting both again in a few weeks for over 3 months. I don't expect any problems when I reconnect them.

 

[scrambler]

Yes

No. You would need to leave the doors unlocked to deactivate the burglar alarm.

Clicking the fob lock button twice deactivates only the motion sensor in the cabin.

Totally safe. I'll be disconnecting both again in a few weeks for over 3 months. I don't expect any problems when I reconnect them.

Thanks again
Much Apreciated!

 

[scrambler]

Here is another questions regarding the 12v battery management.

I read online that when away, if you remotely unlock the car, it would trigger charging for 30mn.

So I opened the frunk and left the car locked for a wile to measure the battery resting voltage, it was 13V which is good
I left the car like that overnight and measured again in the morning it was 12.86V which is not bad.

I then unlocked the car with my phone to see if that would indeed trigger the charge, but the battery briefly dropped to 12.1V then stabilized around 12.4. This means unlocking the door did NOT trigger the charge. I also physically opened the door and no difference.

Question, is that actually normal and these people online are misinformed, or did I miss something?

Next I checked if plugging the EVSE to charge the car would trigger the charge, and it did, the battery went to 14.4V sign of charging.

I next checked if the charge would continue for a while after disconnecting, but 10mn after the battery was back to its resting voltage of 13V, so that did not last.

Thank you for any insights

 

[EvanstonI3]

Question, is that actually normal and these people online are misinformed, or did I miss something?

Yes this is normal and the people you are getting information from are indeed misinformed.
Here is a graph of 24hrs of my car sitting in the garage. The graph is of the voltage of the 12v battery over time.
I decided to test to see if this "unlock the car to charge" myth was true.
So at 1pm I UNLOCKED the car with the Remote, then at 2pm I LOCKED the car with the Remote.
Note on the graph what the voltage of the 12v battery is during that period of time.
The END RESULT is a lower voltage on the 12v battery after doing this. So it is actually counterproductive to do this.

 

[nilknarf]

VW has a solar battery maintainer that plugs into the OBD port to keep the 12v battery charged during long term storage. I believe these were used for cars in dealer lots. I've thought about picking some of these up for when we go on extended vacations. There used to be a lot available on eBay, but I haven't searched in years.

https://vw.snapon.com/SpecialToolsDetail.aspx?itemid=41590163

Edit: I just searched that part # on eBay and didn't turn anything up, but there are a couple with the part # of 1C0915687D. Probably other VW variations as well as generic versions available.

 

[EvanstonI3]

VW has a solar battery maintainer that plugs into the OBD port to keep the 12v battery charged during long term storage. I believe these were used for cars in dealer lots. I've thought about picking some of these up for when we go on extended vacations. There used to be a lot available on eBay, but I haven't searched in years. I'd image there are generic versions out there as well.

https://vw.snapon.com/SpecialToolsDetail.aspx?itemid=41590163

Maybe for an ICE vehicle, but it is not necessary for the i3.
Once the 12v battery discharges to 12.0 volts, the car comes on BY ITSELF to boost the charge to the 12v battery for exactly One Hour. It will repeat this as often as necessary.

I have also tested this on my own i3 to try and determine what early signs of 12v battery failure looked like while the car was OFF.
Here is another graph of my 12v battery as it began to fail. Again, over 24hrs, note how the battery self-discharges and then the car comes on to boost the charge to the 12v battery. This is while the car was sitting in my garage, untouched during this time.

 

[alohart]

I read online that when away, if you remotely unlock the car, it would trigger charging for 30mn.

This was true for our 2014 i3 which turned on the HV system and then the DC-DC converter whenever the doors were unlocked, remotely or in person. It occurred less often on our former 2019 i3. It almost never happens on our current 2021 i3, almost certainly due to its LFP 12 V battery with its 0.5 V higher resting voltage compared with the OEM battery. This behavior might be different for different i3 integration levels (i.e., system software versions) rather than for different model years.

In our 2014 i3, the DC-DC converter would remain on for up to 30 minutes after our i3 was turned off. I timed this at one point. However, at other times, the DC-DC converter might not have remained on as long. It turns off when the 12 V battery is sufficiently charged even with the HV system still on. I've not timed how long the DC-DC converter remains on in our former 2019 i3 and current 2021 i3.

So I opened the frunk and left the car locked for a wile to measure the battery resting voltage, it was 13V which is good
I left the car like that overnight and measured again in the morning it was 12.86V which is not bad.

The voltage difference was likely due to the battery's voltage stabilizing after having been charged. It takes a few hours for this to occur. 12.86 V is probably the true resting voltage of a fully-charged AGM battery at your battery's temperature while 13 V is not. I have 12 V system logs that show that the overnight discharge from vampire always-on 12 V loads does not lower the voltage noticeably.

I then unlocked the car with my phone to see if that would indeed trigger the charge, but the battery briefly dropped to 12.1V then stabilized around 12.4. This means unlocking the door did NOT trigger the charge. I also physically opened the door and no difference.

If your 12 V battery is in good condition, the battery's voltage must not have returned to its resting voltage after the significant short-term load and voltage sag caused by the LV and HV systems starting up. The resting voltage shouldn't decrease from 12.86 V to 12.4 V due to this discharge. I've seen the OEM battery's voltage sag to as low as 11.9 V immediately after unlocking the doors. I don't see a similar voltage sag with my LFP battery. Because of this discharge when the LV and HV systems start up, the 12 V battery charging that could occur after remotely unlocking/locking the doors might not replace the charge lost. Your tests verify this for your i3.

I next checked if the charge would continue for a while after disconnecting, but 10mn after the battery was back to its resting voltage of 13V, so that did not last.

How long 12 V battery charging occurs depends on the charge level of the 12 V battery, the 12 V system power demand, maybe the battery's temperature, and probably other things that aren't documented publicly.

Early on in 2015 when I first stored our i3 for 6 months, I had hoped that maybe remotely locking/unlocking the doors periodically could maintain the 12 V battery's charge level while away because I had noticed that the DC-DC converter always turned on when I unlocked the doors (I had a voltmeter plugged into the auxiliary power port below the center of the dashboard where it was visible through the driver's window). I didn't know at that time that an i3 could automatically, repeatedly charge its 12 V battery for 1 hour while parked when the battery's voltage decreases below a certain limit. However, this behavior isn't documented, so it's safer to disconnect the 12 V battery when an i3 won't be driven for more than a month.

 

[alohart]

VW has a solar battery maintainer that plugs into the OBD port to keep the 12v battery charged during long term storage. I believe these were used for cars in dealer lots. I've thought about picking some of these up for when we go on extended vacations.

If yours is a U.S. i3 and you haven't disabled the OBD alarm via coding, leaving this solar battery maintainer plugged into the OBD port would almost certainly trigger the burglar alarm at some time after your i3 were turned off.