New 12V battery died twice in 3 months

[fauxparisian]

Update: the issue seems to have been a very loose terminal adapter. Solution is to remove the battery and reinstall those with thread locker.

Original post:
I got a 2017 i3 earlier this year that had the original battery (7 years old), which I replaced three months ago. Since swapping in the new battery, I've had two instances where the car has been bricked after sitting overnight. I have a plug-in voltmeter that has been reading consistently low (12.1V-12.4V, rarely around 12.7-12.9), but I'm aware these may not be accurate. It's worth noting that I only drive like 100mi/week and also charge infrequently because of that and not having home charging.

What are the issues that could be causing this? My main suspects are that I might've connected it poorly or the clamps that go on the terminal are dirty. I'm wondering if there are other possible issues like I got a bad battery or something is wrong with how the 12V gets charged. Is it possible that my battery hasn't really been getting charged over 12.4V?

For today, I'm going to just disconnect and trickle charge the battery, clean the clamps if needed, then put the battery back in. Any thoughts on how to avoid having to do this on the regular would be appreciated.

Details below if it helps:

This morning's occurrence
- Main battery was ~75%
- Sat overnight but I hadn't driven 10 days (if anything I was expecting it to be bricked after sitting that long)

First occurrence
- Main battery was >50%
- Sat overnight
- Fixed after I tried to check the voltage with a multimeter and accidentally caused sparks (note: high voltage properly disconnected, might just have a cheap multimeter and am going to avoid doing this again). Accessories powered on and I was able to drive it long enough to get the 12V back up.

 

[mojo]

I have a plug-in voltmeter that has been reading consistently low (12.1V-12.4V, rarely around 12.7-12.9)

When and how are you measuring these voltages? If this is the voltage when the car is powered-on, it is way too low. When the car's DC-DC converter is running, the voltage at the 12V battery should be above 14V.

 

[alohart]

Driving 100 miles/week should be sufficient to keep the 12V battery well charged. It might not be at a 100% charge level at all times because always-on vampire 12V loads gradually discharge the battery (e.g., burglar alarm, keyless entry system, telematics control unit).

If possible, measure the voltage at the battery terminals the morning after leaving the frunk open. When the frunk is opened, the HV system turns on which would allow the DC-DC converter to begin charging the 12V battery, so you wouldn't be able to measure the battery's resting voltage. By measuring the voltage the following morning before unlocking/opening the doors, the battery should not have been charged since the night before.

In addition to tightening the battery clamps, also tighten the Torx screws that attach the SAE battery posts to the battery terminals.

 

[fauxparisian]

When and how are you measuring these voltages? If this is the voltage when the car is powered-on, it is way too low. When the car's DC-DC converter is running, the voltage at the 12V battery should be above 14V.

This is without the DC-DC converter and measured through the plug in voltmeter. When the HV is connected, I see around 14.1 usually.

 

[EvanstonI3]

Check the TORX30 bolts that hold down the Terminal Adapters to the Battery. The original bolts had a small amount of thread locker to keep them from coming loose. If you did not apply any thread locker, it is possible that one or both terminals have vibrated loose, which could cause the symptoms you describe.

 

[fauxparisian]

Driving 100 miles/week should be sufficient to keep the 12V battery well charged. It might not be at a 100% charge level at all times because always-on vampire 12V loads gradually discharge the battery (e.g., burglar alarm, keyless entry system, telematics control unit).

If possible, measure the voltage at the battery terminals the morning after leaving the frunk open. When the frunk is opened, the HV system turns on which would allow the DC-DC converter to begin charging the 12V battery, so you wouldn't be able to measure the battery's resting voltage. By measuring the voltage the following morning before unlocking/opening the doors, the battery should not have been charged since the night before.

In addition to tightening the battery clamps, also tighten the Torx screws that attach the SAE battery posts to the battery terminals.

Overall, I think a loose battery post was the issue and it’s fixed now. Thanks!

I did tighten it in the first install. Just removed the battery and reinstalled it, making sure to triple check that. The car now turns on just fine without any recharging or anything.

The battery is fine, but I get lower voltage readings as I move out from there. Posting these readings in case anyone thinks 2 and 3 are low and may cause future issues.
1. battery alone shows 12.9V reading with multimeter on the terminal posts
2. installed battery reads 12.6V with multimeter wherever I could get the prongs on the terminal clamps
3. plug-in voltmeter reads 12.2V with the car on but no DC-DC converter running

 

[fauxparisian]

Check the TORX30 bolts that hold down the Terminal Adapters to the Battery. The original bolts had a small amount of thread locker to keep them from coming loose. If you did not apply any thread locker, it is possible that one or both terminals have vibrated loose, which could cause the symptoms you describe.

Ah, loose terminal did seem to be the issue. Sounds like it’d be worth it to take the battery out again, clean the threads and add thread locker. Thanks!