12v Battery monitoring

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Jul 22, 2018
Ottawa, Canada
Hi all,
thought I would comment on 12v battery State of Charge loss after hearing from EV owners like tesla, leaf, and others about the 12v battery when left for long periods of time. I gather it's worse with the new vw id3.

So, I was wondering why vw doesn't have a gauge or have the info listed in the infotainment system? ICE cars have (had?) them, why get rid of that parameter/gauge, especially when it's so critical to the cars function?

Anyway, I'm sure you've seen them, but there is a handy little device that connects to the poles of your battery, and via bluetooth to an app on your phone (see the video below or search online for battery monitor. Amazon has some for around $30-$50 CAD). Gives you a consumption timeline graph if you're interested in charge loss over a 24hr period. Very handy. Very interesting.

I was able to see that my 2017 loss when parked, and not plugged in, over a 4 day period was quite a bit more than I thought. Not sure if weather temp was a factor, but it was cold. The 12v battery went from 98% to 69% SOC over those four days, where the main pack didn't seem to change. I can now understand if someone was parked for more than a week at an airport or something that they might have issues "starting" or even getting access to the vehicle.

I found a youtube video with someone with an id3 using one of these devices, and how they were monitoring its SOC.

Just thought I'd pass this along, as I found it very informative and helpful.

I have left my car parked and untouched for weeks at a time with no I’ll effects. It was not super cold. There is a way to view 12 V battery voltage from the gauge cluster, but I don’t remember how. I know the 12 V battery gets charged when driving or charging, but I wonder how many e-Golf owners have had dead 12V batteries in 2020 when many people were stuck at home for extended periods.
It would be interesting to watch the discharge with and without the alarm system active. I have heard that with the car locked and the alarm active it uses more battery as a result. Having said that my car sat idle for over a month and when I checked codes with OBDeleven it showed a fault that the battery bus voltage was 11,9v. No error lights on the dahs about the 12v or having to drive to charge, just saw this fault in the app when i scanned the car.
Interesting idea, though I think the lead acid battery may be better designed to be drained and recharged to 100% (and kept there).

Also, I have to admit that the little monitor I installed may not be the best when it comes to measuring battery level. So the numbers I gave earlier should probably taken with a grain of salt.

Be that as it may, it still shows regular battery consumption for about 15mins regularly overnight when I assume nothing should be running (car locked, not plugged in, no warm-up program running). The alarm and little flashing LED's and such would vampire drain for sure, but the "big drain" wouldn't be that, I don't think, would it?
I just use a USB charging adapter in the 12 v socket in the console. It has an LED readout of the voltage from the 12v battery:


I'm using this device:

Battery Monitor BM2 BM3 Bluetooth 4.0 Wireless Battery Tester 12V Automotive Battery Load Tester,Automotive Charging and Cranking System Monitor Digital Battery Analyzer for Android & IOS

It is permanently connected to the battery and monitors the voltage (using only 1mA). When you launch the corresponding app on your phone, it downloads the data via Bluetooth and displays the information in a graph. You can see the battery voltage over time for the last for the last few days/weeks. $30.- at Amazon.