12 Volt Battery - DRAFT voltage testing procedure

MISSION - I want to be proactive with regard to my 12v battery without being paranoid and replacing the 12v battery every 3 years. I am happy to "test" it every 6 months and recording the readings. Then replace the 12v battery when it shows signs of old age.
OVERVIEW - I charge my 2020 i3S every 3-4 days. It has 40K miles. The 12v battery is probably 5 years old as I have had the car since Aug 2020. NOTE: My former 2014 i3 REX and it's 12v battery was at 60K miles and 6 years old when I traded it in for my i3S.

I had inserted here (and now removed) a draft voltage testing procedure. But a quick response from several members provided a more practical and accurate way to test the 12v Battery. So I removed the draft testing procedure.

I have to admit that I had a difficult time rummaging through all of the good information in this Forum. I'm blaming my search-skills and what appears to be the i3 double-meaning of "battery". I do suggest that anything related to 12v battery issues always be referenced as "12v battery" I think this would make searching for 12v battery issues easier than the ambiguity of saying "battery" when it could be the high voltage battery or the 12v battery. Thanks in advance.

I like the suggestion from Alohart about the bluetooth 12 volt monitor. I will try to investigate the "drainage". If we leave the car for 3 weeks will it drain our battery? Thanks to all of you enthusiastic i3 owners.

Edszhi1 said:

From what I have read, the 12v plug (cigarette lighter) in the center console will provide the non-load value of the 12 v battery after the car goes "dormant" (as long as the fuse hasn't blown.)

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Not true - the 12V outlet (indeed all of the 12V sockets on the car) turn off when the car goes "dormant".

Plenty of people fit a Bluetooth-enabled voltage monitor directly to the 12V battery on their cars, which makes it almost trivial to measure the voltage at any point in time.

I haven't bothered - I've simply fitted an always-on 12V battery display in the passenger footwell that can be seen from outside without needing to open any doors/windows.

You can check it every day and get a graph of the results if you use a Bluetooth Battery Monitor and the free app for it.

Save yourself a lot of time and hassle, and get better diagnoses with this 12 V system monitor. This would show 12 V battery voltage trends that you couldn't see with short-term measurements. Upload 12 V system logs to the included smartphone app periodically via Bluetooth (no need to open the frunk). My 12 V system monitor has helped me detect 2 failing 12 V batteries before they failed so that I could replace them before becoming stranded.

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RafflesNRH said:

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Interesting, thanks. May I ask where you made the battery monitor connections? Directly to the 12v battery in the frunk or the 12v lines under the rear boot floor?

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I wanted to connect to the 12 V terminals in the rear motor compartment which would have been ideal. However, they're covered by insulating caps that would have to be cut to allow the 12 V system monitor wires to be connected. Or maybe the caps would have to be left off. Also, the bolts that attach these cables might be too large for the lugs on 12 V system monitor wires. All of this could have been worked around, but I didn't want to increase the risk of a short-circuit on top of the very expensive EME.

Instead, I installed 12 V system monitor lugs under the battery cable clamp tightening nuts which isn't as convenient as I'd like. It would be difficult to install the positive 12 V system monitor wire on the positive cable clamp without partially pulling the battery out to gain access to the positive terminal that's covered by a red plastic cover. That involves removing and replacing the frunk box and the battery hold-down bracket. This would have to be done only once, so it's not the end of the world.