Disclaimer: I do not sell nor have any interest in products mentioned below. They are mentioned and pictured as reference material only. There are several products on the market that provide the functions mentioned. Readers may search for and purchase these or similar products at their own choice. Suggested search string "bluetooth 12v battery monitor".
These are not certified as scientifically accurate devices. Use them as a general guide for what is happening with your batteries, not as exact voltage reading devices. I found that voltages reported by them to be lower by about 0.7 volt from "Auxiliary battery voltage" reported in WattCat (datum supplied to Jaguar servers by the car). The devices snapshot the voltage readings and can vary by tenths of a volt at any given instant causing ripples in the data as you can see in the graphs below.
Monitoring 12V batteries in EVs would be a wise thing to do. Failures can lead to being stranded, or in the worse cases having a vehicle stop in traffic even though the traction battery has plenty of power. Reference this thread for how to handle 12V battery failures.
In vehicles made before the 2021 model year, there are two 12V batteries. These are located in the frunk under the center removable panel and easily located by the red caps on their positive posts. The larger battery is referred to as the starter battery. This one is oriented differently from...
As a chance at getting a heads-up to a weak 12V battery and get it replaced before there's a problem, one can add monitoring devices. Below I show adding devices that monitor the voltage and capacity of 12V batteries. They require a smartphone for an app that displays the information and can trigger problem notifications on the smartphone. They'll require a Bluetooth connection and location permission.
I already have devices of brand LNEX in use on other 12V battery applications. The app for them is limited to 4 devices so I had to find ones that used a different app (or somehow figure out how to run two separate copies of the same app on 1 phone). I went cheap and got a different brand, Quicklinks, as shown below.
The devices (regardless of brand) come with instructions for installation of the device, installation of the appropriate app, and how t use the app. Read the instructions carefully. I found installation of the first device is easy, but adding a second (or more) device to the app may not be as easy.
I installed one on the starter battery first as seen below. Using a 10mm socket tool, attach the positive wire (red connector) to the battery positive bus bar, and the negative wire (black connector) directly to the negative post clamp. This latter connection bypasses the BMS to prevent any potential interference from one to the other. Model year 2019 is shown. Model year 2021 and following will be slightly different.
Note the orientation of the positive connection. This allowed the red cap to be re-installed on the bus bar (not shown). I tucked the monitor further down before re-installing the cover too.
For the auxiliary battery on pre-model year 2021 vehicles, attach the black connector to the negative post at the clamp for the ground cable, and the red connector to the 13mm nut attaching the fuses to the positive post bar. This latter location allows for re-installing the red protective cap. You may have to adjust it a little from what is shown in the picture as one of the cap's holding clips is located near this location. I had to adjust mine after I had taken this picture.
I used the app to verify the device worked after I installed the starter battery monitor and followed the setup instructions. Then I installed the auxiliary battery monitor and used the app for its setup. You might want to install both monitors, if needed, and then use the app. The sequence doesn't matter and I don't know if it is any easier one way or the other.
Now that the devices are monitoring the batteries I can get the current state of charge/capacity and voltage for each. The app for these devices (and the INEX ones I have on other batteries) can produce a graph for each battery's voltage for a day. The devices will retain up to 35 days of data if you don't happen to look at app for some reason (eg. away on a business trip).
Here's a day's graph for the auxiliary battery on my car. At the left, the car is sitting in the garage unattached to a charger. This was 12.8V A dip to 12.42V occurred when I opened the driver's door. A rise to 14.25V occurred when I started to drive. A dip to 12.48V came at my first stop. A return to 14.1xV happened when I resumed driving but dropped to the stretch of approximately 13V for a period of car sleeping time. The next short trip sent it up to the 14.xV range with a drop to 12.9V when I was back in the garage and shut it down.
As soon as I attached the charging cable, it jumped up to 14.1+V and then slowly tapered down over the course of the long charging session. The dip to the right was caused by a brief power drop to the house (it happens a lot) and you can see the very end of the graph showing the voltage had dropped to what it was before the driving and charging, after the charging had completed.
Here's the same day's graph for the starter battery.
Although similar ups and downs of voltage occur, they are not equal.
I have noticed that everything that causes a current draw of the 12V system may cause a period of "charging" to occur after the momentary drop. Unlocking a door, opening the rear hatch, opening the frunk, etc., may cause a brief jump up in the voltage to the 14.x range for both batteries and the starter battery will show deeper dips in voltage at the start and end of the "event". I haven't had the devices installed long enough to ensure my perceptions are correct all of the time. It will be interesting to see what these reveal. Maybe others have had such devices installed longer and can add insight from their observations.
Some of the monitoring apps (like the one for the INEX brand) allow you export (eg. email) the data (date-time and voltage) and that allows you to examine it on another device. Sadly the one for the Quicklinks monitor does not.