tldr: My theory is the TCU battery only charges when the vehicle is running. If you get the SOS System Not Available warning, try going for a long drive to charge the battery.

I replaced the small TCU battery in my car over a year ago. I had been experiencing the SOS System Not Available warning and with my car being a 2016, figured the battery was running out of life. After the battery change, the warning went away for a few months, but then it returned. I assumed there was something else wrong with the system, since the battery was still fairly new. For background, my car is a sunny weekend only car, and most of my trips are under an hour and only 2-3 times per month. In addition, the car sits all winter to avoid snow and road salt. I keep the car on a CTEK maintainer between drives. Recently, I took a 3 hour drive to NCM Motorsports Park. As usual, the SOS warning came on when I started the car, something I have learned to ignore. About 2 hours into the drive, I noticed the warning triangle was no longer visible. I stopped for gas, started the car back up and still no warning. I put a total of about 600 miles on the car that day and the SOS warning never came on again. It's been about 3 weeks since that trip and I have since done several local trips with my car and the SOS warning has still not appeared. My theory is that the TCU battery only charges while the car is running. Since I normally do short trips, the car doesn't have enough time to charge the TCU battery and it gradually loses charge until it causes the SOS System warning.

Does anyone have any evidence to support or refute this theory?

I replaced mine a few years ago and got the warning a few times shortly thereafter, but that went away after more driving. I'd say that's evidence in support.

I'm in the support group, too. I think it's a cunning ploy by Jaguar to encourage you to drive the car! Who needs encouragement?!

I got the SOS System Limited Functionality warning today and joined the support group. My car is a 2018 so I am assuming the battery just needs replaced. I still have a few months of CPO warranty left. Does anyone know if replacing the battery is covered under the CPO warranty? If not I will replace it myself.

If it is covered by warranty, the dealer will probably just replace the whole TCU. Thus you'll get 4G if the current one doesn't support it (not sure when they switched from 3G TCUs).

A quick update since my original post. I made it about 6 weeks without the SOS warning showing up at all. I have only driven the car for short drives about once a week. Over the weekend, I started the car and the warning popped up. After 2-3 minutes, the light went away and never returned. I am increasingly convinced that the TCU battery only charges when the car is running, so a CTEK or other main battery maintainer does nothing to keep the TCU battery topped off. As someone else stated, this seems to be Jaguar's way of reminding me I need to drive my car more.