Just taken my 2015 V6 S in for a service - I spoke about a "low battery" warning I'm getting even though i'm permanently on a CTEK showing full charge. Battery fairly new, no OBD II issue (red warning triangle goes out) Service receptionist said "it could be the 2nd battery"
Question - I thought my model didn't have a second battery? If it does, and it's dying, would it trigger the warning message? What does it do - is it just for stop/start?
Thanks for any info

The F-Type switched to a single battery system in 2016 so you should have the older two battery system. But yours just might be a MY 2016 built in 2015 with the later single battery system.
Easy enough to check, the second battery is small (motorcycle size) and sits on the right hand side of the boot/trunk up against the rear on the floor of the boot under the carpet. At least on the coupe that's where it is, I think it's the same for the 'vert but I'm not 100% sure of that.
The second battery is purely for the stop/start system and is not needed or used for anything else so you can safely disconnect it or remove it altogether with no effect on anything other than to permanently disable the stop/start system.
The secondary battery does not restart the car after the system shuts the engine down, the main battery is still used to power the starter motor. Instead the secondary battery is used to power the peripherals - lights, HVAC, infotainment etc - when the system commands a restart as the main battery needs all the amps it can muster to fire the starter motor. Then once the engine restarts the main battery takes over again and the secondary battery goes back to sleep.
The secondary battery does not trigger a low battery warning even if it is dead or dying so something else is triggering the warning.

Excellent information, thank you