I was working on the car on Sunday with doors open & rear tailgate up. Rather than disconnecting the battery I decided to use my new CTEK in supply mode. I connected it directly to the battery terminals with the battery in place, selected supply with the mode button and carried on with the work.

Some time later ( perhaps a couple of hours ) car shuts off with a dead battery.

What didn’t I do ? Perhaps I’ve not correctly understood what supply mode is for?

Words of wisdom gladly accepted. TDV8 4.4 2012

Supply Mode is exactly that, the MXS-7 will act as a constant power supply providing 13.6V and up to 5A.
it’s similar to the “Float” (final) phase in the normal charging cycle, except it will not turn off & on as required, it stays on permanently, the idea being that you use it to prevent the battery draining from a constant drain, say you were using a 12V polisher or other 12V tool from one of the car 12V sockets.
Did the battery get hot or gas in this time? The MXS-7 should not cause damage to the battery used in this way, the battery should not demand current from the CTEK unless there is demand from the vehicle, either by equipment (or parasitic drain).
I can only suggest the battery was already on its way out, and you may not have noticed if habitually leaving the CTEK connected to the vehicle, it would mask the weak battery. Other more experienced forum users may offer further insight for you hopefully. “If it ain’t broke, don’t fix it” -where’s the fun in that?

I have the same charger as you. It will output a steady 7amps at 13.6v. Therefore if your battery went flat in a few hours, it was likely weak already, and the draw from the car during that time was well over 7amps. Which it could have been quite easily. Hope that helps.