Suspect vehicle battery failure

After reading Boggie’s problems my Bourton  has come out in sympathy  ( Mercedes 2019 automatic - known to many as the Haunted Snail) has what suspect a partial battery failure. I have power, lights , door unlocking etc, but no starter.I have used a  booster  battery ( two separate units)  to try to encourage it but to no avail.
Replacing the battery is no problem, but by disconnecting the vehicle battery does the rest of the electric system have a hissy fit ,ie radio and its associated systems ?

If you're concerned then it would be prudent to connect a 12V battery or other source of 12V dc to the cable connectors before you disconnect the vehicle battery.  It doesn't need to be high capacity, just enough to keep up the electronics temporarily while you swap over the starter battery.  Make sure to check the polarity of your temporary 12V before connecting it.

Tim

Sorry pardon, more ‘finger trouble ‘  by me.  ( as we called it in the aircraft industry). I now found out the hard way that I need to have my foot on the brake pedal before trying starting the engine. 
Something in 99% of the time I did automatically by putting my foot on the without thinking about it.
That’s the problem when you are used to the older type of vehicle that doesn’t have all these safety features of modern vehicle.
Leaning over and pressing  the Start / Stop  button doesn’t work with the interlocks on the brake pedal not being made. Another vertical learning curve for today.
Many thanks to those who replied offering suggestions very much appreciated.

Same on out Topaz auto
On trying to start there is a pic. of a brake pedal with a foot on it that illuminates on the dash!
frederic