Changing battery -- what happens to electronics?

Nothing else.

Hi, which car, what accessories?

On a W210 (have experiense on that), most likely on a W211 too (although it has a second battery for brakes) you need to visit workshop to clear ABS (etc.) error messages. If you have a sunroof, you need to calibrate it (simple though according to the user manual).

I used to hook a small (like motorcycle or lawn tractor size) 12 V battery to a cigarettle lighter plug (and leave the key in accessory position-- necessary on the vehicle in question) too keep things energized while cleaning battery terminals etc. just to avoid radio station resetting etc. Probably not worth the trouble but if you have a mind to. A computer power supply's 12V output ought to be sufficient to do this too provided the radio is off. Careful with the dangling end of your + battery cable.

Devices affected can include previously mentioned seats, sunroof, radio, mirrors, CD changer, as well as host of other misc trouble msgs which may crop up. These are well documented on MBWorld.org and Benzworld.org...as well as technical service bulletins. Some models seem to be more sensitive to this power down than others.