I'm in the throws of putting together a rear under run on our Compass...and inorder to get things straight & level,it will involve having to tack weld it together whilst bolted/clamped to the vehicle chassis,
Now,years gone by we used to just disconnect the battery(..and if I recall right,the alternator on some vehicles)..but nowadays with all the electronic gadetry,I'm unsure what else needs isolating....
The last thing I want to do is to end up "frying" something expensive!
Similarly,I don't want to disconnect something that then needs to be "re-booted" by the Peugeot main dealer..
as silverwood says,but to be sure both + & - if you are unsure if it is Poss or Neg ground.
------------- Think this year is to follow old meet friends for 2014.
If you cant do someone a good turn,don`t do them a bad one,its nice to be nice you know,and little things mean much more later in life.
..although now,I think I'll design it so that the rear "bumper" section will be a bolt-on component...that way it can be marked-up and drilled off the job(..it could also be removed easily,if it ever takes a "nudge"! )
You should really disconnect the alternator too.
You can buy 'surge protectord' that are supposed to provide protection against voltage spikes when welding but welding very near any sensitive electronic equipment can cause problems.
you can buy a gadget for about a tenner that plugs in to your cigarette ligher that will save any codes or setting, saving you having to redo them,attach to an external battery, diconnect the main one and leisure, and you should be ok, have done a lot of welding over the years even cars with ecu,s without a problem. kieran.