I've welded on hundreds, and I do mean hundreds of vehicles including my own CL65 and have never once disconnected a battery or ECU to mig weld on a vehicle. Never had an issue (OK, once caught a Camaro on fire, long story) but no electrical issues. I'm not afraid to weld on my own $180K Mercedes with 8 computers so I don't think you should you be worried about your ATV.
It doesn't hurt to disconnect the + & - leads of the battery. Also doesn't hurt to disconnect the ECU. Sure, it might take you an extra 15 minutes or so, but on the off chance that something goes awry... You'll be wishing you had....
The ECU on the XP's is mounted on the upper frame crossmember in front of the radiator. You can see part of it in the lower left corner of this pic. I believe you should be able to disconnect the plug on the ECU itself... Never tried it though.
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