Disconnect ALL battery leads and make sure the positive leads are not touching ground. Then disconnect the alternator.
The main thing to remember is ground loop induced current. Which means connect the welder as close as possible to the welding site. Process also matters, SMAW (stick) is the least problematic. FCAW/GMAW are next. Worst is GTAW with HF non-contact start. This process can induce radio frequency through the leads without contact. I've fried electronics without contact, they were just in the loop of the welder lead!
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