Disconnecting Negative, either first or by itself, is sure safer than Positive first or only. And with an open end wrench! I went to help a mechanic take the battery out of a customer's car for winter storage and charging at our shop. Somehow, he either grounded the Positive through the BOX wrench, or got the BOX wrench across the two battery terminals. I forget which but the wrench about lit up across that battery. He got it loose with a rag and tossed it into a nearby kiddie pool. Well, we also drained the customer's pool, through a wrench-shaped hole melted through the bottom.
