Everyone knows that when you disconnect a car battery, you remove the black negative cable first. Also, when you connect a new battery, you start with the red positive cable. But why? You always disconnect the negative terminal first, because it is grounded to the car's entire chassis and body. Removing it first kills the circuit, which means if your wrench slips while you're loosening the positive side, it won't spark fireworks or a fried electrical system. If you start with the positive terminal instead and your tool hits any part of the metal frame- congratulations, you've just built a short circuit, high-voltage handshake. If your wrench touches the car's body while you're doing this, you could send a surge through the system that knocks out anything from your fuses, radio/infotainment, computer or worse, a fire. It's not a best practice, it's basic physics. Electricity needs a path. Remove the ground first (the negative wire that's connected to the car's body), and you break that path. When it's time to install the new battery, you reverse the process- positive goes on first, negative goes on last. Get this wrong, and you're rolling the dice with your car's sensitive electronics. It's the same as hooking up the jumper cables- positive first then the negative. Before buttoning everything up, always check the terminals for corrosion and clean the battery terminals, snug down the clamps, and double-check your connections. One wrong move here doesn't just make for a bad day- it can make for an expensive one.
This is good advice for the general public that wants to replace a bettery of something else that is not mechanical inclined and is not very car smart.
I have always done the positive one first to avoid having the ENTIRE CAR energized at 12V positive, but your method make it so only the negative terminal is a potential shock location, unlike my method, which makes the entire car a potential shock location. And, yes I have divoted my battery terminal with a wrench accompanied by sparks. Hmmm. I learned some new stuff today.
As an addendum, with 120+ volt wiring, the "hot" lead is always removed and the neutral, grounded phase lead is never removed unless necessary for some other reason, as removing the neutral wire from something means you now have the entire device with the "hot" wire still attached as a potential live thing, so the "hot" aka "not attached to ground anywhere in the system" wire is removed first and attached last.
