Wondering in which order you should disconnect your car battery? Should it be positive first or negative? How about the order to reconnect? Here's the answer!

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Order to follow for battery terminals disconnection

Always start by disconnecting the positive (red) terminal first.
Then disconnect the negative (black) terminal.
Forget about the numerous advices stating to disconnect the negative first, so you take no risk of hitting the chassis with the spanner on the positive terminal, thus generating a spark. Safety is the reverse way around, as your negative terminal connects to the car chassis and "grounds" absolutely all metal parts on the car. Although 12V is very low voltage, the good practice dictates to keep the "ground" to the lowest available voltage at all times.
See further below for more explanations.
The negative battery terminal connects to the car chassis and "grounds" everything on your car, like it is visible in this picture. To note that automakers started to use green-yellow wires for the ground, similary to the "earthing" circuits in domestic electrical installations:

Order to follow for battery terminals reconnection

It's the reverse order than for disconnection:
  • Connect Negative side first
  • Then connect Positive clamp

Which battery terminal to disconnect temporarily for intervening on the car, do some repair, or, for resetting an ECU?

It's the Positive battery terminal.
Always keep the negative connector on the battery and take out the positive clamp.
Make sure to move the positive cable and clamp away from the battery and hold it with something so it doesn't accidentally hit the battery terminal while you work on the car.
It's for this reason that automakers that do fit a quick-release, tool-free battery connector on one side of the battery, always do so on the positive terminal.
The science behind it is simple:
When disconnecting the battery clamp from its terminal, you do what a switch does, i.e. turn Off the circuit, while the switch on a DC circuit is always placed where the current exits the battery, the + terminal.
​So it's again and all about good practice!
Quick-release battery clamps are always fitted to the positive terminal:

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