when putting a car battery which terminal first

[DiAndrew]

Always connect the positive terminal first when installing a car battery. This is the fundamental safety rule to prevent a dangerous short circuit. A car's chassis is electrically connected to the negative terminal, essentially making the entire car body a ground point. If you were to connect the negative terminal first and your wrench accidentally touches any metal part of the car while tightening the positive cable, you would create a direct short circuit from the battery positive to ground. This can cause severe sparks, heat, damage to the battery and electrical system, or even an explosion.

The correct, safe sequence is straightforward:

1. Connect the positive (red/+) cable to the positive terminal.
2. Connect the negative (black/-) cable to the negative terminal.

When removing a battery, the order is reversed: always disconnect the negative terminal first, then the positive. This follows the same principle of isolating the circuit from the ground first, eliminating the risk of a short if your tool contacts the chassis.

Beyond the basic order, a few extra steps ensure a safe and successful installation. Always wear safety glasses. Before installation, clean the battery terminals and cable clamps with a wire brush to ensure a good electrical connection, which helps with reliable starting. After both terminals are securely connected, a light coating of petroleum jelly or dedicated anti-corrosion spray on the terminals can prevent future corrosion. Finally, double-check that the battery is secured firmly in its tray to prevent vibration damage.

Safety & Installation Step	Key Rationale	Common Risk if Omitted
Connect Positive Terminal First	Prevents short circuit if tool contacts chassis.	Major sparks, battery explosion, electrical system damage.
Disconnect Negative Terminal First	Isolates the circuit from ground before handling live positive cable.	Same risk of short circuit during removal.
Wear Safety Glasses	Protects eyes from potential battery acid spray or sparks.	Risk of serious eye injury.
Clean Terminals & Clamps	Ensures maximum conductivity for reliable starting.	Poor electrical connection, difficulty starting, premature failure.
Apply Anti-Corrosion Coating	Prevents buildup of conductive corrosion on terminals.	Voltage drop, starting issues, need for frequent cleaning.
Secure Battery in Tray	Prevents physical damage from vibration and movement.	Battery case damage, internal short circuits, terminal damage.

[VanLeonardo]

I learned this the hard way years ago. My dad always said, "Red before black, you'll be right back." Meaning, connect the red positive cable first. If you do the black negative one first and your wrench slips and hits the car's frame, you'll get a huge, scary spark that can weld your tool to the metal. It's all about avoiding that direct short to ground. Just remember: on, positive first. off, negative first.

[StAdelyn]

Think of it as controlling the circuit's path to ground. The car's frame is the ground, connected to the negative terminal. By connecting the positive terminal first, the circuit isn't complete yet. The final step—connecting the negative—is safe because the wrench is already on the grounded terminal. Reversing this order makes the entire chassis live with the positive connection, turning a simple slip into a major hazard. It’s a simple step for a critical safety reason.