No, you should never remove a car battery while the engine is running. This action can cause a significant voltage spike from the alternator, potentially damaging sensitive and expensive electronic components in your vehicle, such as the Engine Control Unit (ECU), infotainment system, and other modules. Modern vehicles rely on a stable electrical system where the battery acts as a crucial buffer, smoothing out the power generated by the alternator.
When you disconnect the battery with the engine on, you remove this buffer. The alternator, which is designed to charge the battery and power electrical systems, can suddenly produce a surge of unregulated voltage—a phenomenon sometimes called a "load dump." This surge can easily exceed the voltage tolerance of your car's computers.
Beyond the risk of immediate damage, you'll also cause the vehicle to stall. The battery provides the necessary power for the ignition system and fuel injectors to operate. Removing it cuts that primary power source. While the alternator can theoretically power the car, the initial interruption and voltage instability almost always lead to the engine shutting off, which can be dangerous if you're driving.
The only possible exception is for professional mechanics performing specific diagnostic procedures, and even then, it involves using specialized equipment to maintain system voltage. For any routine maintenance or troubleshooting, the correct and safe procedure is always to turn the engine off, remove the key from the ignition, and then disconnect the battery, starting with the negative terminal.
