No, it is not safe to disconnect a car battery while the engine is running. The car’s electrical system relies on the battery to provide a stable voltage and power supply. Disconnecting the battery while the engine is running can lead to a variety of issues:

1. Electrical Damage: Disconnecting the battery while the engine is running can cause voltage spikes and fluctuations in the car’s electrical system. This can potentially damage sensitive electronic components such as the alternator, voltage regulator, and various control modules.
2. Surge of Current: Modern vehicles have complex electronic systems that manage various functions such as engine control, emissions control, and safety features. Disconnecting the battery can lead to sudden fluctuations in electrical current, which could potentially damage these components.
3. Stalling or Engine Shutdown: Disconnecting the battery while the engine is running can cause the engine to stall or shut down. This is because the alternator, which generates electrical power, requires the battery to provide a stable load. Removing the battery disrupts this balance and can lead to a loss of power.
4. Voltage Spikes: Disconnecting the battery can cause voltage spikes that might damage sensitive components like the vehicle’s computer (ECU) or other electronics.

If you need to perform maintenance on your car’s battery or electrical system, it’s recommended to turn off the engine before disconnecting the battery. Always follow the manufacturer’s guidelines and recommendations when working on your vehicle’s electrical components to ensure safety and prevent potential damage.

Yes, it is generally safe to disconnect a car battery when the vehicle is not in use, especially if you plan to leave the car unused for an extended period of time (several weeks or more). Disconnecting the battery can help prevent battery drain and extend the overall lifespan of the battery. However, there are a few important points to keep in mind:

1. Safety Precautions: When disconnecting a car battery, always follow proper safety precautions. Wear safety goggles and gloves to protect yourself from any potential sparks or acid exposure. Start by turning off all electrical components in the car, including lights, radio, and accessories.
2. Negative Terminal: If you’re going to disconnect the battery, start by disconnecting the negative terminal (usually marked with a “-” sign) before disconnecting the positive terminal. This helps prevent accidental short circuits and reduces the risk of sparks.
3. Reconnection: When you’re ready to use the car again, reconnect the battery in the reverse order: positive terminal first and then the negative terminal. Make sure the terminals are securely tightened.
4. Memory Loss: Keep in mind that some modern vehicles have complex electronic systems that might lose certain settings or require recalibration if the battery is disconnected. This could include things like radio presets, clock settings, and engine control module adaptations.
5. Security Systems: Disconnecting the battery may also affect the car’s security system. Some vehicles have anti-theft systems that require inputting a security code after the battery is reconnected.
6. Battery Maintenance: If you’re disconnecting the battery for storage purposes, it’s a good idea to periodically check the battery’s charge level. Batteries can self-discharge over time, so using a battery maintainer or trickle charger can help keep the battery charged and in good condition.

Remember that disconnecting the battery is just one approach to preventing battery drain during long periods of inactivity. Alternatively, you can also use a battery maintainer or trickle charger to keep the battery charged without disconnecting it.

Always refer to your vehicle’s owner’s manual for specific instructions and recommendations related to battery maintenance and disconnection.

Yes, you can perform a basic test of the alternator by disconnecting the battery while the engine is running. This is commonly known as the “battery disconnect test” or “alternator output test.” However, I must emphasize that this test should be performed carefully and only for a short duration, as extended disconnection of the battery can lead to potential electrical and electronic system damage, as mentioned earlier.

Here’s how the test is done:

1. Safety Precautions: Before attempting the test, make sure you’re in a safe location and following safety protocols. Wear gloves and safety goggles, and be cautious of moving parts in the engine bay.
2. Engine Running: Start the engine and let it idle.
3. Disconnect Battery: With the engine running, carefully disconnect the negative terminal of the battery. Be cautious to prevent the terminal from touching any metal parts of the car.
4. Observe Engine Behavior: Observe the engine’s behavior immediately after disconnecting the battery. If the alternator is functioning properly, the engine should continue to run smoothly without stalling.
5. Turn Off Accessories: To reduce the load on the alternator, turn off all electrical accessories such as lights, air conditioning, and the radio.
6. Voltage Check: Use a multimeter to measure the voltage across the battery terminals while the engine is running. A healthy alternator should provide a voltage reading of around 13.5 to 14.5 volts. If the voltage is significantly lower or higher, it could indicate an issue with the alternator’s output.
7. Reconnect Battery: Once the test is complete, carefully reconnect the negative terminal of the battery. Avoid creating sparks when reconnecting the terminal.

It’s important to note that while the battery disconnect test can provide a basic indication of the alternator’s output, it’s not a definitive diagnostic method. A more accurate assessment of the alternator’s health would involve using specialized equipment to measure the alternator’s output and perform various tests under different loads.

If you suspect issues with your car’s alternator, it’s recommended to have a professional mechanic perform a comprehensive alternator and electrical system test to accurately diagnose the problem.