When a car runs out of battery, one of the most common solutions is to use jumper cables and connect the discharged vehicle to another vehicle with a charged battery. However, there is a mistaken and dangerous belief: leaving the engine running of the auxiliary vehicle while starting is being carried out.

This practice can cause serious damage to the electrical system and the alternator of the vehicle that provides the power. In this article, I’ll explain the reasons why a battery often fails, why it’s dangerous to start a car from another with the engine running, and how to perform a safe starting process step by step.

Why is turning off the engine of the auxiliary vehicle so important?

• If the auxiliary vehicle’s engine is on, the alternator is generating a constant electrical current.
• When connecting the cables and starting the discharged vehicle, the current flow can be too strong and damage the alternator, battery, and electrical system of the auxiliary vehicle.
• Turning off the motor prevents overloading and protects electrical components.

Reasons why a battery often fails

Car batteries are designed to withstand charge and discharge cycles, but over time they can deteriorate due to various factors. The most common reasons for a dead or damaged battery are:

1. Natural wear and tear

• The average lifespan of a car battery is between 3 to 5 years. After this period, the ability to retain charge decreases markedly.

2. Extreme temperatures

• Extreme cold can reduce the battery’s ability to generate the current needed to start the engine.
• Excessive heat also accelerates the evaporation of electrolytes and the deterioration of the battery’s internal plates.

3. Idle power consumption

• If you leave lights, radio, or devices plugged in while the engine is off, the battery can quickly discharge.
• Even systems such as alarms or proximity sensors can slowly drain the battery charge over time.

4. Alternator Problems

• The alternator is responsible for recharging the battery while the engine is running.
• If the alternator is faulty, the battery will not charge properly and will discharge quickly.

5. Sulfation and corrosion

• Sulfation (buildup of lead sulfate crystals) prevents the battery from retaining and delivering energy.
• Corroded terminals also hinder the flow of current, causing start-up failures.

Essential Steps for Successful Cabling

If you need to start an unloaded vehicle with jump cables, follow these steps to ensure a safe process and prevent damage to the electrical system:

1. Turn off the auxiliary vehicle’s engine

• Before connecting the cables, make sure the auxiliary vehicle is OFF.
• If the engine is running while making connections, the alternator can suffer an overload or voltage spike, damaging the electrical components of both vehicles.

2. Connect the cables in the correct order

1. Connect the red (positive) wire to the positive (+) terminal of the dead battery.
2. Connect the other end of the red wire to the positive (+) terminal of the charged battery.
3. Connect the black (negative) wire to the negative (-) terminal of the charged battery.
4. Connect the other end of the black wire to an unpainted metal part of the engine or chassis of the discharged vehicle (NEVER to the negative terminal of the discharged battery, to avoid sparks).

3. Start the unloaded vehicle

• Once the cables are connected, start the unloaded vehicle.
• If the vehicle doesn’t start after a few seconds, wait a minute and try again.
• Avoid keeping the starter running for more than 5 seconds at a time so as not to damage the starter.

4. Disconnect the cables in reverse order

1. Disconnect the black wire from the chassis or ground point of the unloaded vehicle first.
2. Disconnect the other end of the black cable from the charged battery.
3. Disconnect the red wire from the positive terminal of the charged battery.
4. Finally, disconnect the red cable from the dead battery.

5. Leave the engine running

• Leave the newly started vehicle running for 10 to 15 minutes for the alternator to charge the battery.
• If the vehicle shuts down again quickly, the problem may not be just the battery, but a fault with the alternator or electrical system.

Tips and recommendations

• If the battery is completely discharged or shows signs of damage (such as swelling or excessive corrosion), it’s best to replace it rather than try to recharge it.
• Use good quality jumper cables with enough capacity to support the necessary current.
• If you have to start a car frequently, the problem may be with the alternator or a power leak.
• Avoid starting vehicles with sensitive electronics, such as hybrid or electric cars, using jump cables, as this could damage the internal circuitry.
• If after starting the car, the dashboard lights flicker or the engine runs irregularly, take the vehicle to a workshop for an electrical system check.