can you charge a car battery with a car battery

[Perez]

Yes, you can use one car battery to charge another, but it's technically not a full "charge" in the way a dedicated charger works. The correct term is jump-starting. This process uses the good battery's energy, along with the running engine's alternator, to provide enough power to crank the dead battery's engine. Once the car with the dead battery is running, its own alternator takes over to recharge the battery.

This method is strictly for emergency starting and is not an efficient or recommended way to fully recharge a deeply discharged battery. Attempting to do so can be risky.

Jump-Starting vs. Proper Charging

Method	Primary Goal	How It Works	Best For	Risks
Jump-Starting	Emergency engine start	Transfers short, high current to crank starter motor	Getting a car running immediately after a battery drain (e.g., lights left on)	Reverse polarity (severe damage), sparks, battery explosion, electrical system damage
Using a Battery Charger	Fully restoring battery charge	Applies a controlled, low current over several hours	Deeply discharged batteries, routine maintenance, restoring battery health	Overcharging if left unattended for too long (modern chargers mitigate this)

The key risks of jump-starting involve incorrect connections. Reverse polarity—connecting positive to negative—can cause catastrophic damage to both vehicles' electronic control units (ECUs), fuses, and wiring. Always double-check the cable clamps: red to positive (+) on both batteries, black to a clean, unpainted metal surface on the dead car's engine block (a ground point), not the negative terminal.

For a battery that has been dead for days or is old, a trickle charger or smart charger is the only safe way to restore it to a full state of charge. These devices monitor the battery's voltage and adjust the charge rate accordingly, preventing damage and ensuring a complete charge, which is crucial for battery longevity.

[McLucas]

Yeah, you can jump it. It's a temporary fix to get the car started, not a real charge. Hook up the jumper cables correctly—red to positive on both, black to a good ground on the dead car. Start the good car, let it run for a minute, then try starting the dead one. Once it's running, drive for at least 20-30 minutes so the alternator can put some real charge back in. Don't just let it idle; driving charges it faster. If the battery dies again soon after, it's probably time for a new one.

[DiMatteo]

It's crucial to understand the safety risks. This process forces a high amperage flow between batteries. A single mistake can lead to sparks, battery acid leaks, or even an explosion from ignited hydrogen gas. Modern cars are packed with sensitive electronics; a wrong connection can mean a multi-thousand-dollar repair bill. If you are not 100% confident in identifying the positive and negative terminals and making secure connections, it is far safer to call for roadside assistance. They have the proper equipment and expertise.

[LeIsla]

While jump-starting works in a pinch, it's hard on both batteries and doesn't fully solve the problem. A better investment is a portable jump starter pack. You can find reliable models for under $100. They're compact, don't require another vehicle, and are much safer because they have built-in protections against reverse polarity. For a battery that's completely dead, a proper battery maintainer is the right tool. It slowly charges the battery back to health over several hours, which is what it actually needs, rather than a violent jolt of power.

[JaxonFitz]

I had to do this last winter when my battery gave out in the grocery store parking lot. A guy with a truck helped me out. You feel a mix of relief and anxiety—relief that someone stopped, anxiety that you'll mess up the cables. We got it connected, my car started after a few tries, and I was so grateful. But the mechanic later told me my battery was just old and couldn't hold a charge. The jump-start got me home, but it was a sign. It's a great community thing to help someone, but it's not a long-term solution.