How to Charge a Dead Car Battery Step-By-Step

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An expert-approved guide on charging old vehicle batteries

Co-authored by Duston Maynes and Eric McClure

Last Updated: July 10, 2024 Fact Checked

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Charging a Dead Battery with a Battery Charger

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Jump-Starting the Battery with Another Car

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Video

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Tips

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Warnings

ARTICLE

VIDEO

Whether you accidentally left a light on overnight or you’re trying to resurrect the fixer upper in your garage, you may need to bring a car battery back to life. In order to charge a dead car battery, a set of jumper cables and a functional car with a charged battery are required. However, you may need a float charger to run a trickle recharge if the battery is especially old or hasn’t been used in a while. We spoke with professional automotive mechanic Duston Maynes to help break all of this down for you.

How to Charge a Dead Car Battery

Make sure the battery terminals are tight and corrosion-free.
Hook up the charger to your car battery's terminals.
Turn on the charger and let it charge your car's battery for 12-24 hours.

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Method 1

Method 1 of 2:

Charging a Dead Battery with a Battery Charger

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1
Grab an automotive battery charger to recharge an old battery. If you want to charge a dead battery that has potentially been degraded for some time or jumping wasn’t successful, charging it may bring the battery back to life. All you need is a remote automotive battery charger (sometimes called a float charger.)^{[1]
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- Charging with a battery charger, also known as trickle or float charging, is unique from jumping. While jumping provides a huge burst of power to restart a battery, float charging pumps the battery full of a slow and steady amount of the minimum necessary voltage (for a car battery, 6V or 12V) over a longer period of time. In theory, this will reload the battery without putting any pressure on it.^{[2]
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Connect the red lead to the red positive (+) terminal and the black lead to the black negative (-) terminal. Press the power button on the float charger to power it on.^{[3]
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If you’re charging a battery for a motorcycle, tractor, e-bike, or some other small motor, you’d use the 6-volt setting. Basically all modern car batteries are 12-volt, but read the label on top of the battery just to confirm.^{[4]
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Run the charger and wait 12-24 hours. Refer to the float chargers manual to determine how you run a float charge (these chargers often jump, as well). Once you’re on the right setting, start the float charge. Let the battery charge for at least half a day.^{[5]
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- There’s no real way to determine whether a float charge was successful or not, so the longer you wait, the more likely you’ll be to successfully charge the battery.
If the battery works, you’re good to go. However, if the battery loses voltage again in the next few trips, it’s a sign the battery is sulfated. This is a fancy way of saying it’s no longer capable of holding a charge, so it's time to discard and replace it.^{[6]
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Method 2

Method 2 of 2:

Jump-Starting the Battery with Another Car

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1
Inspect the battery to make sure it isn’t leaking. Your battery should be intact with no cracks, and should not visibly leak any battery acid. If it is leaking, you cannot safely jump or reuse your battery—replace it with a new battery.^{[7]
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- Do not attempt to jump-start your car if your battery is oozing liquid, covered in leak stains, or misshapen, as you may cause injury to yourself or others if you do so.
- Wear safety goggles and rubber gloves.
- While you're there, confirm that the cables attached to your car's battery are secure and free of corrosion. If any of cables are loose, it could be the source of your battery woes. Tighten the loose cables with a wrench, if necessary.
- If your battery cables are corroded, clean them as best as possible using a brush with stiff bristles and some baking soda.
2
Drive a functional car into position next to the dead car. Don’t let the vehicles touch one another. Ideal positions for this task are either placing the cars closely next to one another facing the same direction, or facing one another head-on, or nose-to-nose.^{[8]
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- Verify that the distance between each car's battery is close enough for the jumper cables to connect the cars together. The length of jumper cables varies greatly depending on their style and manufacturer.
- Do not attempt to connect two different pairs of jumper cables together if your first pair isn’t long enough. This can melt the jumper cables and start a fire.
Put the car in park and cut the power.
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Pop the hood on each vehicle. Use the hood release on each vehicle.
- Can’t find the battery? Duston Maynes explains that it’s probably in the trunk. Some makes and models do not place the primary battery in the engine bay.
Positive terminals will be indicated by the plus symbol (+) and the color red. Negative terminals will be indicated by a minus symbol (-) and the color black. Maynes points out that these are your jump points.
Maynes explains that the order in which you attach the jumper cables is important, so follow this pattern: First connect one end of the positive jumper cable to the dead battery, then connect the other positive end of the jumper cable to the charged battery.
In most cases, the negative jumper cable is black. Hook up the cable to the negative terminal on the working battery first.^{[9]
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Ground the remaining jumper cable. Maynes explains that you can use either the negative terminal of the dead battery, or attach the terminal to any unpainted metal on the chassis. “Generally, a good spot is like the top of the strut tower nut.” This will ground the car that contains the dead battery upon jump-starting. You can attach the ground cable to the frame, chassis, or another component that is reasonably clean and free of paint or oxidation.
- If you don’t ground the remaining cable, Maynes explains that it could cause a spike in the voltage that destroys your alternator.
Upon starting the engine, its charging system will begin to charge the dead car battery through the jumper cables.^{[10]
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This will give the dead battery time to build up a charge of its own, although it may take longer to fully charge the battery if you haven’t used it in a while.
11
Try to start the engine of the car that contains the dead battery. Maynes explains that if the jumper cables and the battery you are charging have enough power, the car engine should turn over easily and start.
- If the engine in the car with the dead battery fails to start, allow five more minutes to pass for the dead battery to charge.
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Disconnect and remove the jumper cables from each car. Do this in the reverse order in which you connected them. This will prevent sparks or an explosion from occurring.^{[11]
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- Disconnect the grounding cable first, then the cable attached to the negative terminal on the battery, then the cable attached to the positive terminal of the battery of the car that provided the jump, and finally the cable attached to the positive terminal on the previously dead battery.
This will give your car plenty of time to recharge.
This is plenty of time to give your alternator and battery time to fully recharge and synchronize. If you haven’t used the car in a long time (say, 6-12 months at least), drive for at least 20-30 minutes just to play it safe.^{[12]
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Tips

Some Ford cars have been known to suffer a power surge due to jump starting. To avoid electrical problems from this, make sure that your car heater is on with the heating fan turned up all the all the way up and turn on the defroster. If there is a surge, the fuse to the fan will blow, and having the heating/fan on will absorb excess current to avoid electrical damage.^{[13]
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Larger wire size in jumper cables allow a faster charging rate.

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Check the electrolyte level in the dead battery to make sure each cell is properly filled.

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Warnings

Never allow the positive and negative jumper cables to touch or connect with one another at the same when they are connected to the battery; especially while you are handling them. If you allow the cables to touch, you may melt the cables, damage the batteries, or even start a fire.

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Recharging batteries can create the explosive gas, hydrogen

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If you have a manual transmission, use the clutch carefully.

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