How to charge a car battery
You can charge a car battery three ways: with a battery charger plugged into a wall outlet, by driving the car so the alternator recharges it, or by jump-starting it from another vehicle. The method you choose depends on whether your battery is completely dead, how much time you have, and whether you have the right equipment on hand. A wall charger is slowest but safest; driving is free but only works if the battery has enough charge to turn the engine over; jump-starting is fastest but requires another car and jumper cables.
Before you start, make sure the battery terminals are not corroded so badly that you cannot reach them. If white, blue, or green crusty buildup covers the posts, clean it off with a wire brush and a mixture of baking soda and water, then dry everything thoroughly. A corroded connection will prevent charging no matter which method you use.
Key Takeaways
- A wall-outlet battery charger takes 4 to 24 hours depending on how dead the battery is, but it is the safest method and requires only the charger, cables, and an outlet.
- Driving the car for 20 to 30 minutes lets the alternator recharge the battery, but only works if the battery has enough power to start the engine in the first place.
- Jump-starting from another vehicle takes 5 to 10 minutes and requires jumper cables, but the other car's battery must be in good condition and the cables must be connected in the correct order.
- After any charging method, test whether the battery holds a charge by turning on the headlights and interior lights for a few minutes to confirm they stay bright.
Using a battery charger at home or in a garage
A battery charger is a box that plugs into a standard wall outlet and connects to your battery terminals with two cables—red for positive, black for negative. Most chargers sold at auto parts stores cost between $30 and $150 and come with instructions specific to that model. Before you plug anything in, read the manual for your charger, because some require you to remove the battery from the car and others allow you to charge it in place.
Set the charger to the correct voltage for your battery. Most car batteries are 12 volts, but some older or specialty vehicles use 6 volts. Check your owner's manual or look at the battery itself—the voltage is printed on the top or side. Connect the red cable to the positive terminal (marked with a plus sign or the letter P), then connect the black cable to the negative terminal (marked with a minus sign or the letter N). Make sure both connections are tight. Plug the charger into the wall outlet and turn it on. The charger will show a light or digital display indicating it is running.
Leave the charger connected for the time recommended in its manual. A standard charger typically takes 4 to 24 hours depending on how completely discharged the battery is. Some chargers have a "fast charge" setting that speeds this up to 1 to 2 hours, but fast charging generates heat and can shorten battery life, so use it only when you are in a hurry. When the charger indicates the battery is full, turn it off, unplug it from the wall, and disconnect the black cable first, then the red cable. This order matters because disconnecting black first prevents sparks.
Recharging by driving the car
If your battery has enough charge to turn the engine over and start the car, you can recharge it by driving. The alternator—a component that generates electricity while the engine runs—will restore charge to the battery as you drive. This method is free and requires no equipment, but it only works if the battery is not completely dead.
Start the car normally and drive for at least 20 to 30 minutes at highway speeds or steady city speeds. Avoid stop-and-go driving in heavy traffic, because the alternator charges most efficiently when the engine is running at a consistent speed. If you have a short commute, take a longer route or drive around for an extra 15 minutes. Turn off the radio, air conditioning, and other electrical accessories while driving to let the alternator focus its output on the battery rather than powering those systems.
After you finish driving, turn off the engine and wait a few minutes. Then turn on the headlights and interior lights for a minute or two. If they are noticeably brighter than they were before you drove, the battery has taken a charge. If they are still dim, the battery may be too damaged to hold a charge, or the alternator may not be working properly—in either case, you will need a charger or a replacement battery.
Jump-starting from another vehicle
Jump-starting uses another car's battery to give your battery enough power to start your engine. Once your engine is running, your alternator takes over and recharges your battery. You will need jumper cables (usually $15 to $40) and another vehicle with a working battery. The other car should be parked close enough that the cables can reach both batteries comfortably—usually within 5 to 10 feet.
Turn off both cars and open both hoods. Locate the positive and negative terminals on both batteries. Connect the red cable to the positive terminal of the dead battery first, then to the positive terminal of the working battery. Connect the black cable to the negative terminal of the working battery, then to an unpainted metal surface on your car's engine block or frame—not directly to the negative terminal of your dead battery. This final connection to bare metal instead of the battery terminal reduces the risk of sparks near the battery.
Start the working car and let it run for 2 to 3 minutes. Then start your car. If it does not start after 30 seconds, wait another minute and try again. Do not crank the starter for more than 30 seconds at a time, because this drains the other car's battery. Once your car starts, let both engines run for another minute or two. Then disconnect the cables in reverse order: black cable from your car first, black cable from the working car, red cable from the working car, and red cable from your car last. Drive for at least 20 minutes to let your alternator recharge the battery.
What to do if the battery will not charge
If you have tried charging with a wall charger and the battery still will not hold a charge, or if the charger shows no activity at all, the battery may be dead beyond recovery. Batteries typically last 3 to 5 years, and age is the most common reason they stop accepting a charge. Extreme cold also temporarily reduces a battery's ability to charge, so if you are in freezing weather, move the car to a warm garage and try again after a few hours.
If the charger shows activity but the battery drains again within hours of charging, the problem may be a parasitic drain—an electrical component drawing power even when the car is off—or a failing alternator that is not recharging while you drive. These issues require a mechanic to diagnose. If you need to drive the car before you can get it to a shop, jump-start it and drive directly to the mechanic without stopping, so the alternator has continuous time to charge the battery.
Preventing future battery discharge
A battery discharges fastest when you leave lights on, run the radio or air conditioning with the engine off, or leave the car parked for weeks without driving. To avoid a dead battery, turn off all lights and accessories before you exit the car. If you will not drive the car for more than two weeks, disconnect the negative cable from the battery terminal to stop parasitic drain. Reconnect it when you are ready to use the car again.
In cold climates, park in a garage if possible, because cold reduces battery performance. Keep the battery terminals clean by checking them every few months for corrosion and wiping them with a dry cloth. If corrosion appears, clean it with baking soda and water as described earlier. A battery in good condition and kept clean will last closer to 5 years than 3.
Frequently Asked Questions
Can I charge a battery while it is still in the car?
Yes, most wall chargers are designed to charge a battery in place. Make sure the car is off and all lights and accessories are turned off before you connect the charger. Some chargers require you to remove the battery, so check the manual first. If you are jump-starting, both cars must be off before you connect the cables.
How do I know if my battery is completely dead?
A completely dead battery will not turn on the headlights, interior lights, or dashboard lights at all, and the engine will not crank when you turn the key. If the lights are dim but still on, or if the engine cranks slowly but does not start, the battery has some charge left and may respond to driving or a quick jump-start.
Is it safe to charge a battery indoors?
A wall charger is safe to use indoors in a garage or basement as long as there is good ventilation and no open flames nearby. Batteries release hydrogen gas during charging, which is flammable but disperses quickly in open air. Never charge a battery in a sealed room or near a furnace or water heater pilot light.
What if the jumper cables get hot while I am jump-starting?
Hot cables usually mean the connection is loose or corroded. Stop when ready, turn off both cars, and check that both ends of each cable are tight and making full contact with the terminals. Clean any corrosion with a wire brush. If the cables are still hot after you reconnect them, do not attempt the jump-start—the battery or cables may be damaged.
Can I overcharge a battery with a wall charger?
Most modern chargers have an automatic shutoff that stops charging when the battery is full, so overcharging is unlikely. Older chargers without this feature can overcharge if left connected too long, which generates excess heat and can damage the battery. Check your charger's manual to see if it has automatic shutoff, and if not, set a timer to remind you to unplug it.