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How to charge a car battery

Car batteries can’t read minds, yet they always seem to fail at the worst possible time. It’s when you’re late, tired, and caught in a rainstorm that nothing happens when you turn the key. Don’t panic; you’re not as stuck as you imagine. Here are several ways to charge a car battery, and how to change one if it’s truly dead.

With jumper cables

If you’re in a hurry, you will want to get juice into the battery quickly and get on with your day. Connecting jumper cables to another car with a good battery will get you moving, but there are portable jumpers available as well. The good news is that the alternator will begin charging the battery once the engine turns on. Driving for about 20 minutes should do the trick. The bad news is that if your battery can’t hold a charge anymore, you’ll have to jump it every time you want to get somewhere. We don’t recommend this option unless you really have no other choice.

Jumping a car is straightforward, but you need to follow basic precautions to avoid damage and injuries. The first step is finding the battery. It’s usually under the hood, but in some cars it’s hidden under the rear seats or tucked away in the trunk. If that’s the case, there might be a terminal in the engine bay. Locating it beforehand will save you the hassle of setting everything up and realizing your jumper cables aren’t long enough. Found it? Good, you can move on to the next step.

Put both cars in park if they have an automatic transmission, or in neutral if they have a manual transmission, and engage both parking brakes. Turn off the ignition in both cars and remove anything covering the battery terminals, including protective plastic covers and seats. Start with the positive (red) cable; connect one end to the positive terminal on the dead battery, and carefully clamp the other onto the good battery’s positive terminal. It’s usually marked with a + symbol, or with the letters POS. Then, connect the negative table to the good battery’s terminal, which is often marked with a – symbol, or with the letters NEG. Finally, connect the other end of the negative cable to an unpainted metal part in the engine bay, preferably one that is not right next to the battery. It’s very important to make sure the cables are on tight; the last thing you want is a cable coming loose as you start the engine.

If everything looks good, start the car with the good battery and let it run for a few seconds to send juice to the dead battery. Next, start the car with the dead battery. If it starts, great, but make sure to keep it running (and, preferably, drive it around for 15-20 minutes after removing the cables) because it might not start again if you don’t. If it doesn’t start, either your battery is completely dead or your problem comes from elsewhere. What happens (or doesn’t happen) when a car refuses to start tells you a lot about the problem. A clicking sound sometimes indicates a starter motor issue; the engine turning over but not firing signals a fuel delivery or a spark issue (like bad spark plugs).

With a portable charger

If time permits, or if you don’t have access to jumper cables, you’ll either need a portable battery charger or you’ll have to remove the battery and take it to an auto parts store (more on that in a moment). Investing in a portable charger is a good idea — many feature flashlights and USB ports for smartphone charging — but most parts store will top up your battery for free. To recharge via a portable charger, simply attach the positive clamp to the positive battery terminal (red to red) and the negative clamp to the negative terminal (black to black). Then, plug the charger into a regular household outlet and turn it on. Your car isn’t a laptop; keep in mind charging a battery can take eight or more hours depending on the type of charger you’re using and how much electricity it needs. Modern battery chargers usually have lights that tell you when the battery is fully charged.

Replacing a dead unit

A battery that won’t take a charge needs to be replaced with a new one. Luckily, that’s straightforward, too. You can save a good deal of cash by changing it yourself. You need basic wrenches (often a 10mm, but double-check because some cars require a different size), and we recommend buying sandpaper to clean the terminals. Keep in mind battery acid is corrosive; it can burn your skin and it will chew right through your clothes. You can avoid both by being careful, but it is always a good idea to wear gloves and old clothes you don’t care about damaging.

Put your car in park, or engage first gear if you have a manual transmission, and set the parking brake. Assuming you already found your battery, use the correct wrenches to loosen the battery terminals. Check for white or blue-ish buildup around the terminals, wipe it off using a rag, and remove any additional buildup on both the car and the battery terminals. Once they are off, you are ready to remove the battery.

Find the bolts that secure the battery to the battery tray and remove them. In some cars, a bolt holds down a metal plate near the base of the battery. In others, two bolts connected to metal rods firmly clamp down the battery. Either way, make sure not to lose anything you remove. Set everything aside and you’re ready to pull the battery out. It’s relatively heavy, so be prepared. Installation is the reverse of removal. Tighten the bolts that hold down the battery and the ones on the terminals, make sure everything looks good, and start the car. It should fire right up.

You can’t simply throw your old battery away. You need to take it to a place that will recycle it properly. Most auto parts stores will do that. Alternatively, if you are feeling adventurous, you can take your battery to a scrap metal place and get cash for it. How much they give you largely depends on the price of scrap metal, which fluctuates like the stock market. Don’t expect a fortune unless you’re bringing two dozen batteries, but anything that offsets the price of a new battery qualifies as a win in our book.

Editors' Recommendations

Ronan Glon
Ronan Glon is an American automotive and tech journalist based in southern France. As a long-time contributor to Digital…
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