Why Is the Battery Light On?

An illuminated battery light means there’s something wrong with the car’s charging system, which primarily consists of the battery and alternator, along with some associated components such as the regulator, fuses and wiring.

Related: How Long Do Car Batteries Last?

What Parts Are in a Car’s Charging System?

One simple way to think of the components involved is that the battery starts the car and the alternator keeps it running. (An alternator is a modern version of what used to be called a generator; both are driven by the engine and produce electric power.) A more thorough explanation is that the battery can produce the large jolt of power needed to start the engine, while the alternator — which is turned by the engine via a belt — supplies the electrical power needed to keep the engine running, along with powering accessories such as the lights, wipers and radio, while also recharging the battery. Without the alternator taking over the electrical needs of the car, the battery would soon be drained of power.

Thus, if the alternator or its associated components malfunction, the battery can only keep things running for a while on its own. That amount of time varies based on several factors (it might be about 30 minutes to an hour), but it can be maximized by turning off electrical loads including the lights (in daytime), seat heaters, air conditioner, fan and radio, as well as not using the power windows. It takes a lot of electrical power to start the engine, so if the battery light comes on, it’s best to leave the engine running if at all possible — though keep in mind the clock is ticking. While the safest bet is to call for a tow truck, that might not be feasible, and you may need to drive the car home or to a shop if close by.

What Should You Do if the Battery Light Comes On?

If the battery light comes on while you’re driving, it’s best to start looking for a place to pull off the road and stop, but leave the engine running. Once you’ve stopped, check to see if there are signs that the engine is overheating — your best indications being the temperature gauge rising or reading high, or the temperature warning light coming on. If that’s the case, it’s likely due to the serpentine belt, which drives the water pump and the alternator, broke. If the engine is overheating, it can cause expensive damage, and you should turn it off and call a tow truck.

If the engine is not overheating, leave it running. That’s because the battery can keep the car running on its own for a short time, but if the battery is dying, it may not have enough power to restart the engine. Yet as long as it’s working properly, the alternator can produce the power needed as long as the engine keeps running.

The trouble is, without special equipment, you don’t know which part is bad. If there’s a battery gauge on your car’s instrument panel, check it to see if the needle is lower on the scale than usual; if so, that’s an indication the problem is with the alternator or its associated components. That’s because a fully charged 12-volt battery actually puts out about 12.6 volts, while a properly functioning alternator puts out about 13.5-14.5 volts. Thus, if the needle normally sits exactly in the middle of the gauge’s scale (where it would be reading the alternator’s 13.5-14.5 volts), it will be sitting slightly below that if the car is running off just battery power.

About the only fix you could try at the side of the road is to check to see that the battery clamps are tight, and that’s only worthwhile if you have a wrench to tighten them. However, if you can get the car home, you can also check to see if there’s corrosion (usually a white, foamy substance around the terminals) that can prevent the battery’s power from getting to the car or the alternator’s power from recharging the battery. If so, you can remove the clamps and clean them with a wire brush. It’s important that whether you’re working at the side of the road or in your garage, be aware that the corrosion, or an acid film, can burn your skin, eyes and even clothes. Use gloves or a rag when touching any of them, put on protective glasses, and be very careful what you touch afterward.

You can also try testing the battery with a voltmeter (some versions are called a multimeter because they can check several things), which can help determine whether the problem is with the battery, alternator or perhaps a fuse.

Keep in mind that batteries typically last about up to five years and may run down gradually before failing altogether. Slow cranking when you start the engine is an indication the battery is on its way out. It could also mean the alternator isn’t keeping it charged, but that should cause the battery light to go on.

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Disconnecting the Battery

Check your vehicle’s owner’s manual, shop manual or do an internet search to see if there are any special instructions for disconnecting the battery.

It’s important to note that you remove the battery clamps to clean them, but battery power will be lost to the car and cause the various memories — for the clock, radio presets, navigation system, memory power seats and other functions — to be lost. In fact, some cars might have to go back to the dealer to have the computer reset. (Keeping these memories alive is why the battery can go dead if the car is left to sit for a long period without the engine occasionally being started and let run awhile.)

There are ways you can connect another battery to the car’s electrical system before disconnecting the regular battery in order to keep the memories alive, however. A battery jumper pack with a plug that can connect to the car’s interior power outlet is a good way to do this. (A jumper pack is not the worst thing to carry in the car anyway.) However, you have to make sure the outlet is “live” in order to transfer power; you may have to turn on the ignition (but not start the engine) to make this work.

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