Dead battery or bad alternator? How to tell

What do the battery and the alternator each do?

The battery and the alternator share one job, keeping your car's electrical system powered, but they handle different parts of it. The battery supplies the surge that cranks the engine. It also runs things like the lights and radio while the engine is off. Once the engine is running, the alternator takes over. It turns engine power into electricity, runs the electrical system and recharges the battery as you drive.

That is why the two get confused. When the alternator stops charging, the car keeps running on whatever is left in the battery until the battery is drained. AAA notes that driving on a failing alternator can also permanently damage the battery. So a car with a bad alternator often ends up with a dead battery too.

What are the signs of a dead or failing battery?

A battery problem usually shows up when you try to start the car, not while you are driving. Age matters a lot in Texas. AAA says heat speeds up the rate at which a battery breaks down, so in hot southern climates a battery typically lasts about three years, compared with five years or more up north. AAA recommends a test every year once a battery reaches its third year, and notes that some failing batteries show no obvious symptoms at all.

These are the common signs of a failing battery, according to AAA:

• The engine cranks slowly, as if it is struggling to turn over.
• You hear rapid, repeated clicking when you turn the key or press the start button.
• Corrosion has built up on the battery terminals.
• The case looks swollen or bulging. AAA notes this usually comes from overcharging, so the charging system deserves a look too. Don't jump-start a swollen battery.
• The car runs fine for a long time after a jump, then won't restart once you shut it off. The battery isn't holding a charge.
• The battery is more than three years old.

What are the signs of a failing alternator?

An alternator problem usually shows up while the engine is running. The car may start fine in the morning and then give you trouble on the drive. Watch for these signs, drawn from AAA's guidance:

• The battery or charging warning light comes on while you drive. In some cars, the check engine light or a check charging system message comes on too.
• Headlights, dash lights or the dome light dim, flicker or get unusually bright.
• The radio, screen or other electrical features start acting up.
• You notice an electrical burning smell.
• A squeal gets worse as you turn on more accessories.
• The car starts with a jump but dies again right away.
• The engine stalls while you are driving.

What does a jump start tell you?

A jump start is the simplest test there is. Once the car is running on its own, what happens next says a lot about which part is failing.

• It runs and restarts normally later: the battery was probably just drained, often by a light left on, a car that sat unused or a string of short trips.
• It runs for a long drive, then won't restart: the battery likely can't hold a charge anymore.
• It dies within minutes of removing the cables: the alternator is probably not charging.
• A single click with bright dash lights: AAA links this to a bad starter motor, so a new battery may not fix it.
• No start after three tries: per AAA, the battery has most likely failed completely.

How do you check the battery and alternator with a voltmeter?

If you have a basic digital multimeter, two readings at the battery terminals narrow it down. A low reading tells you where to look, not exactly what failed. A load test on the battery or a bench test on the alternator gives the final answer.

Keep your hands, tools, loose clothing and the meter leads clear of the belts and fan while the engine runs. OPTIMA Batteries gives these numbers as a guide:

1. With the engine off, set the meter to DC volts and touch the red lead to the positive terminal and the black lead to the negative terminal.
2. A reading of 12.6 volts or more means the battery is charged. A lower reading means it is low, though not necessarily bad.
3. Start the engine and measure again. A healthy charging system shows about 13.7 to 14.7 volts at the terminals.
4. A running reading well below that range points to the alternator, a belt, a pulley or the wiring.

What else can look like a battery or alternator problem?

Not every no-start means a bad part. A new battery won't fix a slipping belt, and a new alternator won't fix a corroded cable, so it pays to test before you buy parts. AAA and OPTIMA both point to causes that can mimic a dead battery or a failing alternator:

• Loose or corroded battery cables and connections.
• A worn or damaged drive belt or tensioner pulley, which can keep the alternator from charging the battery.
• Parasitic drain, when something like a light or an alarm keeps drawing power with the car off.
• Mostly short trips, which may never give the battery time to fully recharge.
• A car that sits for weeks, since a parked car's battery slowly loses charge on its own.

What should you do if the battery light comes on while driving?

Treat it as a sign that the car may be running on battery power alone. AAA's advice is to pull over safely, because a failing alternator can make the car stall in traffic. Get off the highway at the next exit or into a parking lot while the car still runs, rather than waiting for it to quit in a live lane. If it does stall, AAA warns that a jump may restart it but not for long.

If the car stops in a lane of traffic, or anyone is hurt, call 911 first. Once you are stopped somewhere safe, turn on your flashers and stay well clear of traffic. On a highway, never stand between the car and passing cars.

How Pit Stop helps when your car won't start

When your car won't start anywhere in the Dallas area, Pit Stop Roadside Services is open 24/7 at (214) 367-5302. You hear the price on the call before anyone drives out, and that is the price you pay for the job you described. In Dallas, a technician typically arrives in 30–45 minutes.

After a jump start, the technician can check the battery and charging system, so you leave knowing more about which part is the problem. If the battery is worn out, the technician can replace it with a new one where the car is, once the dispatcher confirms the right battery for your car and when it can get there, and once you accept the price. If the jump starts the car, you pay the quoted price even if the alternator turns out to be the real problem. If the car can't be started, the $35 to $75 service fee replaces the quote and the rest is refunded. The dispatcher tells you the exact fee for your job before you pay. The technician tells you what they found so you can take it to a shop.