Bad Alternator or Battery? How to Tell Before You Buy

A bad alternator or battery can look the same, from a car that won't start to the battery warning on the dash, which a weak battery can light too. A battery stores the charge and an alternator makes it, and only a test tells you which one stopped.

Bad alternator vs bad battery: how do you tell?

A car that dies once the jumper cables come off points at the alternator, while no lights at all points at the battery or a connection. Most other signs overlap, because either failure leaves the car short of electricity the same way.

A handful of signs of a bad alternator or battery do lean one way:

A squeal, whine or grind that rises with engine speed is the belt drive or the alternator's bearings. Our guide to a whining noise when accelerating covers it.

The alternator, and why it whines

Car won't start: battery, alternator or starter?

A jump start tells you the most. If the car starts on the cables and dies soon after they come off, nothing is replacing the charge, which points at the alternator.

If it starts and keeps running, the battery was flat, and maybe worn too, and East Penn says a flat one has to be charged before a load test means anything. No lights and no click points at the battery or a connection, and one loud click with bright lights points at the starter, which our car won't start guide covers.

Is it safe to drive with a bad alternator?

Only as far as a safe place to stop, because once the alternator stops charging, the car runs on what's left in the battery. The car may then stall, Jeep's and Ram's manuals warn, and Kia's and Hyundai's say the steering can turn heavy.

Chevrolet's Cruze manual allows only a short drive with the accessories off. If the battery light comes on, stop somewhere safe and check the belt, and see a dealer at once if it's loose or broken, as Nissan's manual puts it.

If the car stalls or the steering turns heavy, call for roadside help rather than drive on. Our car shuts off guide covers a stall.

How can you test the battery and alternator for free?

The big auto parts chains publish battery, alternator and starter testing as a free walk-in service, and one chain's store page says it tests all three on the car or off it. Call ahead before a long drive, because services vary by store.

Go in this order, and stop at the first step that gives you the answer:

1. Open the hood and look at the terminals. Crusty or loose, clean and tighten them, then see whether the problem goes away. Cost: nothing, and you never leave the car.
2. If you own a multimeter, take two readings in the driveway. Engine off and rested, then engine running, as the multimeter section below explains. Cost: nothing, if you already own the meter.
3. Get the free test at a parts counter. Drive there only if it's close, or carry the battery in. Ask for the charging system test, not just the battery test, because it checks the alternator. Cost: nothing, against the price of the wrong part.
4. Buy the part the test names, and only that part. If a shop quotes both at once without a test, ask what the charging system read. Before you pay, run the car through our free VIN checker for recalls on its model year.

When you work on the terminals, wear eye protection, keep sparks and flames away, and take the grounded (negative) cable off first and put it back last. If a quote arrives with two parts on it, our guide to reading a repair estimate covers what else to ask.

A load test draws hard on the battery and watches the voltage, and the charging test then checks the alternator. East Penn and Delco Remy, two battery and alternator makers, load the battery at half its cold-cranking amps (CCA), its cold-weather rating, for 15 seconds.

Don't pull a battery cable with the engine running to test the alternator, an old trick that can backfire. Denso says disconnecting the battery with the engine running, or during a jump start, can damage the alternator's diodes, which feed the battery.

Drive it there, or carry the battery in

How do you test a battery and alternator with a multimeter?

Take two readings, engine off and then engine running. A basic multimeter is all it takes, in your own driveway:

• Engine off, and rested. A full battery reads 12.6 volts if it's a flooded one and 12.8 if it's AGM, says East Penn, and one under 12.4 volts needs charging. A battery just charged carries a surface charge, so a reading straight after a drive can flatter it.
• Engine running. Many newer cars lower the running voltage on purpose to save fuel, so read it with the lights and blower on, where it should rise. A reading of 13.8 volts or more clears Delco Remy's first check, and one under 13.0, the bottom of Denso's window, points at the charging side. One over 14.5, its top, points to a failed regulator, among other causes, and one in between needs the free charging test.

A battery that reads fine at rest but sees no rise when the engine starts points at the charging side, not the battery. A real charging fault can also light the battery lamp or store a code.

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Why does my new battery keep dying?

Either the alternator isn't recharging it, or something drains it with the key out, and a failing alternator can be that drain too. A test with the engine running can miss a key-out drain, so ask for a parasitic draw test, which measures it with the key out.

Short trips, and accessories left running with the engine off, can flatten a battery too, as the 2017 Altima manual warns. And on some newer cars the battery monitor has to relearn a new battery: the 2018 Escape needs 8 hours parked and asleep after a replacement.

A battery near the end of its three to five years that fails a test has simply worn out. The same result on a one-year-old battery points at the charging side, and the alternator has no replacement interval to go by.

What can mimic a bad alternator or battery?

Look at the path between the battery and the alternator first, because four other faults can pass for a bad alternator or battery. The charge runs from the alternator to the battery and out to the starter, and each of these blocks it along the way:

• A corroded or loose terminal. East Penn lists loose cable connections and corrosion among the first things to check, and Denso cleans the terminals before it tests the charging system. Cleaning them costs nothing.
• A bad ground strap. The strap from the engine or body to the battery's negative side corrodes or works loose. Delco Remy tests the cables and grounds for voltage drop and replaces any with too much.
• A loose or broken belt. The belt turns the alternator, so a slipping or snapped one stops the charging as surely as a dead alternator. Nissan's owner's manual says to turn the engine off and check the belt when the charging light comes on.
• A parasitic drain. A good battery, a good alternator, and the car still flat after sitting means something is drawing current with the key out. That's a diagnostic job rather than a part, and it's the one case here where a shop earns its hourly rate.

Should you replace the battery and alternator together?

Not without a test, because guessing wrong between the battery and the alternator costs money in both directions:

• Guessing battery when it was the alternator. The fault is still there, and the bad alternator can flatten or damage the new battery too (the FAQ below has Denso's reason).
• Guessing alternator when it was the battery. That money goes on a car that only needed a battery, and the battery still has to be bought.

Sometimes it is both, though. A battery that a faulty alternator left flat may need replacing as well, even after the alternator is fixed, and the test shows whether it does.

How much does a new alternator or battery cost?

For a battery the part is most of the bill, while an alternator adds labor that depends on how hard it is to reach. Get the free test first, so the part you buy is the one it names.

Ask for the total with labor, tax and shop fees before you agree. A new and a remanufactured alternator can cost about the same, so ask which one the quote is for.

Before you buy a battery, check the old one's warranty and ask about the core charge, which comes back when you return the old battery. GM's own store, for one, sells its battery with a 42-month, unlimited-mile warranty on the battery itself.

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Common questions

Asked most often

Can a bad alternator ruin a new battery?

It can damage one. Denso lists a failed regulator among the causes of a charging voltage above 14.5, so test the charging system before you fit a new battery.

Is the battery light on the dash about the battery?

Mostly not, since the owner's manuals tie it to the charging system, and Ford's says the battery isn't charging properly. Denso adds that a weak battery or a low idle can light it too.

What causes an alternator to go bad?

Parts inside it wear with use, and the conditions it works in decide how fast. We set them out, with the mileages they reach, in how long an alternator lasts.

This article is general information, not advice about your specific vehicle. See the disclaimer.