Ten minutes and a voltmeter before you buy another battery

Quick answer: With the engine off, a healthy battery reads about 12.6 volts. With the engine running at around 2,000 rpm, the alternator should push it to 13.8 to 14.7. If the running number sits near the resting number, the alternator isn't charging, and a new battery will just die again.

The mistake that costs you twice

The car won't start Monday. You jump it, drive to the parts store, and the guy at the counter says the battery's weak. You buy a new one and it works great. By Thursday it's dead again.

That battery was never the problem. It was the victim. A failing alternator stops feeding the system, the battery covers the gap until it's empty, and the car quits. Swap the battery and you've reset the clock on the same failure. You paid for a fresh victim.

A digital multimeter costs about what a tank of gas does. Buy one. You'll use it for the rest of your life, and it settles this question in the time it takes to drink a coffee.

Set it up

Turn the dial to DC volts, the 20 setting if it's not auto-ranging. Pop the hood.

Touch red to the positive post and black to the negative. Use the metal post itself, not the cable clamp, because corrosion on a clamp lies to you. If there's white or blue crust on the terminals, scrape it off with a wire brush first. That crust alone can mimic a dead battery.

Two safety notes. Take off any ring or watch before you reach near the terminals, because metal across a battery makes a bad day. And keep your hands, sleeves and the meter leads away from the belt and the fan whenever the engine's running.

Test one: the battery at rest

Engine off, at least an hour since you drove it. If you just parked, turn the headlights on for a minute and shut them off. That burns away the surface charge that makes a tired battery look better than it is.

Read the number.

• 12.6 or higher: fully charged.
• About 12.4: fine, a bit down.
• About 12.2: roughly half charged.
• 12.0 or below: discharged. Charge it fully and test again before you condemn it.

A low reading here doesn't tell you whose fault it is. It only tells you the battery's empty. Keep going.

Test two: cranking

This one needs a second person. Leave the meter connected, and have your helper turn the key while you watch the display. Better yet, prop the meter where the helper can see it and stand clear yourself.

Voltage will crash the instant the starter pulls. What matters is the floor. On a healthy battery at normal temperature it should stay above about 10 volts. If it dives into the 8s or lower, the battery or its connections can't deliver, and that's one point against the battery.

Cold weather drags every number down. A battery that sags at 10 degrees may be perfectly fine in June.

Test three: the alternator running

Start the car and let it idle. Read the meter.

You want 13.8 to 14.7 volts. Rev it gently to 2,000 rpm and hold it. It should stay in that same range.

What the numbers mean:

• 13.8 to 14.7: the alternator's charging. Good.
• Within a half volt of your resting number, 12-something: the alternator isn't doing its job. Check the belt before you check anything else.
• Above 15: the regulator's failed and it's overcharging. That cooks a battery from the inside, and it's the quiet reason some new batteries die young.

Test four: load

Alternators often pass the easy test and fail the hard one. With the engine at 2,000 rpm, turn on the headlights, the blower on high, the rear defroster, and the radio. Read the meter again.

It should hold above about 13 volts. If it sags toward 12 as you add loads, the alternator's too weak to carry the car, even though it looked fine at idle. That's the classic failing unit: strong enough to run the engine, too tired to run everything else.

Read the whole picture

Put the four readings side by side and the answer usually shows itself.

Low at rest, bad on cranking, good charging voltage: the battery's finished. Buy one.

Good at rest, fine cranking, weak or no charging voltage: the alternator. Leave the battery alone.

Low at rest and weak charging: both may be tired, or the alternator killed the battery. Replace the alternator, then charge or replace the battery, and don't skip the order.

What the meter can't see

Some newer cars have smart charging. The computer deliberately drops the alternator voltage when the battery's full and the load is light, and you can see 12.8 or 13.2 at idle on a perfectly healthy system. That's why the load test matters. Turn the accessories on and see whether the number climbs. If you drive a start-stop vehicle, a hybrid, or anything where the battery sits in the trunk or under a seat, read the owner's manual first. Those systems can need a specific battery type and a scan tool to register the new one.

And don't test by pulling a battery cable off with the engine running. It's an old shade-tree trick. On a modern car, a voltage spike can fry expensive electronics.

Before you blame the alternator, look at the simple things. A loose or glazed serpentine belt will mimic a dead unit. Corroded terminals and a rusty ground strap will do the same. Twist the clamps by hand. If one moves, you may have just found the whole problem for free.

When to hand it off

Many parts stores will test a battery and an alternator on the car for free, and it's a fair second opinion. Use it. A good mechanic can also run a ripple test for bad diodes inside the alternator, which a basic meter check can miss. If your readings don't add up, or you smell something hot, stop and take it in. And if you don't trust your own hands near the belt, that's a perfectly good reason to let a pro do it.

Pay it forward

If a neighbor jumped you at six in the morning, the right move isn't a text saying thanks. Test your own system, fix the real problem, then return the favor. Check their battery with your meter some Saturday. Bring the coffee.

And if you borrow someone's multimeter, put fresh batteries in it before you hand it back.

For more on keeping a car on the road, see the car servicing that can't wait, and the kind that can. If you're shopping, twenty minutes with the engine off tells you most of it. And if the goal is making a car last, what it really takes to get a car past 250,000 miles covers the long game.