Why does my car battery keep dying?

Get it checked soonUpdated October 16, 20263 sources2 min read

Short answer

A car battery that keeps dying is usually being drained while the car is off (a light, charger or faulty part), not being recharged enough (short trips or a failing alternator), losing contact through loose or corroded terminals, or simply worn out with age. A voltmeter check with the engine off and then running tells you whether to look at the battery or the charging system.

Safety first. Wear gloves and eye protection when you work near a battery; the white or green crust on terminals is corrosive. If you're not comfortable testing it yourself, a repair shop can check the battery, alternator and any drain for you.

On this page

Step by step
The five usual suspects
What the voltmeter is telling you
When to see a pro
Follow-up questions
Sources

Step by step

1
Rule out the obvious drains
Make sure nothing was left on. Headlights, dome lights, glove box, trunk and vanity mirror lights can all run a battery down overnight, and so can a phone charger or other accessory left plugged in.
2
Look at the battery terminals
With the engine off, check that both cables are tight and look for crusty white or green buildup. Corrosion can keep the battery from charging and delivering power properly. A mix of 1 part baking soda to 3 parts water cleans it off.
3
Check the battery voltage with the engine off
Set a multimeter to DC volts and touch the red lead to the positive terminal and the black lead to the negative. A fully charged battery reads about 12.6 volts or a little more. Around 12.2 volts or lower means it needs charging, and a battery that won't come back up to about 12.6 after a full charge is likely worn out.
4
Check the charging voltage with the engine running
Start the car and measure again. A healthy alternator usually keeps the reading around 13.5 to 14.5 volts. If it stays near the engine-off number, the alternator or its belt may not be recharging the battery.
5
If the battery and alternator test fine, look for a parasitic drain
A battery that tests healthy but is dead after the car sits points to something drawing power with the key off, such as a stuck relay, a light that won't shut off, an aftermarket stereo or subwoofer, or LED accessories. Finding it takes a current-draw test, which a shop can do if you don't have the tools.

The five usual suspects

Almost every “my battery keeps dying” problem comes down to one of these:

Something stays on. A light, a plugged-in charger, or a faulty part keeps drawing power after you shut the car off. Mechanics call this a parasitic drain.
It never gets fully recharged. Lots of short trips, or a car that sits for weeks, don’t give the alternator time to replace what starting used. If the car will sit all winter, see how to keep a parked car’s battery and tires healthy.
The alternator isn’t charging. A failing alternator or a slipping belt can’t keep the battery topped up, so it slowly runs down even while you drive.
Bad connections. Loose or corroded terminals stop power from getting in and out of the battery properly.
The battery is just old. Batteries lose their ability to hold a charge over time, and heat speeds that up.

What the voltmeter is telling you

Reading	Engine	What it means
About 12.6 V or a bit more	Off	Fully charged
About 12.2 V or less	Off	Needs charging
About 13.5 to 14.5 V	Running	Alternator is charging
Close to the engine-off reading	Running	Charging system may not be working

Readings above roughly 12.9 volts with the engine off can mean the battery is being overcharged.

When to see a pro

Have a shop take a look if the battery tests good but still goes dead overnight (that’s a drain only a current-draw test will find), if the battery warning light comes on while driving, or if the battery looks swollen, cracked or is leaking.

Common follow-up questions

How long should a car battery last?

Firestone says a typical car battery lasts up to about five years, but extreme heat or cold, frequent short trips and everyday wear can cut that to two or three years. If yours is in that range and keeps dying, age alone may be the reason.

Can short trips really drain my battery?

Yes. Starting the engine takes a lot of power, and the alternator needs time to put it back. Les Schwab says lots of trips under 15 minutes can wear a battery down. A drive of at least 15 minutes once a week helps keep it charged.

Why did my battery die after the car sat for a few weeks?

Many modern cars keep drawing a small amount of power for onboard systems even when they're off. Over weeks of sitting that can flatten a battery, especially an older one. A longer drive now and then, or a battery maintainer on a car you store, prevents it.

Does cold weather kill car batteries?

Cold is when weak batteries finally fail to start the car, but heat does much of the damage. Les Schwab notes that summer heat breaks a battery down and the problem often shows up as slow starts once winter arrives.

Is it the battery or the alternator?

Test both. If the battery won't hold about 12.6 volts after charging, it's the battery. If the battery is fine but the reading doesn't rise to roughly 13.5 to 14.5 volts with the engine running, suspect the alternator or its belt. A car that dies while you're driving, with the battery light on, points to the charging system.

Sources

General information only, not a diagnosis. Your owner’s manual comes first. How we write these