Why Does My Car Battery Keep Dying?

The Battery Is Usually Not the Real Problem

Here is the hard truth: when a battery keeps dying, the battery is often the victim, not the cause. A battery is just a storage tank for electricity. If something keeps emptying the tank, or nothing is filling it back up, it does not matter how new the tank is. It is going to come up empty.

I have seen plenty of people buy a second and third battery in the same season, only to be stuck in the same parking lot again. The battery was never the root problem. Something else was.

Think of your car's electrical system as three parts working together:

• The battery stores power and gives the starter a big push to crank the engine.
• The alternator is a small generator driven by a belt on the engine. Once the engine is running, it makes the power the car uses and refills the battery.
• Everything else (lights, radio, computers, alarm) uses that power. Some of it keeps sipping power even when the car is off.

A battery that keeps going dead means one of those three parts is not holding up its end. Your job is to figure out which one, or get it in front of someone who can test it.

Common Causes of a Battery That Keeps Dying

1. Something Is Draining It While the Car Is Off (Parasitic Drain)

This is the one people never suspect. After you shut the car off, the computers inside it take a little while to power down and "go to sleep." If one of them never goes to sleep, or something else stays on, it keeps pulling power all night.

Common sources include:

• A dome light, trunk light, glove box light, or vanity mirror light that stays on
• Aftermarket add ons like alarms, remote starters, amplifiers, subwoofers, or dash cameras
• Phone chargers or other accessories left plugged in
• A relay (an electrical switch) that sticks in the on position
• A computer module in the car that fails to shut down

The clue: the car runs and drives fine, but it is dead after sitting overnight or for a couple of days.

2. The Charging System Is Not Refilling It

If the alternator, the belt that spins it, or the wiring and terminals between the alternator and the battery are not working right, the battery never gets fully recharged. Every start takes a little more out than the drive puts back. Eventually it is empty.

This is why a brand new battery can go dead in a week. The battery was fine. The system that feeds it was not.

The clue: dim headlights, slow cranking, a battery or charging warning light on the dash, or the car losing power while you are driving. Our guide to bad alternator symptoms goes deeper on this.

3. Short Trips

Starting the engine takes a big burst of power. Driving puts it back, but only if you drive long enough. If most of your trips are a few minutes to the store and back, the alternator may never have time to fully recharge the battery. Les Schwab says too many trips under about 15 minutes can wear a battery down over time.

Cars that sit for long stretches without being started have the same problem. The normal small drain slowly empties the battery while it sits.

The clue: it happens more in weeks where you only drive short distances, or after the car has been parked for a while.

4. Loose or Corroded Battery Connections

The cables that clamp onto the battery posts have to be clean and tight. Corrosion (that crusty white, green, or blue buildup) and loose clamps get in the way of power going in and out. The battery may not charge fully while you drive, and it may not deliver enough power to start the car.

The clue: you can see crust on the terminals, or a clamp moves when you wiggle it by hand.

5. The Battery Is Simply Worn Out

Batteries do wear out. Heat, cold, vibration, and being run completely dead over and over all shorten their life. An old battery cannot hold as much charge as it used to, so a drain or a short trip that a healthy battery would shrug off is enough to leave you stranded.

Les Schwab makes a point most people do not expect: heat, not cold, does most of the damage. The cold morning is just when a battery that was already weakened by summer finally gives up. For how long batteries typically last and the signs one is wearing out, read our car battery lifespan guide.

The clue: the battery is several years old, and the problem showed up gradually with slower and slower starts.

What You Can Safely Check Yourself

You do not need tools or experience for these. They take a few minutes and can save you a trip.

• Walk around the car after you lock it. Look for any interior light still glowing, especially the dome light, trunk light, and glove box light. Some vanity mirror lights stay on if the cover is left open.
• Unplug everything. Phone chargers, dash cameras, and other plug in accessories can keep drawing power. Unplug them overnight and see if the problem stops.
• Turn your headlights off yourself. Even if your car turns them off automatically, getting in the habit removes one possibility.
• Look at the battery terminals. With the engine off, look for crusty buildup on the posts and clamps, and see if a clamp moves when you try to wiggle it. Do not touch both terminals at the same time with anything metal.
• Think about when it happens. Overnight only? After a week of short trips? After a new stereo or remote starter was installed? Write it down. This is the information a technician needs.
• Watch the dashboard. If a battery or charging light comes on while you drive, that points at the charging system. See our guide on the battery light.

When It Becomes a Safety Problem

A dead battery in your driveway is an inconvenience. These situations are more serious:

• The car loses power or stalls while you are driving. That points to the charging system, and it can leave you without power on the road. Get off the road safely and do not keep driving it. Have it towed in to be checked.
• The battery or charging light comes on while driving. The car may be running on battery power alone, and it will not last long. Head somewhere safe and have it looked at.
• The battery case is cracked or leaking, or you smell rotten eggs near it. Battery acid burns skin, and a rotten egg smell can mean the battery is being overcharged and giving off a toxic gas. Leave it alone and have it handled by someone with the right protective gear.
• The battery may be frozen. Do not jump start a frozen battery. Mazda's owner's manual warns it may rupture or explode.
• You depend on the car for long drives or in bad weather. A battery that keeps dying will eventually pick the worst possible moment.

When to Go to a Shop

If you checked the simple stuff and the battery still keeps going dead, it is time for testing. Do not just buy another battery and hope.

Here is what a good shop will do, based on the process described by the Motorist Assurance Program (an industry group for repair shops):

1. Test the battery. They check whether it is charged, whether it is healthy, and whether it can still deliver power under load.
2. Test the charging system. They check the alternator, belt, cables, and terminals to make sure the battery is actually being refilled while you drive.
3. Run a parasitic draw test. If the battery and charging system pass, they measure how much power the car uses while it is parked and shut off. This takes patience, because the car's computers need time to go to sleep before the reading means anything.
4. Track down the circuit. If the draw is too high, they narrow it down to the fuse, relay, module, or accessory causing it, fix it, and test again to confirm.

Finding a parasitic drain can take time, so give the shop as much information as you can. That leads to the next part.

What to Tell the Technician

The more you can tell us, the faster we find it. Bring answers to these:

• How long does the car sit before the battery is dead? Overnight, a few days, a week?
• Does it start fine after a long drive?
• How old is the battery, and has it been replaced recently?
• Have any warning lights come on, especially a battery or charging light?
• Has anything been added or changed? A remote starter, alarm, stereo, dash camera, or recent repair work?
• Do you notice anything odd, like a light that stays on, a radio that stays lit, or a fan or clicking noise after the car is off?
• What does a normal week of driving look like for you? Mostly short trips, or longer drives?

Not sure how to put it into words? Describe what your car is doing to the free APEX diagnostic tool and take its notes with you to the shop.

How to Keep It From Happening Again

• Drive it long enough, regularly. Les Schwab suggests driving about 15 minutes at least once a week to keep the charge up.
• Keep the terminals clean and tight. Look at them every time you check your oil.
• Be careful with add ons. Every accessory that stays powered when the car is off is one more thing that can drain it. Have aftermarket equipment installed by someone who knows how to wire it correctly.
• Park in the shade or a garage when you can. Heat is hard on batteries.
• Get the battery tested before winter if it is getting older. A weak battery is much easier to deal with in your driveway than in a cold parking lot.

Want to see how technicians hunt down a drain? APEX Tech Nation has a technician level walkthrough of parasitic draw testing, and a plain explanation of how the charging system works.