Why Does My Car Battery Keep Dying? Causes and How to Find Them
A car battery that keeps going flat is frustrating, especially when you have already replaced it. A dead battery is often a symptom, not the cause. Something is either draining it while the car is off or failing to charge it while you drive.
Short version: check the obvious things first, like lights left on and loose terminals. Then test the charging voltage to see if the alternator is working, and check for a hidden drain that runs while the car is parked.
Common causes
- Lights or accessories left on. Interior lights, boot lights and glove-box lights can stay on if a switch is faulty.
- A worn-out battery. Batteries wear out. See signs your car battery is failing.
- A faulty alternator. If the alternator does not charge, the battery runs down while you drive.
- Loose or corroded terminals. A poor connection stops the battery charging properly and can stop the car starting.
- A parasitic drain. Something keeps drawing current after you switch off.
- Short trips. Starting the engine uses a lot of energy, and a short drive may not put it back.
- Heat. Hot conditions wear batteries out faster. See how heat affects car batteries.
- Long periods of inactivity. Even a healthy battery slowly loses charge when the car is parked for weeks.
Common sources of parasitic drain
- Aftermarket accessories, such as alarms, dash cameras, trackers, amplifiers and extra lights, especially if they were wired straight to the battery
- A stuck relay or faulty switch
- A door, boot or bonnet switch that tells the car something is still open
- A faulty control module that never goes to sleep
- A phone charger or device left plugged into a socket that stays live
Step 1: check the basics
Make sure nothing is left on. Look at the terminals for corrosion and check that the clamps are tight. Look at the drive belt, which turns the alternator. If it is loose, cracked or squealing, the battery may not be getting charged.
Step 2: test the charging system
With a multimeter set to DC volts, measure across the battery terminals with the engine off, then with the engine running. A healthy system shows about 12.6 V at rest and roughly 13.7 to 14.7 V while running. If the running figure is close to the resting figure, the alternator is likely not charging.
Step 3: look for a parasitic drain
This test is for people comfortable with a multimeter. If you are not, take the car to an auto electrician.
- Turn everything off, close all doors and remove the key. Wait for the car's systems to go to sleep, which can take 20 to 45 minutes.
- Disconnect the negative battery cable.
- Set the multimeter to measure amps and connect it in line between the negative terminal and the disconnected cable.
- Read the current. A normal drain on a modern car is typically well under 50 milliamps. A much higher reading means something is drawing power.
- Pull fuses out one at a time. When the reading drops, that circuit contains the fault.
Take care not to start the car or open a door while the meter is connected, because a large current can blow the meter's fuse.
When to get professional help
If you find a drain that you cannot trace, or you suspect the alternator, get it tested by a workshop. Many will check the charging system for free. If the alternator is faulty, replacing only the battery will not fix the problem, and the new battery will be ruined by repeated flat spells.
In the meantime, keep the car running with a safe jump-start if you need one.