Car batteries rarely fail without warning. Most give signs for days or weeks before they leave you stranded, especially in cold weather. Here's how to recognise those signs, how to test the battery yourself, and how to tell a battery problem from a charging problem.
How long do car batteries last?
Battery life depends on climate, driving habits and battery type. Many conventional car batteries last somewhere around three to five years, but some fail sooner and some last longer. Heat tends to shorten battery life, and cold weather exposes a weak battery because engines need more power to start when it's cold.
Cars with start-stop systems usually use enhanced batteries (EFB or AGM). These need to be replaced with the same type, and on some cars the new battery has to be registered with the car's computer.
Signs your car battery is failing
1. Slow or labored cranking
The engine turns over more slowly than usual before starting, especially first thing in the morning. This is one of the earliest and most common signs.
2. Clicking when you turn the key
A rapid clicking with no cranking usually means there isn't enough power to turn the starter motor. It can also be caused by loose or corroded terminals, or a faulty starter.
3. Dim lights and electrical glitches
Dim headlights at idle, a flickering dashboard, a slow radio or electric windows, or random warning lights can all point to low voltage.
4. The battery warning light is on
A battery-shaped light while driving usually means the charging system isn't working properly. That's often the alternator or its drive belt rather than the battery itself. A car with a charging fault will run on the battery until it's flat, then stop. See our dashboard warning lights guide.
5. Corrosion on the terminals
White, green or blue crusty deposits on the terminals can increase resistance and cause starting problems. Cleaning helps, but heavy corrosion can also be a sign of an ageing or leaking battery.
6. A swollen or leaking case
A battery case that bulges, cracks or leaks needs replacing straight away. Don't charge or jump-start a visibly damaged battery.
7. A rotten-egg smell
A sulphur smell near the battery can indicate it's overheating or leaking. Keep away from it, with no sparks or flames, and get it checked.
8. Age
If your battery is older than the typical life for your climate and you notice any of the signs above, it's sensible to test it before winter.
How to test a car battery
Option 1: Free test at a parts store or garage
Many parts stores and garages offer free or low-cost battery and charging-system tests using a proper load tester. This is the most reliable quick test, because it checks how the battery performs under load, not just its resting voltage.
Option 2: Test with a multimeter
A basic digital multimeter is inexpensive and gives a useful first check.
Resting voltage (engine off):
- Make sure the car has been off for at least an hour, ideally several, so the reading isn't skewed by a recent charge. Turn off all lights and accessories.
- Set the multimeter to DC volts (20V range).
- Touch the red probe to the positive (+) terminal and the black probe to the negative (−) terminal.
Approximate guide for a typical 12-volt lead-acid battery:
| Reading | Approximate state of charge |
|---|---|
| about 12.6 V or higher | fully charged |
| about 12.4 V | roughly three-quarters charged |
| about 12.2 V | roughly half charged |
| below about 12.0 V | discharged |
These figures are approximate and vary slightly by battery type and temperature. A battery that reads low may simply need charging. One that won't hold a charge after a full charge is likely failing.
Cranking voltage: with a helper starting the engine, watch the reading. If it drops very low during cranking (commonly cited as below roughly 9.6 to 10 volts), the battery may be weak.
Charging voltage (engine running): with the engine running, a healthy charging system typically shows roughly 13.5 to 14.8 volts, depending on the car. Much lower suggests a charging problem, and much higher suggests the alternator's regulator may be faulty. Some modern cars use smart charging that varies voltage, so check your manual.
Battery, alternator or something else?
- Battery tests weak and won't hold a charge: replace the battery.
- Battery is fine but goes flat while driving, or the battery light is on: likely the alternator or drive belt.
- Battery goes flat overnight or after a few days: possibly a parasitic drain, such as a light left on, a faulty module or an accessory. Cheap OBD adapters left plugged in can contribute. See how to read OBD-II codes.
- Clicking, but lights stay bright: could be the starter motor or a poor connection.
Low voltage can also trigger confusing engine codes and warning lights. Our list of common causes of a check engine light includes it.
Safety tips
- Wear eye protection and gloves when working near a battery.
- When disconnecting, remove the negative terminal first. When reconnecting, connect it last.
- Never let a metal tool touch both terminals at once.
- Follow your car's manual for jump-starting. Some cars have specific jump points under the bonnet.
- Recycle old batteries. Most parts stores and recycling centres accept them.
Replacing a battery
Battery prices vary with size, type (standard, EFB or AGM) and brand. Replacement is a simple DIY job on many cars, but harder on cars where the battery is under a seat or in the boot, or where it must be registered with the car's computer. A garage can do it if you're unsure.
If your car won't start and you're not sure whether it's the battery, describe what happens on GrepKit's free car checker. It ranks the likely causes and shows typical local repair prices.
Frequently asked questions
How do I know if it's my battery or my alternator?
If the car won't start but runs fine after a jump and keeps running, the battery is the likely culprit. If the car dies while driving, or the battery warning light is on with the engine running, suspect the alternator or charging system. A load test checks both.
Can a car battery die without warning?
It can, especially in cold weather or if an internal fault develops, but most batteries show signs first, such as slow cranking or dim lights. Testing an older battery before winter is a good habit.
Does driving charge the battery?
Yes, the alternator recharges the battery while the engine runs. Very short trips may not put back as much charge as starting used, so a car used only for short journeys can slowly run down.
Is it worth testing a battery that's still starting the car?
Yes, if it's a few years old or you've noticed slow cranking. A test can show a weak battery before it leaves you stranded, particularly before winter.