Battery or alternator?
Your car won't start, or the battery keeps going flat – but is the battery to blame, or the alternator? The signs below will help you tell the difference before you spend money on the wrong part.
Signs to look for
Look at what your car is doing. Most problems point clearly to either the battery or the alternator.
- Clicks or turns over slowlyThe engine struggles to turn over, especially on cold mornings when a weak battery has the least power.
- Starts with a jump, then runs fineOnce it's running the car drives normally, but it won't start again on its own. The battery is no longer holding its charge.
- The battery is three to five years old, or olderMost car batteries last three to five years. After that, it may simply be worn out.
- White or green crust on the terminalsThis is corrosion. It stops power flowing properly, even if the battery itself is still good.
- The radio or clock resetsIf your settings are lost each time you start the car, the battery is dropping too low while the engine cranks.
- Flat after a few days parkedA healthy battery easily lasts a few days without driving. If yours can't, it's no longer holding its charge.
What to do: bring your car in for a free battery test. Often the battery only needs a proper charge, and we'll tell you if that's the case. What the test results mean
- The battery light comes on while drivingThis warning light means the battery isn't being charged while the engine is running.
- Lights dim or flicker as you driveYour lights need a steady supply of power. Dimming or flickering means not enough is coming from the alternator.
- The car dies while it's runningWithout the alternator, the car runs on the battery alone until it's empty – and then the engine cuts out.
- A new battery keeps going flatIf a new battery goes flat again soon, it's usually not being charged properly, or something is draining it.
- A whining noise or squealing beltA whine can come from a worn alternator. A squeal usually means the belt that drives it is slipping.
- A swollen battery or a rotten-egg smellThe battery is being overcharged, which damages it from the inside. Have it checked straight away.
What to do: come in for a free charging test. If the alternator is faulty, we'll explain what we found and refer you to an auto electrician – or you can book a diagnostic scan.
How it works
Your battery and alternator work as a team, and each one depends on the other.
Think of your phone. The battery stores the power, and the alternator is the charger. The difference is that this charger only works while the engine is running. If the charger fails, even a brand-new battery will soon run flat.
When the charging goes wrong
The alternator sends a steady flow of power back into the battery, so it stays full and starts the car every time.
Too little power reaches the battery, so it runs down a little more on every trip. Even a brand-new battery will eventually go flat.
Too much power is pushed into the battery. It overheats, can swell, and wears out long before it should.
- With the engine off, a fully charged battery reads about 12.6 V. Below 12.4 V, it needs charging. What the voltage tells you
- With the engine running, the right voltage depends on the battery type. AGM and EFB batteries charge a little higher than standard SMF batteries, and cars with stop/start vary the voltage as you drive. Charging by battery type
It might be something else
These problems cause the same signs as a bad battery or alternator – and they're often quicker and cheaper to fix.
What we check – free
Visit either of our branches in Brackenfell or Stellenbosch. You don't need to buy a battery to have it tested.
If the alternator is the problem, we'll explain what we found and refer you to an auto electrician. If it's the battery, we'll help you choose the right one for your car.
Common questions
Will a new battery fix a bad alternator?
No. If the alternator isn't charging, a new battery will go flat as well – usually within a day or two of normal driving. That's why we always test the charging system too.
Can a bad battery damage the alternator?
It can. A weak battery makes the alternator work harder all the time to keep up, which can wear it out sooner.
Can I keep driving with the battery light on?
Only as far as you need to go to get help. The car is running on battery power alone and will stop once the battery is empty – often within an hour, and sooner if your lights and air-con are on.
Why does my car start fine in summer but not in winter?
Cold weather makes a battery weaker and the engine harder to turn over. A battery that's starting to wear out often shows it first on cold mornings.
Not sure which it is?
Bring your car to Brackenfell or Stellenbosch for a free battery and charging test. We'll tell you honestly what's wrong – even if you don't need a new battery.