When your car won't start, the two usual suspects are the battery and the alternator. They cause similar symptoms but need very different fixes, so it pays to know which one you're dealing with before you spend a cent. Here's how to tell the difference — and how our free on-site test settles it for certain.

The Quick Version

As a rule of thumb: a battery problem shows up when you try to start the car, and an alternator problem shows up while you're driving. The battery gets the engine going; the alternator keeps everything running and recharges the battery once you're moving. If one is failing, the clues are usually different.

Signs It's the Battery

Slow, laboured cranking — that "wrr... wrr..." sound when you turn the key. See our full list of battery warning signs.
A single click or rapid clicking and the engine won't turn over.
It's fine after a jump start, then flat again the next morning — classic tired battery, especially if it's over three years old.
Nothing at all — no lights, no dash, no crank — often points to a completely dead battery or a bad connection.

Batteries age faster here than in cooler states. In Brisbane's heat, most only last three to four years.

Signs It's the Alternator

The battery or charge warning light comes on while you're driving.
Headlights dim or flicker at idle and brighten when you rev the engine.
The car starts fine, then dies shortly after — the engine is running purely on the battery because it isn't being recharged.
A brand-new battery goes flat again within a day or two. If you've already replaced the battery and it keeps dying, the charging system is the more likely culprit.

Battery vs Alternator at a Glance

Clue	Likely Battery	Likely Alternator
When it happens	At start-up	While driving
Warning light	Sometimes none	Charge/battery light on
After a jump start	Runs, but flat next day	Dies soon after
Headlights	Dim at start	Dim/flicker at idle

The Only Way to Be Sure

Symptoms overlap, so guessing can be an expensive mistake — there's no point buying a battery if the alternator is the real problem. When we come out, we run a proper battery and charging-system test: we measure the battery's health, the alternator's charging output and the starter draw, so you only replace what has actually failed. The test is free, and if you do need a new battery we fit it on the spot.

If it turns out to be the alternator or starter — something we don't replace roadside — we'll tell you honestly so you can get it sorted properly, rather than sell you a battery you don't need.

Frequently Asked Questions

How do I know if it's my battery or my alternator?

A failing battery usually cranks slowly and struggles at start-up, while a charging fault often lets the car start but then goes flat within a day or two, frequently with a charge warning light on. The certain way to know is an on-site test of the battery, alternator output and starter draw — which we do for free before recommending anything.

My new battery keeps going flat. Is that the alternator?

Often, yes. If a healthy, recently fitted battery keeps discharging, the charging system usually isn't putting enough back in — or something is draining the battery while the car is parked. We can test the charging output on the spot to confirm.

Can you replace the alternator too?

We specialise in mobile battery replacement, so alternator repairs are better handled by a workshop. What we will do is diagnose the fault accurately and tell you honestly whether it's the battery or the charging system, so you don't waste money on the wrong part.

Not Sure Which One It Is? We'll Find Out

Don't guess — get it tested. Call 0423 845 744 or request a callback and we'll come to you anywhere in Brisbane and surrounds, test the whole system for free, and fit a new battery on the spot if that's what you need. Available 24/7.

Battery or Alternator? How to Tell What's Failing