About half the "flat battery" callouts we go to don't have a flat battery. The alternator died, and the battery just took the fall for it. From inside the car the two failures look almost identical, which is exactly why people end up spending $200 on a new battery to "fix" a $600 alternator problem. Or, even more annoying, doing it the other way around.
Here's how to tell which one is actually broken BEFORE you spend money on the wrong part.
What each part actually does
The battery has one main job: deliver a big punch of current to crank the engine over. Once the engine is running, the alternator takes over — it powers everything (lights, fans, fuel pump, stereo, dashcam, you name it) AND tops the battery back up. So if the alternator dies, the car keeps running on the battery alone until the battery drains and everything stops. If the battery dies, the car won't even start in the first place. Two different parts, two different failure modes, totally different bill.
Symptom-by-symptom: which one is it?
| Symptom | Battery | Alternator |
|---|---|---|
| Car cranks slowly or just clicks | ✓ Most likely | Maybe (if battery already drained) |
| Lights and accessories work, but engine won't crank | ✓ Classic battery | Unlikely |
| Engine starts, then stalls after a few minutes | Unlikely | ✓ Classic alternator |
| Dim or flickering headlights while driving | Possible | ✓ Most likely |
| Battery warning light (red battery icon) on dash | Possible | ✓ Most likely |
| Burning rubber smell from engine bay | No | ✓ Slipping belt — alternator |
| Whining noise that changes with revs | No | ✓ Failing alternator bearing |
| Jump start works, car runs for a day, then dies | ✓ Battery | Unlikely (would die sooner) |
| Jump start works, car dies again within minutes | Unlikely | ✓ Alternator not charging |
The 5-minute headlight test
If your car will start (even with a jump), this is the test you can do in the driveway. Takes 5 minutes, gets it right about 90% of the time:
- 1Start the car. Let it idle.
- 2Turn the headlights on high beam.
- 3Watch the brightness as you rev to about 2,000 RPM and back down to idle.
- 4Lights get noticeably brighter as you rev? Alternator is probably alright, but it's not quite keeping up at idle. Could be a worn belt or a tired battery making it work harder than it should.
- 5Lights stay dim no matter what, or get dimmer when you idle? Alternator is on the way out.
- 6Lights are bright and steady at every RPM? Alternator is fine. Your problem is the battery.
The voltmeter test (the one that actually doesn't lie)
A $20 multimeter from Jaycar or Bunnings is the proper diagnostic. Numbers don't have feelings. Read them like this:
- Engine OFF, key out: a healthy battery reads 12.4-12.8V. Below 12.2V means it's discharged or weak.
- Engine ON, idling: should read 13.8-14.4V. That's the alternator pushing charge in. Below 13.5V and the alternator isn't doing its job. Above 14.6V and the voltage regulator is faulty (which will literally boil your battery dry — replace it now).
- Engine ON, headlights and aircon ON: should still stay above 13.5V. If it drops below 13V under load, the alternator can't keep up. Replace it before it strands you on the Tonkin.
Haven't got a multimeter?
Most auto parts shops in Perth (Repco, Supercheap, Burson) will test the battery and alternator for free if you bring the car in. We do it on-site at every callout — free, even if you don't end up needing a battery. No "diagnostic fee", no "call-out fee for the test", nothing.
Why people end up replacing the wrong one
Because a dying alternator quietly drains the battery while you drive. By the time you actually notice the problem, the battery is also flat. So you jump-start, drive home, battery dies again overnight, and you assume "yep, battery's stuffed". You buy a new one. Same thing happens a week later because the alternator still isn't charging anything. We see this exact cycle every single week.
It can also go the other way. A dying battery has high internal resistance, so the alternator works overtime trying to push charge in, and the alternator burns out early from doing too much. You replace just the alternator, drive happily for a few weeks, then the original battery finishes off the new alternator the same way it killed the first one. Now you've spent $800 instead of $200 because you didn't replace both at once.
What to do right now
Stuck somewhere and don't know which one died? Get the car running with a jump and drive it straight somewhere safe — home, not a workshop the other side of town. If the car dies again within 20 minutes, it's the alternator and you're not going any further without help. If it makes it home fine and only dies overnight, it's the battery (or a parasitic drain). Either way, don't go and buy a battery until something has actually been tested.
When to call us
We test the battery AND the alternator on every single callout with proper instruments. If the battery is fine and the alternator is the real culprit, we tell you straight and refer you to one of the mobile auto-electricians we work with across Perth. We're not interested in selling you a battery that won't fix the problem — you'll just call us back angry next week.
Frequently asked
Can a bad alternator drain a good battery overnight?+
Not directly — when the car is off, the alternator isn't drawing power. But if the alternator has been undercharging while you drive, the battery is left sitting in a discharged state and will go flat overnight on its own. Same outcome, different mechanism.
How much does an alternator replacement cost in Perth?+
Usually $400-900 fitted, depending on the car. European cars and 4WDs are at the top of that range, sometimes higher. We don't fit alternators ourselves but we'll point you at honest mobile auto-electricians we trust — no kickbacks, just blokes who do the job properly.
Will the battery warning light always come on if the alternator dies?+
Usually yes — that light is wired to the charging system, not the battery itself. But some older cars and a few modern ones won't show it until the voltage has dropped critically low, so the light coming on can mean you've got minutes, not hours.
Is it dangerous to drive with a failing alternator?+
Once the warning light is on, you've usually got 30 to 60 minutes of driving before the battery is too flat to power the engine management. Then the car dies and won't restart. Get off the freeway and somewhere safe (not stuck in the right lane on the Mitchell) before it gives out.
Common service areas for this guide