A flat battery and a faulty alternator can look exactly the same from the driver’s seat: the car won’t start, or the battery light comes on. But they need very different fixes, and buying a new battery won’t help if the alternator is the real problem.
What each part does
The battery gives your car the burst of power it needs to start. Once the engine is running, the alternator takes over. It powers the electrics and recharges the battery as you drive. If either one fails, you end up stuck.
Signs it’s the battery
- Slow cranking or clicking, especially on cold mornings
- The car starts fine after a jump and keeps running normally
- The battery is more than three years old
- Swelling, leaks or corroded terminals
Signs it’s the alternator
- The battery light comes on while you’re driving
- Headlights brighten and dim with engine speed
- The car starts after a jump, then dies again a short time later
- A whining or grinding noise from the front of the engine
- A brand-new battery keeps going flat
A quick voltage check
If you have a multimeter, these rough readings tell you a lot. Readings vary between cars, so treat them as a guide, not a diagnosis.
| Reading | Engine | What it usually means |
|---|---|---|
| 12.6V or more | Off | Battery fully charged |
| Around 12.4V | Off | Partly charged, worth watching |
| Below 12.2V | Off | Low or failing battery |
| About 13.7V to 14.7V | Running | Alternator is charging |
| Under about 13.2V | Running | Alternator may not be charging |
Why testing first saves you money
A proper test checks the battery under load, the alternator’s output and the starter’s draw. It takes a few minutes and tells you exactly which part has failed. That’s why every RBA callout starts with a test. You see the result and approve the price before we fix anything.
A failing alternator can also kill a new battery. If you’ve replaced your battery recently and it’s flat again, ask us to check the charging system.
