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The car will not start. Often it is the battery. Sometimes it is the alternator, and replacing the battery then means the new one goes flat again.
The three failures that look the same
| What you experience | What it usually is |
|---|---|
| One click, then nothing | Battery voltage collapsing under load, or the starter solenoid |
| Slow, laboured cranking that gets worse over weeks | Battery losing capacity, often from heat or age |
| Starts fine after a jump, then dead again next morning | Charging fault, or something draining the battery overnight |
The test sequence
1. Resting voltage. Measured with the car off and nothing switched on. A healthy, fully charged battery sits in the low 12s. A reading in the low 11s means it is discharged; it does not yet tell you whether it can be recharged.
2. Load test. The battery is loaded to something close to real cranking current. This is the step that separates “flat” from “finished”: a discharged battery recovers, a failed one collapses.
3. Charging output. With the engine running, the alternator should be lifting system voltage clearly above resting level. If it is not, the alternator (or its belt or wiring) is at fault, and no battery will fix it.
4. Drain check. If the battery is healthy and the charging system is healthy, something is consuming current while the car is parked. That is a diagnostic job, not a parts job.
What each outcome means
- Battery failed, charging fine → replace the battery. Test again after fitting.
- Battery fine, charging low → the alternator or its circuit is the fault. A battery is a waste of money here.
- Both fine, drains overnight → find the drain. Replacing anything is guesswork.
Why we show the readings
Because “your battery is dead” is unfalsifiable otherwise. When the numbers are on the screen in front of you, the decision stops being a matter of trust. That is also why every job here starts with a test of the battery and charging system, including the ones that end in a replacement.
