Symptom · Diagnosis
Is It the Battery or the Alternator?
Both faults end the same way — a car that will not start. The difference is what happens after a jump. A car with a tired battery and a healthy alternator will run and keep running once the engine is turning. A car with a failed alternator will start on the pack, then die again within a few miles as the battery drains to nothing.
We settle it with measurements rather than guesswork: resting voltage before the jump, charging voltage at idle, and charging voltage held at about 2,000 rpm with the headlights and blower loaded. Those three numbers tell us whether you need a battery, a charging system repair at a garage, or nothing at all beyond a proper long drive.
Key points at a glance
- Healthy resting voltage
- Around 12.6V engine off, settled
- Healthy charging voltage
- Roughly 13.8V–14.6V with the engine running
- Classic alternator sign
- Battery light stays lit while driving
- Classic battery sign
- Starts fine after a jump and stays fine
What this means in practice
- We take a resting reading first — jumping the car straight away destroys the most useful piece of evidence.
- Charging voltage that sits below about 13V under load points at the alternator, wiring or belt rather than the battery.
- Charging voltage above roughly 15V is also a fault — an overcharging alternator cooks a battery quickly.
- A slipping or glazed auxiliary belt can mimic a dead alternator; we look and listen before condemning parts.
- If it is the alternator, we say so and get you moving safely rather than selling you a battery that will flatten again.
Questions we get asked
Can I drive home if the alternator has failed?
Sometimes, for a short distance, with all non-essential electrics off — but the engine will stop when the battery is empty, including power steering assist on some cars. We will tell you honestly whether the drive is realistic or whether recovery is the safer call.
Will a new battery fix an alternator fault?
No. The new battery will run flat exactly like the old one, usually within days. That is why we test the charging system before recommending a battery.
Do you replace alternators?
We are a mobile jumpstart and battery service, not a roadside garage. We diagnose and confirm the charging fault, then you can take it to your own garage with a clear picture rather than a guess.
Not sure which one it is?
Call us and we will test the battery and the charging system on site, then tell you exactly what needs doing.
Related help & advice
Category: Symptoms & diagnosis · Our services · Battery replacement · Areas we cover