4.913,260 reviews on Google

Battery or Alternator: How to Tell Which Failed

Tell a bad battery from a failing alternator or starter with simple checks and a multimeter: 12.6V rested, 13.8 to 14.7V charging. A guide for Dubai drivers.

WhatsApp us +971 54 309 1378

Pay your way

CardCash

Battery or Alternator: How to Tell Which Failed

Vans on the road in Arjan now

12.6VHealthy battery

Up to 24 monthswarranty
20 to 35 minarrival in Arjan
Open 24/7every day

Pay your way

CardCash

The short answer

If the car starts with a jump and keeps running, the battery is the likely fault. If it starts with a jump and dies within minutes, or the battery light stays on while driving, suspect the alternator. A healthy rested battery reads about 12.6V and a working alternator charges at roughly 13.8 to 14.7V. Arjan Battery tests both at your door in Arjan, Dubai.

By the Arjan Battery technical team. Updated 1 October 2026. 6 min read.

Two parts, one circuit

The battery stores energy and starts the engine. The alternator, driven by a belt from the engine, then powers the car's electrics and recharges the battery. When either one fails, the symptom looks the same from the driver's seat: the car will not start. Which part failed decides whether you need a battery at your door or a workshop visit.

Replacing the battery when the alternator is at fault buys a few days at most. The new battery is never recharged, so it goes flat in turn, and repeated deep discharge damages it. That is the reason a proper battery test always includes the charging voltage with the engine running.

Symptoms compared: battery, alternator, starter

	Battery	Alternator	Starter motor
When it shows up	First start of the day, or after the car has stood	While driving, or soon after a jump start	At any start, often with the engine hot
Cranking	Slow, or rapid clicking	Normal until the battery runs down	One click or silence
Dashboard	Usually no light until it is very low	Battery light on with the engine running	Usually no light
Lights	Dim before starting, normal once running	Fade as you drive, may change with engine revs	Stay bright when you turn the key
After a jump start	Starts and keeps running	Starts, then stalls within minutes	Still does not crank
Noise or smell	None, or a sulphur smell if it has been overcharged	Whine or growl from the belt side, hot rubber smell	Grinding or a free-spinning whirr

Voltage readings with a multimeter

Engine off, rested: about 12.6V: Fully charged and healthy
Engine off, rested: about 12.2V: Half charged
Engine off, rested: under 12.0V: Discharged. Charge and retest
While cranking: Should usually stay above about 9.6V
Engine running: 13.8 to 14.7V: Alternator charging normally
Engine running: close to battery voltage: Not charging
Engine running: well above 15V: Overcharging, usually a regulator fault

Smart alternators on newer cars vary their output on purpose and can read lower for a while, so one low reading on its own does not prove a fault.

Simple checks you can do in the car park

1
Watch the dash at switch-on
The battery light should come on with the ignition and go out once the engine runs. If it stays on, the alternator is not charging.
2
Listen to the crank
Slow turning or rapid clicking points to the battery. One solid click with bright lights points to the starter.
3
Try the headlight test
With the engine idling at night, switch on the headlights and AC. Lights that brighten clearly when you raise the revs suggest weak charging at idle.
4
Measure at rest
After the car has stood for a few hours, put a multimeter across the battery terminals with everything off.
5
Measure with the engine running
Read again at idle, then with the AC, headlights and rear demister on. The voltage should hold in the charging range.
6
Look at the belt and the clamps
A glazed, cracked or loose alternator belt slips under load. A crusted or loose battery clamp blocks current in both directions.

When it is the starter instead

A failing starter motor is the third suspect. The giveaway is strong electrics with no cranking: the headlights stay bright, the dash is lit, and when you turn the key you hear one solid click or nothing. A jump start changes nothing, because the battery was never short of power. Starters often act up hot, after a long drive and a short stop.

Bad connections imitate all three faults. A loose or corroded battery clamp, or a tired earth strap between the engine and the body, can pass enough current for the lights and still fail under the load of the starter. We check the terminals and the main earth before condemning any part.

Do not pull a battery cable with the engine running

An old test was to lift a battery cable off with the engine running to see whether the alternator kept it alive. On a modern car this can send a voltage spike through the control units. Use a multimeter or a proper charging system test.

Terms used in a charging test

Resting voltage: Battery voltage with the engine off after the car has stood for a few hours.
Charging voltage: Voltage at the battery with the engine running, which shows what the alternator is delivering.
Voltage regulator: The part of the alternator that holds its output steady. When it fails the battery is undercharged or overcharged.
Smart alternator: An alternator controlled by the engine computer, which raises and lowers its output to save fuel.
Earth strap: The cable that joins the engine to the body and the battery negative. Starter current returns through it.

Battery or Alternator: How to Tell Which Failed: common questions

Not answered here? Ask on WhatsApp. A person replies, at any hour.

How do I know if it is my battery or my alternator?

Jump start the car. If it then runs normally and restarts later the same day, the battery was flat or weak. If the battery light is on, or the engine stalls a few minutes after the leads come off, the alternator is not charging. A voltage reading with the engine running settles it.

Can a bad alternator ruin a new battery?

Yes. An alternator that undercharges leaves the battery partly discharged, which sulphates the plates. One that overcharges heats the battery and boils off electrolyte. In both cases a new battery can be damaged within weeks, which is why we test charging output before fitting one.

Will a car run with a bad alternator?

Only for as long as the battery lasts, and that depends on its charge and what is switched on. With the AC, lights and fans running, a good battery can be drained quickly. Switch off what you can, head for a safe place to stop, and do not turn the engine off until you are there.

Can Arjan Battery fix an alternator fault?

We test the alternator's charging output at your door as part of every battery test in Arjan. If the alternator is at fault, the repair is booked through the same WhatsApp number. Smaller jobs are done where the car is parked, and bigger ones go to a workshop with pickup and drop-off from Arjan arranged.

What voltage should a car battery read?

About 12.6V with the engine off after a few hours of rest. Around 12.2V means half charged and under 12.0V means discharged. With the engine running you should see roughly 13.8 to 14.7V at the terminals, though smart alternators move around within and below that range.

All battery guides

More than batteries

While we are at your car, or any other day, the same number books these.

All car garage services in Arjan

What drivers around Arjan say

4.913,260 reviews on Google

HassanArjan, Nissan Patrol
2 weeks ago
The Patrol would not start in the tower basement before work. The technician found the bay from the pin and level number I sent, tested the old battery and showed me the reading before changing it. He took the old one away with him.
Battery replacement
JamesMotor City, Volkswagen Tiguan
1 month ago
My Tiguan has start-stop, so I asked if it needed an AGM battery. He checked the one fitted, confirmed it was AGM and fitted the same type, then coded it with a scan tool. Clear explanation of what he was doing at each step.
Battery replacement
LaylaJVC, Kia Sportage
3 weeks ago
I thought the battery was finished because the car was slow to start. He tested the battery and the alternator and said the battery still had life in it and only needed a proper charge. He did not try to sell me a new one.
Battery testing
MuhammadAl Barsha South, Toyota Corolla
5 days ago
I left the interior light on overnight. I sent my location on WhatsApp early in the morning and the car was running again before I had to leave for work. He told me to drive it for a while so the battery could recharge.
Jump start
NouraDubai Hills, Lexus ES 350
2 months ago
The price was confirmed on WhatsApp before he set off and the invoice showed the same amount. The battery was changed in our driveway, and the radio and clock settings were still there afterwards.
Battery replacement
OmarArjan, Mitsubishi Pajero
1 week ago
The battery gave up late on a Friday night after dinner. I did not expect anyone to answer, but they replied on WhatsApp and a technician came to the building car park. I paid with Tabby.
Battery replacement
SarahStudio City, Mazda CX-5
4 days ago
My car had been parked for three weeks while I was travelling and it was completely flat. The technician jump-started it, then checked the charging voltage with the engine running to make sure the alternator was doing its job.
Jump start
AaravDubai Science Park, Hyundai Tucson
3 months ago
The car would not start in the office car park at lunchtime. He replaced the battery while I went back to my desk and sent me a photo of the charging reading when he was done. The invoice has the warranty period written on it.
Battery replacement
MaricelSports City, Nissan Sunny
10 days ago
My car did not start after my night shift. The technician jump-started it and tested the battery. He said it was weak and would probably need changing soon, but he left the decision to me.
Jump start