How can you know if it's a dead battery or alternator?

Determining the exact cause can be tricky, however, but the signs are subtly different.

Main signs that it's a battery issue

1. The engine won't start or is slow to start

If your engine will not turn over or seems to take much longer than usual to start, it could be that the battery is dead. The best way of checking this is to reach for your jumper cables and attempt a jump-start. If, when attempting a jump-start, it stays running for a while but then again fails to start the next time you try, it is probably a battery issue. If it immediately stalls, it is more likely to be a problem with the alternator.

2. Dim dashboard lights or headlights

When the engine is switched off, the vehicle's battery is responsible for powering all of the electrical components. Therefore, a key sign that the battery is dead or dying is if the headlights or dashboard lights are dim or do not come on at all. A weak or failing battery will result in the lights being dimmer than they should be, whilst a dead battery will result in no lights coming on at all.

3. There's an odd 'eggy' smell

If your lead-acid battery is leaking, it will release sulphuric gases, emitting a smell which resembles rotten eggs. Leaking batteries need to be replaced as a matter of urgency.

Reasons for battery failure

There are numerous reasons why a battery may fail. Some are human error and some are mechanical.

One of the most common is that the lights have been left on for an extended period of time (usually overnight) and have subsequently drained the battery.

Another reason a battery can die is if it is exposed to exceptionally cold conditions and has actually frozen. This is more likely to happen if the battery isn't fully charged.

Car batteries last, on average, five years if they are well-maintained, so another possible reason for battery failure is simply old age.

A car battery can become swollen or bloated if there is a build-up of gases.

Another easily fixable reason for a failing battery can be corrosion. If the battery terminals are corroded, this will reduce the electrical energy being transferred. Corroded or loose battery cables can also cause issues.

Main signs of a faulty alternator

If you have ruled out a dead battery diagnosis, then the start-up issues could well be linked to an alternator problem. The key things to look out for are:

1. Frequent engine stalls

A failing alternator will have trouble charging the car battery. This may leave the battery with insufficient charge to start the car. This is why one of the key signs to look out for is that your car stalls when you start it up, immediately after a jump-start, or if it frequently stalls whilst driving once you have got it going.

2. Issues with headlights/interior lights

If your headlights are behaving strangely, perhaps flickering or dimming/brightening unevenly, this may be an indication that the alternator is unable to deliver a consistent level of power and could be beginning to fail. If, when you rev your engine, the headlights brighten at higher revs and then dim when you reduce them, the alternator is likely to be underperforming.

If your interior lighting and the lights on your dashboard dim gradually when the engine is on, this could also suggest that there is insufficient power being generated by the alternator.

3. A dead battery

A dead battery can actually be a symptom of alternator charging issues rather than the cause in itself. This is because a faulty alternator will be unable to charge the battery during normal driving.

4. Malfunctioning electrics

If the alternator is failing, it may manifest itself in issues with electrical components, such as strange sounds from the stereo, a speedometer showing strange readings, or even a slow-closing electric window.

5. Squealing or growling noises

Strange noises are rarely a good sign in any vehicle. An issue with the alternator may cause a squealing noise that gets louder when the sound system or heater is on. This could indicate a failing alternator or a slipping alternator belt.

6. A burning smell

As the alternator belt wears excessively, it can cause a burning smell. This burnt smell can be caused by an alternator that has damaged or excessively worn internal components.

Automotive expertise

In addition to vehicle maintenance and servicing, Protyre are one of the largest suppliers of tyres in the UK and we have an extensive network of garages across the country. All of our garages are staffed by experienced and qualified fitters.

When it comes to the tyres available from Protyre, we offer premium products from leading names such as Michelin, Pirelli, Goodyear, and Bridgestone; mid-range offerings from brands including Firestone, Avon, Sumitomo, and Hankook; and economy options from manufacturers such as Ovation, Budget, and Autogrip. Most of the brands offer summer, winter, and all-season versions of most models - with some also offering extra load (XL) and run-flat versions.

Beyond tyres

Protyre garages also offer MOTs, servicing, and vehicle repairs, together with a number of free safety checks for your vehicle.

Simple to arrange and book

You can book any of these services and can purchase your tyres online or over the phone. To find your nearest Protyre garage, simply enter your postcode into the 'Find your local garage you can trust' search tool on our website.