Battery or alternator?

cornbb

Right, my battery is dead once again, as it often is. However this time I definitely didn't leave any lights on. So I'm fairly sure I need either a new battery or alternator. How can I check which is bust? I could go to a mechanic but that would seem like a waste if I could just check myself.

Anan1

cornbb wrote:

Right, my battery is dead once again, as it often is. However this time I definitely didn't leave any lights on. So I'm fairly sure I need either a new battery or alternator. How can I check which is bust? I could go to a mechanic but that would seem like a waste if I could just check myself.

If you drive the car some distance, will it start if you try immediately but not if you leave it, say, overnight?

cornbb

I left my lights on for about 24 hrs on tues so I got a jump start yesterday. Left it for half an hour, started it again, and it was grand. Then I left it overnight, it started again grand this morning. Then it failed to start at lunchtime. Which is pretty confusing. And I'm full certain I didn't leave my lights on this time

RoundyMooney

It's the battery, almost definitely, I'd imagine.

They can only stand so many deep discharge cycles (unless specifically designed for same), and 24 hours of constant headlamp load is definitely enough to discharge almost any battery.

bazz26

Replacing a battery is alot cheaper than replacing an alternator. I would replace the battery first and take it from there.

overdriver

The simplest way is to disconnect the battery tonight ( the negative side) and reconnect it in the morning. If it's still dead, then it's probably the battery.

You can test the alternator by starting the car and placing a multimeter ( about 10 quid for one from Maplin's) across the battery terminals. It should be over 12 volts, and closer to 14. If that's not happening, it's not charging. Dodgy alternator is the prime suspect there.


If the battery is older than 5 years, change it anyway.
If the battery has been getting discharged over and over and over by a dodgy alternator, I would change it anyway too.


Hope it helps.

anthony4335

When you are driving is the battery light on , if so then it is the alternator. If not most likely a bad battery.

The Novacastrian

dont waste your money on a battery yet!!

You said you got it jumped, and then started it a few times. if a battery goes flat it needs to be recharged, which can require going for a decent drive, not leaving the car ticking over in your driveway.

Also, check each cell for water, if empty fill with DI water from chemist. (personally I use boiled and cooled tap water without any problem but DI should be used)

daedalus2097

There could be an electrical problem too draining the battery over time. A faulty lambda sensor relay had me tearing my hair out a while ago, tried different alternators, batteries, until I eventually traced it to the relay which would sometimes stick in the on position.

To see if it's the battery, like someone else said, disconnect the negative every night and every time it'll be left for several hours and see if it's still full when you reconnect it.

The alternator light might not come on to show a dodgy alternator. For example, if the alternator indicator bulb blows, not only do you not get any warning, but the alternator also will not work as that bulb is used as part of the priming circuit in most cars to start the alternator.

It can take some time for a battery to charge after it's been run totally flat, your best bet would be to get a loan of a charger and leave it to charge overnight if possible. This is better for it than an alternator charge. If you have a multimeter, a good battery should measure 12.2 - 12.6V when the car's been left for some time, and 13 - 14V when the engine's running. Try to get a better idea of the problem before you start throwing parts at the problem, it could save you a fair few quid!

cornbb

Cheers for all the tips guys! I should have time to take a proper look at it this afternoon. A workmate recommended that I jumpstart the car and rev it with the headlights on; according to him, if the intensity of the lights coincides with the engine revs then that would indicate they're running from the alternator and the battery is knackered. I'll let ye know what happens either way.

overdriver

Your workmate may not be right. That will happen anyway, and I think it may be a way to check the rectifier, which is a seperate unit in some cars.

The multimeter is the only reliable way to know if your car is charging. It's simple as.

Hotblack Desiato

goslie wrote:

(personally I use boiled and cooled tap water without any problem but DI should be used)

That will eventually kill your battery. Boiling and cooling it makes little or no difference to the dissolved ions which are the problem. De-ionised water is cheap enough in a motor factor's.