How do I tell if the alternator is kaput?

[mookers]

... apart from getting a new battery and discovering that it dies shortly?

My car was running fine up to yesterday. This morning it wouldn't start, no lights, door locks wouldn't work, etc.

Jump started it and it was fine, I just came back from driving it round for 30 minutes. I'll see if the battery is holding any charge by starting it up again in a while.

What I want to know is, what would cause the battery to go flat overnight? As far as I know, no lights were left on, no doors open etc. I can buy a new battery but I'm worried the alternator is gone.

I need my car every day, so if I can get something sorted before it gets fully borked that would be nice.

 

[Guest]

get a volt meter,

the voltage with the car of should be approx 11.5v-12v
(if its lower the battery is porked or somethings draining it)

with the car at idle with no accessory's on approx 13v-14v
(if its lower the alt is on the way out)
(if its higher than this it porked and will have killed your battery aswell)
(if its in this range the alt is fine but your battery has probably dropped a cell and will not hold voltage long)

if you still have the orignal battery the car came with,
its most likely new battery time as a typical battery only lasts 5 years

 

[bradc]

Yep, should be around 14v at idle. Much less than 13.5v and there is a problem.

 

[mookers]

Thanks for that.

Where do I measure the voltage from? Just across the battery terminals?

 

[bradc]

yep

 

[mookers]

Well I haven't got round to measuring the voltage yet, but the car started fine twice this afternoon. I have no idea what happened last night...

Hopefully it won't be dead in the morning!

 

[Johny]

cant you just do the whole start the car and then disconnect the earth and if its still running the alternators working trick?

 

[mookers]

Well the car started just fine this morning. I'll do the voltmeter check when I can but I guess for now it's ok. I have no idea what happened, maybe one of my kids left a door ajar and I didn't see it.

 

[qball]

The same ting happened to me last week. Flat battery over night,Jump started and has run fine since

 

[naughtika]

same thing here as well. Friday last week.. my battery just died.. so replaced it with a new one.. I believe i just need to recharge my old one and should still run fine..