Quick battery/alternator question

[STORM 2 0]

Something that I'm too stupid to figure out...

If I connect a fully charged battery that holds charge [12.99V] on its own (not connected to anything) to the car and come back 24 hours later the voltage reads [5.4V]. If I charge it up and leave it disconnected for 24 hours the voltage remains OK.

 

Excluding current drain in the normal way (fuse pulling debacle) can a dodgy alternator simply draw current with the car standing doing nothing overnight?.

 

Cheers

[davidwort 0]

yep, does the alt get warm all by itself?

[Supercharged 2]

A light bulb stuck on would do this - need to start pulling fuses while measuring the drop...

[STORM 2 0]

A light bulb stuck on would do this - need to start pulling fuses while measuring the drop...

 

Yes.. already did this.. and have excluded a current drain of such magnitude. I mean you'd need a major drain to flatten the battery dead within 12 hours surely?

What I wanted to know was if a faulty alternator can draw such a large current even though everything is switched off inc. the engine.

I tried checking for voltage drop across the battery terminals while the engine was running whilst switching on the electrics:

 

ENGINE RUNNING------------>13V

ENGINE RUNNING AND LIGHTS AND VENT ON---------->12.7V

 

So, are we talking new alternator?

[STORM 2 0]

yep, does the alt get warm all by itself?

 

hi - when do you mean?

by itself do you mean if I came to the car in the morning and lifted the bonnet without starting the car and put my hand on the alternator?

 

If so - I haven't done that yet

cheers

[STORM 2 0]

Incidentally what do people think of disconnecting the battery while the cars running to test the alternator ? :scratch: I heard this could be damaging

[davidwort 0]

A knackered alternator can drain the battery overnight, the energy dissapated as heat, so if the alternator feels warmish to the touch in the morning when everything else is stone cold then the alt is kaput.

 

13v is too low with the engine running, I think you have diode rectifier problems inside the alternator.

[STORM 2 0]

cheers for that David - can the diode rectifier be 'rectified' without replacing the whole thing :)

[davidwort 0]

I don't know about the VR6 type alternator but on the bosch units the plate and all of the diodes that form the AC-DC rectifier are one unit connected to the outer windings, it would be very hard to repair, not like just swapping over a brush pack unfortunately.

[dr_mat 0]

Correct. Alternator output should be 13.8 to 14.3 volts. But that doesn't mean the alternator is faulty, just that the output, by the time it gets to the battery, is only 13v. It may well be that there is a current drain resulting in the alternator being unable to maintain 13.8 v output.

 

And note that it would only take a 30W drain to flatten your car battery from fully charged to zero in twenty four hours by the way.. 30W is about the same as running the sidelights ....

 

And btw, your car battery doesn't really put out 12.99 volts. Leave it for thirty minutes or so after taking it off charge and you'll see that it *really* puts out 12.6V. (Assuming your meter is properly calibrated ...)

[STORM 2 0]

So i ran the car with everything connected and disconnecting the +ve lead of the battery did'nt do anything - the idle ran smoother ------------still the alternator?

[dr_mat 0]

I already said I didn't think it was your alternator...

[STORM 2 0]

Sorry dr mat ---so you think there may be a drain or some sort of load resistance in between the wiring from the alternator to the battery?

 

[Warning : the above message may contain confusion, a lack of understanding and outright incompetence]

[dr_mat 0]

Yes I think so. You may even have something going pretty much to short out, to pull the alternator's output down that much.

As for what, well that's rather more difficult to locate. Likely suspects are anything you've had fiddled with since the problem started occurring.. Alarms? ICE? Lights been disconnected/reconnected?

 

At the most, you might have a voltage regulator problem on the alternator. But if the windings had actually failed it wouldn't put out any volts at all..

[sparks 0]

Warning about runing car and disconnecting the battery

 

The battery act as a smoothing device on the car to stop any peaks or not quite dc wave forms going around the car electrics runing the car without a battery is very bad for the ecu as any spikes will not get sucked into the battery but run around the looms spiking the electronics first !!!!

 

if one diode is gone on your pack (true dc requires 4 diodes half wave dc only 2 i dont know how many are in the bosch unit )

it will act as a short hence draining the battery hence the warm alternator

 

just though i throw my 2p in

 

cheers

stu ...

[davidwort 0]

...if one diode is gone on your pack (true dc requires 4 diodes half wave dc only 2 i dont know how many are in the bosch unit )

...stu ...

 

there's about 6 in my old bosch alternator :? :confused4: - more than my GCSE physics can cope with...

[dr_mat 0]

It depends how many windings there are on your alternator, and apparently:

 

"To provide direct current with low ripple, automotive alternators have a three-phase winding."

 

Three phase winding means you need six diodes.

[kawark 0]

Hi,

 

I have had had problems with my own vehicle relating to the charging circuit and diagnosis has certainly taught me a few things!!

 

If you open the bonnet (assuming a fully charged battery) and test with a multi meter on the battery terminals you would expect around 12 volts.

 

If you then turn on the ignition you would see a very small change in this reading.

 

Then turn the key and start the engine and read the meter again and it should increase to between 13.8 and 14.8 at tick over.

 

This proves the alternator is functioning.

 

If you have no battery light on the dash check the D+ wire from your loom to the alternator. Normally black and only one with a thick cable which is the positive to the starter.

 

I agree with some of the other comments on the drain which is a lot harder to trace.

 

Cheers

 

Mark