Altenator or battery problem?

Submitted: Wednesday, Apr 07, 2010 at 23:04
ThreadID: 77541 Views:2545 Replies:11 FollowUps:8
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Twice in the last 3 weeks, my daughters car wouldn't start.
Battery was flat both times.
The first was after the lights were left on for 10 minutes, I connected the battery to a charger for a few hours, and everything was fine until today.
Today, the car wouldn't start after it was parked for 10 minutes, with only the radio on.
RACQ guy said the alternator required replacing.
He jumped the car, and after a 2km drive and parking for 2 hours, it started. I put a multimeter on the battery and it showed 12.95 volts, but dropped 0.1 volt every 10 seconds for half a volt drop.
I started it, and checked the battery cables, and it registered 13.8 volts, which I assume indicates the alternator is fine.
Does anyone have information based on this whether :
a. The battery is stuffed
b. The alternator is stuffed
c. The alternator leads or drive belt are loose
d. any multiple of the above
The car needs to go to Roma on the weekend to live for a few months, and parts are cheaper in Brisbane.
Thanks in advance
Ian
I'll get there someday, or die wanting to.

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Reply By: Member - Allan B (QLD) - Wednesday, Apr 07, 2010 at 23:19

Wednesday, Apr 07, 2010 at 23:19
Hi Ian, try this.........

Bring the battery up by the charger or by running the engine.

Stop the engine and disconnect the + terminal of the battery.
Observe the battery voltage and if its voltage is falling as before then it is faulty.

If not falling then reconnect the + terminal and again observe voltage.

If it is now falling then current is flowing from the battery. It may be a vehicle circuit or it may be the alternator. Investigate further.

Cheers
Allan

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AnswerID: 412053

Reply By: Steve and Viv - Wednesday, Apr 07, 2010 at 23:27

Wednesday, Apr 07, 2010 at 23:27
If you have such a high discharge with nothing at all running then you r battery is stuffed. This can usually be tested very easily at a 12volt shop, they can load test and most won't charge as they expect you to purchase a new battery if faulty.

If, while the car is running you can read a voltage of 13.8 volts then I think your alt is fine. mine outs out 14.2.

You could check this if you want by disconnecting the leads from the car and then charge it with a charger. Once up to spec, let it sit and then test again. The point I'm trying to make here is you want to rule out an appliance draining the battery, lights, radio etc. does it have rear windscreen heater, stuff like that.

Anyway. Sounds like your battery to me, classic symptoms
AnswerID: 412054

Follow Up By: Member - Allan B (QLD) - Wednesday, Apr 07, 2010 at 23:33

Wednesday, Apr 07, 2010 at 23:33
Steve, is your email still as per your Blogspot?

Cheers
Allan

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Follow Up By: Steve and Viv - Thursday, Apr 08, 2010 at 00:11

Thursday, Apr 08, 2010 at 00:11
Got your email. Just been out today.....Will reply soon

Steve
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Reply By: Member - Stuart P (WA) - Wednesday, Apr 07, 2010 at 23:37

Wednesday, Apr 07, 2010 at 23:37
should only have0.2amp current draw when key is on the off position. otherwise something is staying on
AnswerID: 412055

Reply By: pop2jocem - Wednesday, Apr 07, 2010 at 23:40

Wednesday, Apr 07, 2010 at 23:40
Ian, sounds like the battery may be at the replacement stage unless as Alan has said something else in the car is drawing a hefty amount out of it when parked. A healthy battery should be able to run the headlights for a lot longer than 10 minutes with the engine off and should run a radio for days. If the battery voltage gets up to 13.8 volts with the engine running the alternator should be ok.
I presume you have checked things like battery lead terminals and belt tension.

Cheers Pop
AnswerID: 412056

Reply By: brushmarx - Thursday, Apr 08, 2010 at 09:23

Thursday, Apr 08, 2010 at 09:23
Many thanks for the responses.
The car is a Saab, with security and computers etc, and the first RACQ guy said that with the lights on, in conjunction with the ignition on accessories, there is a fair draw on the battery.
I'd much rather replace the battery than the alternator, but with two different RACQ "mechanics" giving completely different answers I don't want to replace the wrong one.
With the stupid Swedish batteries having recessed terminals, I'm either paying $250 for a replacement battery, or $700 plus for an alternator.
I think a trip to a battery place for a battery test is on the books.
Cheers
Ian
I'll get there someday, or die wanting to.

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AnswerID: 412079

Follow Up By: Tenpounder (SA) - Thursday, Apr 08, 2010 at 09:38

Thursday, Apr 08, 2010 at 09:38
Hi there. I hate to say it, but it could be both!! Happened to me a while back: symptoms of a stuffed battery; replaced it; still problems because the new battery wasn't charging up; fault traced to alternator; replaced that too: Ouch!!
I wonder if, perhaps, an alternator which is playing up means more strain on the battery (deeper discharge etc). Even dicky diodes may aggravate a failing battery, perhaps?
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FollowupID: 682060

Follow Up By: Andrew-rodeo - Friday, Apr 09, 2010 at 23:44

Friday, Apr 09, 2010 at 23:44
Hi Brushmarx, have you tested the alternator output with load on (lights on, heater fan on and rear demister on) output should be atleast 13.8 volts otherwise the battery is discharging, with no load it should be a touch over 14 volts. Also if someone is charging you $250 for a DIN55 or DIN 66 battery they are stitching you up quite nicely.
Cheers Andrew
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FollowupID: 682332

Reply By: Member - Troopytrek - Thursday, Apr 08, 2010 at 10:34

Thursday, Apr 08, 2010 at 10:34
Hi Brushmarx

I would also like to suggest if the battery has had its fluid checked as I had the same problem with my car and I topped up the fluid and then charged it and have had no problems with it since.Some people can foget this vital mantanance. Thats if the battery is not a sealed battery. The fluid should just cover the plates at the top,use a torch to see.
Hope this helps.
AnswerID: 412095

Follow Up By: Member - Troopytrek - Thursday, Apr 08, 2010 at 10:39

Thursday, Apr 08, 2010 at 10:39
This problem i had was on a BMW and all European cars have a miner drain from the battery supply at all times even when sitting. Yes I think it is to do with the computers they install. Have had european cars in our family for quiet some time(30 to 40years) new and old and they all do the same thing.
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FollowupID: 682068

Reply By: Shaker - Thursday, Apr 08, 2010 at 10:54

Thursday, Apr 08, 2010 at 10:54
There was a case recently of A range Rover battery going flat every time it was parked at home.
It turned out to be caused a 'Cent-a-Meter' in the house meter box, it apparently affected the security system in the vehicle causing battery drain.


AnswerID: 412097

Reply By: mechpete - Thursday, Apr 08, 2010 at 21:32

Thursday, Apr 08, 2010 at 21:32
get the battery man or an auto electicion to load test the battery .
had one the other day .it showed good voltage with the multimeter .
but it wouldn,t take the load of the starter motor .
mechpete
AnswerID: 412192

Reply By: Member - Charlie M (SA) - Thursday, Apr 08, 2010 at 22:00

Thursday, Apr 08, 2010 at 22:00
Hi
A alternator should normally have a output of between 13.8 and 14.2 volts for correct operation.

If battery was low on fluid it's reserve capacity will be very low.

If it was in my vehicle I would replace battery.

Cheers
Charlie
AnswerID: 412196

Reply By: Member - Jeremy W (SA) - Thursday, Apr 08, 2010 at 23:41

Thursday, Apr 08, 2010 at 23:41
You can get a good idea as to the condition by measuring the internal resistance of the battery, but as this number is so low one has to do quite accurate measurements with a good digital multimeter.

Accurate method.

1, Measure the battery voltage with no load.

2. Put a 10A load on the battery and measure the battery voltage under this 10A load.

3. The internal resistance of the battery is the difference in the above voltages divided by the load current (ie. 10A)

ie. ( No_Load volts - load volts) /load current.

Normally this number will be about 0.005 much higher than this and there are battery problems.


Trial method

1. Measure the battery voltage in the car.

2. Switch on the low beam lights (for about 5 secs) and measure the battery current and this load voltage.

3. The internal resistance is, once again, the difference between the voltages (noload volts - load volts) divided by the current.

If the battery volts drop by say 0.5V there are possible battery problems.

AnswerID: 412212

Follow Up By: Member - Jeremy W (SA) - Friday, Apr 09, 2010 at 09:26

Friday, Apr 09, 2010 at 09:26
Just for completeness and for those who are not averse to using their calculators, this is what I do:

One does not have to measure current to find the internal resistance of a battery.

I made up a 1.2 ohm 120W resistor (12V/10A) - call this Rl

Here's what one does:

1. Measure the no load voltage (Vnl)
2. Measure the load voltage with the 1.2 ohm load (Vl)

then the internal resistance (Rint) is

Rint = {(Vnl - Vl) / Vnl} * Rl

Or

Putting it in words: The internal resistance is the change in voltage divided by the original voltage and multiplied by the load resistance ... Voila!!
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FollowupID: 682198

Follow Up By: Steve and Viv - Saturday, Apr 10, 2010 at 10:52

Saturday, Apr 10, 2010 at 10:52
Good stuff Jeremy, haven't played with that for a while. Thanks for bringing it up.
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FollowupID: 682352

Follow Up By: Member - Jeremy W (SA) - Saturday, Apr 10, 2010 at 11:25

Saturday, Apr 10, 2010 at 11:25
Thanks Steve and Viv.

There is a third way to measure Internal resistance of a voltage source but this is a classic case of NOT to practice the theory.

So for those reading this:

PLEASE, PLEASE, PLEASE DO NOT DO WHAT THE SIMPLE EQUATION SUGGESTS. The effect is what one sees in bad sci fi movies - melted screw drivers and spectacular fires.

OK,

Rinternal = Vopen_circuit / Ishort_circuit

LoL

Jeremy.
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FollowupID: 682356

Reply By: brushmarx - Saturday, Apr 10, 2010 at 14:23

Saturday, Apr 10, 2010 at 14:23
If anyone is interested, I tested the battery voltage with a multimeter, and it showed a slow drain. I turned the lights on low beam, and the volts dropped approx 0.1 volts every couple of seconds.
I put in a replacement battery from an older Saab I plan on selling, and all looks good.
Apparently the computer system gives a warning if the alternator is not up to scratch, so I'm hoping the battery was the cause.
I have it on a charger now, and I will see what happens.
Thanks for the help
Cheers
Ian
I'll get there someday, or die wanting to.

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AnswerID: 412378

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