Alternator or battery?

tdonegan1990

Hi all. Just had my alternator reconditioned recently. While on a long drive I noticed that the voltage from the alternator went from 14.1 (start of journey) to 12.9 (end of journey). Is this normal or should it always be outputting at least 13.5v? Car is a 05 Vauxhall astra h 1.7cDTI.
Thanks.

tdonegan1990

Any thoughts?

Cee-Jay-Cee

Have you only used the car once since the alternator was recon'd?

The alternator charges periodically while driving and then runs down again as accessories drain it and then it charges again and so on.

So long as it starts each time and doesn't sound like its dying then I wouldn't be too worried about what the actual voltage is.

9935452

tdonegan1990 wrote: »

Hi all. Just had my alternator reconditioned recently. While on a long drive I noticed that the voltage from the alternator went from 14.1 (start of journey) to 12.9 (end of journey). Is this normal or should it always be outputting at least 13.5v? Car is a 05 Vauxhall astra h 1.7cDTI.
Thanks.

A healthy alternater will give out between 13.5 and 14.5volts. a healthy battery on the shelf will give out 12.8 volts.

Are you taking the readings with the car ticking over with a multimeter or dash readout.
The car can run slow enough for the alternator to 'cut' out without the light coming on.
Rev the car a bit and seeif the voltage changes

tdonegan1990

9935452 wrote: »

A healthy alternater will give out between 13.5 and 14.5volts. a healthy battery on the shelf will give out 12.8 volts.

Are you taking the readings with the car ticking over with a multimeter or dash readout.
The car can run slow enough for the alternator to 'cut' out without the light coming on.
Rev the car a bit and seeif the voltage changes

Using the dash readout. When I rev the voltage goes up slightly?
I'm just confused why it drops so much while driving?

TallGlass

It will be drawing power while driving so will drop. You don't want to draw to much power and overcharge the battery either so it might be dropping for a reason.

Special Circumstances

Some modern cars can disconnect the battery out of the charging circuit unless there is a certain minimum current draw. So you may need to turn on parking lights or dipped headlights or something to be sure that the battery is connected to the alternator.

First, try revving it as suggested above.

Ah just saw you are using a dash voltage reading. Try turning on some lights, it may be a smart alternator thing.

tdonegan1990

Thanks for all replies folks!

jca

tdonegan1990 wrote: »

Thanks for all replies folks!

All of which are complete rubbish.... The battery is only required for starting with the alternator handling all electricial demands as soon as the engine starts. The voltage should be 14.4 V all the time while driving. The voltage might drop slightly if in prolonged heavy traffic with everything turned on, HRW, lights,wipers,heater fan etc but should bounce up to 14.4 immediately on picking up the revs even only slightly. I think your recon alternator is a dud. Have a quick check for bad or burned connections around the alternator and make sure the belt is tight enough too. If the alternator is struggling this time of year it's a bad sign.

jca

Cee-Jay-Cee wrote: »

Have you only used the car once since the alternator was recon'd?

The alternator charges periodically while driving and then runs down again as accessories drain it and then it charges again and so on.

So long as it starts each time and doesn't sound like its dying then I wouldn't be too worried about what the actual voltage is.

None of that is true.

tdonegan1990

jca wrote: »

None of that is true.

Thanks jca.. Is it possible a bad battery could cause the drop in voltage or is the alternator the no.1 suspect?

jca

I'd suspect the alternator. What happened the alternator in the first place? Have you got a voltmeter wired in permanently, if so, what is the battery voltage first thing in the morning before you start the engine? A lot of garages/ auto factors will do a battery test for you.

TallGlass

jca wrote: »

All of which are complete rubbish.... The battery is only required for starting with the alternator handling all electricial demands as soon as the engine starts. The voltage should be 14.4 V all the time while driving. The voltage might drop slightly if in prolonged heavy traffic with everything turned on, HRW, lights,wipers,heater fan etc but should bounce up to 14.4 immediately on picking up the revs even only slightly. I think your recon alternator is a dud. Have a quick check for bad or burned connections around the alternator and make sure the belt is tight enough too. If the alternator is struggling this time of year it's a bad sign.

If its a case of it been rubbish, then why did my Corolla start up fine from a jump, but try to apply any type of load of the engine and it cut out. My alernator is fine its was a recon unit the car had no battery. I think your wrong and with the battery and it dose play a part with the electrics once running. Although my Morris Minor had no issue running and driving without a battery. So maybe some cars can and others can't function without the battery.

jca

TallGlass wrote: »

If its a case of it been rubbish, then why did my Corolla start up fine from a jump, but try to apply any type of load of the engine and it cut out. My alernator is fine its was a recon unit the car had no battery. I think your wrong and with the battery and it dose play a part with the electrics once running. Although my Morris Minor had no issue running and driving without a battery. So maybe some cars can and others can't function without the battery.

Of course the battery is integral to the electrics of the car. What i meant was, that from a supply point of view it's the alternator that does all the work. The battery simply acts as a backup in the very rare case where demand is larger than supply. You should never run any car without a battery in circuit whether it's a Morris Minor or an alternator equipped car. You're lucky you didn't do damage to your recon alternator when you took the other vehicles battery out of the circuit and left the system without a battery.

TallGlass

jca wrote: »

Of course the battery is integral to the electrics of the car. What i meant was, that from a supply point of view it's the alternator that does all the work. The battery simply acts as a backup in the very rare case where demand is larger than supply. You should never run any car without a battery in circuit whether it's a Morris Minor or an alternator equipped car. You're lucky you didn't do damage to your recon alternator when you took the other vehicles battery out of the circuit and left the system without a battery.

Both cases of start without a battery where just to test something not to drive the car. But good information.