Car alternators are pretty standard voltage out across a standard battery. Each cell of a 6 cell battery is 2.2v equating to 13.2v but they are never 100% efficient even brand new so call it 12.5v. An alternator will put out what ever current its rated at but will be at the nominal voltage. You should see at least 14v steady and with a current meter, a good few amps even at idle.

 

Between 13-15 volts, ideally 13.5-14.5 volts

Roger.

 

Very vague and misleading statement. In fact worthless. My dying alternator in P1007 was 13.2 to 13.4 volts and the battery was notoriously undercharged.

 

Another fun test to do it to compare headlight brightness between engine off, engine idle and reving the engine.

Engine running should be a bit brighter than engine off.

Start the engine. Brightness should be constant when revving the engine and should be no appreciable dimming at idle.

 

Thanks everyone for taking the time to try and help.

I am getting 14.01 volts which I suspect is not enough to be charging my battery properly.

This is a good read.
https://www.w8ji.com/battery_and_charging_system.htm

 

This is a good read.
https://www.w8ji.com/battery_and_charging_system.htm

For somebody lecturing on battery charging, he has some pretty manky battery connectors.

I also don't agree with the bit about the field current increasing at slower speeds to keep the same mechanical load. The field current is controlled by the output voltage. It reduces when less output is required.

 

Hi...14 volts is plenty to charge a battery...question must be is the battery good enough to store/hold the charge its being given?

 

How old is the battery?
Certainly 14.1v is adequate for charging purposes, though you may not get 100% charge at that.

But what was your original problem? Why did you need to check the voltage?

 

That is a very good article, and explains that 14.1 V will charge a battery very slowly, so it would take a long time to replace the charge consumed by the starter.

However, there is a snag in checking alternator output volts - some newer cars now control the output, turning down the volts if the battery charge is good, to save fuel. So, if your car is one of those, first check the battery voltage with engine off, and only if it is below about 12.6V, and alternator output is below about 14.3, suspect the alternator.

 

I bought a mondeo as a tow car and the alternator light is on constantly it charges at 14.6 volts occasionally the battery light goes out it then charges at 12.6 !

Other strange thing is when the battery light goes out the heated screen works if the battery light is on it wont work modern electrics are very strange

Smart charging is not very smart on this one !

 

14.5 volts is peak voltage from an automotive alternator. If you are getting 14v its fine. Your voltage is basically a unit of electrical pressure and should stay within its window unless currant increases or decreases significantly, thats when internal resistance starts to effect it. With a meter across the battery and a currant clamp on the output of the alternator you should see your nominal voltage and currant flow to the battery. In an healthy battery it will take a significant top-up currant at idle after cranking on a cold engine, decaying after a few minutes as the battery charges(all loads off, ie : fans, lights, etc). If there is none or little currant showing then you have a suspect diode(s). It can also be the windings of the armature but either way, its duff. But saying all that, if the battery is on its way out, it wont accept the charge the alternator is putting out. Chargers can only offer charge, they cant force it into a storage cell so the battery is really in charge of the whole process and its status must be
determined first.

 

No need to overthink this. 14.1 volts with the engine running is good.
Why do you ask?, is the battery going flat, or do you have electrical issues?.
If so tell us your symptoms and we will see what we can do to help.

Roger.

 

Thanks everyone for your input!

My reason for checking the alternator was this,
When I bought the car I took it for a four hour round trip to see some friends, when I got home I left the lights on and it killed the battery, it would not hold charge. Fair enough I didn't know the history of the battery no big deal.

Instead of going and buying a new battery straight away I took the one out of my Golf that had the same rating and was only a year old and put that in the car to get me by.

After a week of driving to work and back (3Hrs round trips) I pulled over to take a call, shut the motor off and left the lights on. After five minutes on the phone I went to start the car and got the dreaded click clicks.. and had to jump start it

That is why I suspected the alternator. When I first metered it I was getting 14.01 across the battery with the car running.

I have since cleaned and tightened all of the terminals and am getting:
14.40v Engine running - no load.
9.8v Cranking.
12.2v after engine running for half an hour then shut off for 20 minutes.

 

Your alternator sounds ok with 14.4V. Your battery with 12.2V looks like its suffering ageing effects. If you have the use of a charger that will allow a reconditioning (gassing) charge, try using it.

Batteries should, ideally, have a gassing charge once a year - most don't, and that helps the battery manufacturers' profits.

 

Well yes, but the whole article is really about a racing set up where you are trying to reduce loading as much as possible.

 

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