Have I got a bad alternator?

[davidmiller]

Hello, I bought a new battery recently thinking mine was a goner.
This morning I thought I'd just check the alternator. It gave the following readings.

12.34 with engine off
14.40 engine on
14.12 engine on, radio on, lights on, blower on
14.40 lights etc off. Engine on
12.80 engine etc off
12.37 a few minutes later

Am I right thinking the new battery reading 12.34 is to low due to the alternator?
Cheers.

 

[Harry Bloomfield]

12.80 engine etc off
12.37 a few minutes later

That drop in voltage, suggests to me that either the battery is almost dead, or you have a discharge somewhere. You said a new battery, so you need to check for a discharge, the discharge, might be the alternator.

 

[Stivino]

Voltage guide for 12V batteries.

 

[Johntheo5]

Hello, I bought a new battery recently thinking mine was a goner.
This morning I thought I'd just check the alternator. It gave the following readings.

12.34 with engine off
14.40 engine on
14.12 engine on, radio on, lights on, blower on
14.40 lights etc off. Engine on
12.80 engine etc off
12.37 a few minutes later

Am I right thinking the new battery reading 12.34 is to low due to the alternator?
Cheers.

The battery should really be disconnected for a hour or so when checking the terminal volts but 12.35V connected is ~ 75% charged, mine regularly read as low as this and all lasted at least 10 years and used to charge at a constant 14.2V to 14.4V, your alternator is fine IMO.

 

[Harry Bloomfield]

The battery should really be disconnected for a hour or so when checking the terminal volts but 12.35V connected is ~ 75% charged, mine regularly read as low as this and all lasted at least 10 years and used to charge at a constant 14.2V to 14.4V, your alternator is fine IMO.

I would be concerned, if my battery read that low, immediately after being on charge at >14.5v, as the OP seems to be suggesting. 12.35v would be of less concern, if the battery has been stood for several hours unused, or if there is a load on it - hence my suggestion that there might be a parasitic load left on.

Battery voltage, alone, never tells the entire story, despite how some web sites attempt to simply it.

 

[davidmiller]

Thanks for the replies. I'm not sure how to check what's happening. I did disconnect the battery as soon as I got back from a short drive. And about half hour later it read 12.80. I'll check again tomorrow. I noticed the other day the radio turned itself on! But that wasn't the case today?

 

[Harry Bloomfield]

And about half hour later it read 12.80. I'll check again tomorrow. I noticed the other day the radio turned itself on! But that wasn't the case today?

Nowt wrong with the battery then, you need to find the parasitic drain.

 

[Johntheo5]

This gives a rough idea of the battery SOC, open circuit.

 

[Jurassicspark]

Start by looking at all the lights to see if any have been left in the on position.

 

[Old Salt]

What car is it?
Does it have a “smart” alternator?
How long was the car run for before checking the voltage?
When did it last have a decent run?
Had the car gone to “sleep” before checking the voltage?

Some modern cars only pump out volts to the battery when decelerating, coasting etc. So for a lot of these cars you cant really just rely on the voltage reading.

 

[davidmiller]

What car is it?
Does it have a “smart” alternator?
How long was the car run for before checking the voltage?
When did it last have a decent run?
Had the car gone to “sleep” before checking the voltage?

Some modern cars only pump out volts to the battery when decelerating, coasting etc. So for a lot of these cars you cant really just rely on the voltage reading.

It's a 17 year old car so not a fancy alternator.
When I did the test yesterday it had a short run beforehand then after about 10 minutes with the engine off I just took a reading then started the engine to test the alternator.
The car has hardly been used lately, just a mile or 2 every other day.
I didn't test the battery because it had died, I just thought I ought to check it as I'd bought the new one without checking the alternator.

I did a reading this morning. The battery is still disconnected from yesterday. It said 12.53.

I checked the light switches.
Apparently the radio comes on on its own when I get out the car according to my passengers fairly often.

 

[Old Salt]

Freind of mine had a car with an ameter which basically shows the amps in the battery. He used to drive about 18 miles each way to work and on arrival the alternator had just replaced the charge used to start the car.

So a 1 or 2 mile trip every other day will not be enough to keep the battery charged. Add in wet weather, rear screen heater, wipers, lights, air con, radio memory etc and the battery will struggle. Give the battery a good charge and keep an eye on it.

 

[Harry Bloomfield]

The car has hardly been used lately, just a mile or 2 every other day.

A mile or two, is not nearly enough to make up for the current consumed in starting the engine. Depending on how much cranking time is needed, it can take 15 minutes to an hours run, to replace what has been taken out, in starting the engine.

 

[Harry Bloomfield]

Freind of mine had a car with an ameter which basically shows the amps in the battery. He used to drive about 18 miles each way to work and on arrival the alternator had just replaced the charge used to start the car.

An ammeter, suggests a car fitted with a dynamo. Once alternators were fitted, due to the higher currents involved, they switched to just measuring battery voltage - none-the less, the same rule applies, in time/distance needed to recharge.

 

[davidmiller]

Thanks gents. I'll give it a long charge for now and a long run in a few days.