Failing alternator or... ?

[1Scotty1]

Hello,

first of all, sorry if this is the wrong section. I want to ask for guidance with my C3. Roughly two weeks ago I started getting the battery indicator on my dash during driving. It would light up and after a few (2-3) minutes it turned off. I paid no mind to it, but this week it got worse - the indicator lights up roughly 5 minutes after starting the car and driving and turns off in 30 minutes of driving. Furthermore I noticed, a few seconds before the indicator turns off I notice a slight slowdown of the car, as if I stepped on the brake lightly.

Yesterday I took the car to a local service shop, and he measured the battery and alternator with a voltmeter and everything seemed fine, 14,6V on all points of measuring.

Can I do something else to remedy this situation or is this a sign of a failing alternator? Or something else?

Thank you for helping and if I missed any information, feel free to ask

[Ozvtr]

The Battery symbol on the dash tells you “there is something wrong with the electrical power/charging system” but it doesn’t tell you what the problem is. So it’s not quite useless (close). It’s also used for eco mode and engine lock out mode. Apparently they couldn’t afford to put more warning lights on the dash so they doubled up to confuse the driver but I digress.

Believe it or not the battery light doesn’t (typically) come on if the alternator fails. That’s because the regulator is the bit that sends the signal to the dash to turn on the light but guess which bit typically fails in an alternator? Yep the regulator. Hmm do I sound a bit sardonic?

If you are getting 14.6V at the battery terminals, then your alternator seems to be OK. At least while the guy took the voltage readings! Was the battery light on when he took the readings? You should try and check voltage with the battery light on (if practical). If you are getting 14.6V with the battery light on then it’s not your alternator it’s some other part of the charging system.
I would suspect the battery from what you say but you need to load test it to be sure. Just checking it with multimeter is not good enough. A load tester (as the name implies) puts a load on the battery and measures the voltage at the terminals. A bad battery’s voltage will drop under load. In the old days you could check the battery cells’ specific gravity with a hydrometer but most batteries now are sealed and I would not trust the "dolls eye" indicator if it has one. If you have access to a multimeter you could check the battery voltage in the morning, after it’s been sitting overnight. Without being charged, a bad battery will drop voltage fairly quickly but again that’s a bit subjective and might be difficult to interpret the results. Sounds, from your description, like the “flat” battery is drawing excessive current for a few minutes after starting until the terminal voltage comes up then the light goes out. This does not mean the battery is charged, only that the alternator has "dragged" the terminal voltage up enough to stop whinging.

I wouldn’t discount the alternator or charging path but you have to start somewhere and without access to test equipment (I.E. a multimeter or clamp meter) there is not much you can do..

The Electronic Control Unit’s used by the car are very finicky about voltage and the car may do strange things without a strong electrical power supply. This includes both the alternator and the battery being in good condition. A classic symptom of electrical power mediocrity is loss of the power steering but other random stuff can happen.

[1Scotty1]

...

Hello,Ozvtr

sorry for the late reply. When the mechanic was measuring the voltage, the battery light was not on. So I thought, what would be the easiest way to measure voltage, when the battery light comes on while driving? Answer was pretty simple - I bought one of those small voltage meters that go in a cigarette lighter and it displays the voltage on a small display. So, during normal behavior I am getting 14.2V on it and while the battery light is on, I am getting only 11.5V! That could be a problem.

BUT!... got one more question... can high humidity be also a culprit? Because I noticed my issue is mainly when there is high humidity outside. For example, for a few days now we were having very good weather here (10-15C average), and not a single issue or battery light. But once the temperature goes down to nearly 0C and there is high humidity, the issue comes back again. So that got me wondering...

Thank you for your time and help.

[Ozvtr]

I bought one of those small voltage meters that go in a cigarette lighter and it displays the voltage on a small display.

That's brilliant, I never would have thought of that! I'm going to use that for fault diagnosis in the future. Thanks.

Well, now it's looking like your alternator.
Batteries don't typically work, then stop, then work again. It is possible one of the cells has an intermittent short and is dragging the voltage down but that is highly unlikely.
The classic alternator failure is worn carbon brushes.
The rotor in an alternator is basically a spinning electro magnet and the way they get the current to it is via spinning copper slip rings (on the rotor) and static carbon brushes (on the housing). The carbon brushes wear down and eventually fail to make electrical contact with the slip rings. There can be this sort of "twilight zone" between where the brushes are making contact and when they fail altogether. The alternator will work then it wont work, then it will work and so on, until the brushes fail completely. A similar thing can happen with starter motors.
Yes, moisture in the air might effect contact between the brushes and the slip rings when they are barley touching.
The brushes can be replaced. They can be replaced by themselves or in a module that includes the regulator. BUT the carbon brushes can also wear the slip rings down. The slip rings can also be replaced but they are not as easy.
Auto electricians are reticent to repair things these days and just tend to sell you a new or recon alternator.
There are plenty of YouTube videos for Valeo alternator repairs if you are interested.