Battery light stays on for the first 10 min of a journey.

[J3JCV]

Just a quick check before I try and diagnose further.

The battery light stays on for roughly the 1st 10 min of the car running, and only if its the first journey of the day or there has been a good 4 / 5 hours since the last run.

I had suspicions of the battery being worn out so replaced that, but this hasn't rectified the issue. I haven't yet put a multimeter (as I have lost mine) across the terminals when the light is on to see what volts are there but will do next time it happens. Got to work and checked it, but the light was off by then and there was 13.8v showing as one would expect.

I just wanted to know if this is something that has happened to other people and what the issue was, alternator wearing out, lose belt, anything really??

Thanks in advance!

[magpie215]

Voltage regulator??

[GreenV8S]

If it's an old fashioned no-charge warning lamp it'll be connected between the ignition switched 12V at the dashboard, and the alternator 12V output. This would normally glow when the alternator isn't charging. It can also glow if the ignition switched 12V line is low, for example because the ignition switched circuit voltage is dipping under load. It can also glow if the alternator output is too high.

If the warning lamp is one of these new fangled ECU controlled things, it means the ECU has detected a fault in the charging system and you would probably find a faulty code to go with that.

[Dave.]

Get one of these - https://www.amazon.co.uk/Cigarette-Lighter-Voltage...

It'll let you see what's happening while you're using the car, rather than just sticking a voltmeter across the battery while stationary etc.

[J3JCV]

Thanks;

Its a 2008 car, so not new but not that old I guess.

So when I started it this evening the light came on and so i measured the voltage at the battery terminals and it was a constant 12.3V, so I drove for a bit and this time after about 5 min the light when out and when I stopped about 15 min later and measured the voltage it was at the expected 13.9V. Car was stationary for 1.5 hrs and then when I started it again the light stayed off the rest of the way home.

It seems to need to warm up or something - very odd.

Unless it's not odd and this is the warning signs of an alternator or voltage regulator packing up - I just don't understand how that happens though?

[t400ble]

What car is it?

[J3JCV]

t400ble said:

What car is it?

Ah - good point. A mazda5 2.0l petrol Sport.

[Penelope Stopit]

Is the warning light on at full brightness or slightly dimmer?

[Penelope Stopit]

Penelope Stopit said:

Is the warning light on at full brightness or slightly dimmer?

If one of the brushes are worn out there is the possibility that the worn brush makes contact with the slip ring when the temperature has risen and there's some expansion here and there

You can check for a worn brush by removing the alternator belt, turn the ignition on (don't start engine), have someone look at the warning light while you slowly turn the alternator pulley through 360 degrees, if there is a worn brush the battery warning light will flicker on and off or dimly and brightly

Try the above when the engine is nice and cold

[J3JCV]

Thanks - will try that when I get a chance, but you have to take the wheel off, a panel in the wheel arch and then the AC drive belt before changing it (which I was going to do next). Unless the belt tensioner can be slacked off enough to pop it off? I have never used one of those on an engine before as all other cars / boats have relied on the alternator creating the tension.

[GreenV8S]

J3JCV said:

you have to

If it's that awkward, you might be better off putting the car in top gear and nudging the engine over.

[Penelope Stopit]

You can use the starter to crank the engine through 360 degrees slowly slowly little by little, a quick turn to crank and off ....and again....and again

I intentionally didn't mention this in my top post as many people misunderstand and moan that the engine keeps starting

[gordmac]

If you give the engine a good rev does it go out? Belt slipping?

[J3JCV]

The light doesn’t go off if you give it a lot of revs, but when it gets close to going off properly (after around 7min) it occasionally can be intermittent when you are say pulling away in a low gear.

This morning I rolled it down a gentle slope in 5th with the ignition on, turned it over a few times without starting and the light was on solidly, no flicker at all. It has never changed brightness if that makes any difference as well?

[Penelope Stopit]

The fault isn't the brushes

[J3JCV]

I’m going to have to take the tray off the bottom and get underneath to make sure are the connections are perfect - I think it must be heat related expansion making the earth connection good - or something like that!

[Penelope Stopit]

Good idea, make sure the positive and earth are both good and also check the plug with the thin wires isn't burnt inside at the terminals

[Mignon]

J3JCV said:

I’m going to have to take the tray off the bottom and get underneath to make sure are the connections are perfect - I think it must be heat related expansion making the earth connection good - or something like that!

Possibly something eventually gets jiggled into contact again after the car has been moving for a while.

[GreenV8S]

Does this car have a fusible link? I've known those to fail mechanically leaving a dry joint.

[J3JCV]

GreenV8S said:

Does this car have a fusible link? I've known those to fail mechanically leaving a dry joint.

No idea - would that make it work intermittently or not at all?