Battery light

[Cats]

What's the thoughts on the battery light coming on intermittently when driving i.e. not idling stationary. In the dark noticed the headlights dimmed slightly at the same time.
Car has just been serviced.
2002 450.

[over_the_hill]

If you have a voltmeter a few simple checks would help to eliminate a few things

Check direct at the battery with the engine off - should be 12.6 volts or close to this
If not your battery is low and may not be charging properly

Switch on the engine and at the battery you should get 13.5-14.0 volts
If not you have a bad connection from the alternator or a bad alternator

So leave the engine running and check at the alternator outputs - again should be 13.5-14.0 volts
If it is you have a bad connection from alternator to battery
If it isn't you may have a faulty alternator

If all of these check out there could be a bad earth somewhere else - good luck with that -
although more knowledgeable people will be along shortly I'm sure.

[QBee]

When I had this happening, it was the alternator packing up.
I have also had the 100 amp fuse fail (under the car, straight down from the alternator), which stops the alternator charging the battery and could put the light on.
Also a slipping aux belt can do this.

You can test the alternator operation with a multimeter as said above.

I have a simple device in my car - a voltmeter that plugs into the fag lighter socket. Gives you a constant and accurate idea of your battery voltage

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2...

[ianwayne]

I would add that with the engine idling, turn all the lights on (full beam included) heater motor, stereo etc to provide as much load as possible. If the output from the alternator drops off to under 13V, it's on the way out.

The 100A fuse may have a hairline crack in it. This happened to me a couple of yrs ago and the first indication was the charging light glowing slightly.

[Cats]

Thanks folks
Strangely this post has repeated itself from last week so don't know what's going on there.
Car is going back to garage to check this out. This was on the drive home from collecting it so sort of pissed off.
Thoughts are fuse or alternator but either way it will be done foc.
I'll let you know the outcome.

[Cats]

''Twas not alternator or fuse. It was a loose connection on a battery isolator switch that was hiding behind the bird nest wiring loom!
Who'd have thunk it??