Alternator, Low output voltage ?

[MattShaw]

Hi Guys/Gals,

Car was a little slow to turn over after been stood for 8-9 days without use and has a yellow battery warning light on the dash, I thought that I needed a new battery.

Went to Halfords, and they tested the battery and they said it was ok. They then tested the battery voltage with the car running (tick over) and it was about 13.9 V. Switching on the lights, heated seats etc dropped this to 13.7 V. They thought this was a little low, would have expected it to be about 14.5 V.

Car starts, and turns over quickly if used every couple days.

Does the voltage sound low, what should it be ?

Any ideas of what could be wrong ?

Thanks in advance for any help.
Matthew

Aug 2007 RS4 B7

[amanda1]

Mine is a 6 but mine was changed because it was spiking .....if the battery is good and the car is going flat it is because the altinator isnt charging the battery..get a new one the rs6 is approx £500 fitted not sure what the rs4 one would cost.

[neckarsulm]

...errr I think you should try a battery first.
You wont be wasting your money if it doesnt fix problem as the original will be past its best
I wouldn't trust the diagnosis of Halfords
From what you say it sounds like battery
If you start the car after 2 days and it's OK but it wont start after 9 days, in those 7 days the alternator hasn't had any influence anyway.

[PetrolDave]

The alternator voltage is only 14.5V when it's initially charging after starting. If you had driven a few miles then 13.7-13.9 is about right.

Agree that it's worth trying a new battery first.

[MattShaw]

If you start the car after 2 days and it's OK but it wont start after 9 days, in those 7 days the alternator hasn't had any influence anyway.

I thought that, but that battery wouldn't be charging fully at 13.8 V, not sure that it would make enough difference though (9 days compared to 5 weeks that others leave their cars).

[MattShaw]

The alternator voltage is only 14.5V when it's initially charging after starting. If you had driven a few miles then 13.7-13.9 is about right.

Interesting, I thought lead acid cells were charged with a constant voltage. Always good to learn something new .

Thanks,
Matthew

[PetrolDave]

Interesting, I thought lead acid cells were charged with a constant voltage.

Nothing like constant, it's heavily dependent on temperature and when the battery voltage reaches a level that indicates a full battery the alternator drops the voltage to maintain the charge without over gassing the battery.

Constant voltage charging isn't too bad at low charge rates, but at higher charge rates the voltage and current need to vary to avoid damaging the battery.

[MattShaw]

That makes a lot of sense. A new battery isn't too bad an outcome, better than a new alternator or some other part of the charging circuit.

More great info, thanks.