New Battery not Charging but Alternator "Fine"

[RossCormack]

Hi, I recently posted a question about removing the ignition barrel on the w123. Thanks to you guys, I got it out then jump started the car and tried to charge it up. The battery is not charging up properly. After a good long drive, the car died as I was parking it. 20 minutes later, measured the battery and it was completely flat. Even if the alternator is a bit low, this doesn't explain such a fast drain. I don't get it, has anyone found something like this to happen before? I feel like I am goin to be stuck in Ibiza forever.

 

[EDZ649]

Sounds like a broken earth strap to me.

 

[iscaboy]

nothing in your description gives any reason to think the alternator is "fine"

 

[Diesel Benz]

nothing in your description gives any reason to think the alternator is "fine"

On the contrary, it could be a failed diode draining the battery or something similar.

 

[Ted]

W123 alternator is simple to test.
Does the ignition warning light illuminate as this is used to provide the initial excitation of the rotor on these?

 

[m7m]

You can detect if there is an excessive drain on the battery by connecting an ameter between the negative terminal of the battery and ground.
If the current is relatively high, you can start pulling fuses out (one at a time) to find out through which fuse does the draining current flow.

 

[Frank O' Phile]

I wouldn't have thought that a drive, even a good long one, would be enough to charge a flat battery. That's not the job of the alternator, it just keeps it topped up.

A good battery will measure about 12.6v. (2.1 volts per cell)
With the engine running it should be around 13.5 - 14.0v

Get the battery on a proper battery charger and go from there.

 

[RossCormack]

Thanks for the suggestions. About the alternator, I forgot to mention that I called the AA a couple of weeks ago and the guy told me the alternator was "fine" and that the battery was flat because it was just old and needed replaced, hence, new battery.

 

[jaymanek]

so have you put on a new battery?

 

[RossCormack]

lol, yes, that was done on the advice of the AA man before I drove from Glasgow to Ibiza

 

[RossCormack]

I have bought a battery charger and charged it up. Now, I find that the alternator does seem to be too low. I can only think that the AA man was wrong. The alternator charge wire is at 12.6V, going up to 13.3 and climbing when reved. So, at the moment, the alternator is lower than the battery. The battery warning light is on faintly at idle and goes off when I rev it. Time for a new alternator?

 

[Ted]

Sounds like a rectifier diode in the alternator.
Get it tested by a reputable auto-electrician, but it sounds like a replacement is required.

 

[MarkJunedd]

+1 . If the light is on dimly then youre not chargeing anywhere near enough. Change the alternator. Also ,as an aside, an alternator will not charge a completley flat battery as it needs a minimum voltage applied to it to switch the rectifier etc .