Ford Focus battery light showing on the dash

Check the wiring / plug on alternator ?

I have had bad connections on back of instrument cluster causing similar problems,remove cluster and clean circuit board and wiring plug connector

I had exactly the same problem. Around 12 months ago I had the alternator and battery replaced on my Ford Focus 1.6 LX 1999, after the battery light started coming on intermittently. This seemed to fix it, but recently the battery light started coming on whilst driving, and it got progressively worse so that it came on 30 seconds after starting the engine on every occasion. I took it to Kwik Fit and they tested the alterator and it was working fine. They suggested it might be a break in one of the 4 smaller wires which come from alternator, two of which (I discovered from the Haynes manual) go to the ECU, and the signal to the indicator light on the dash comes from the ECU.

It therefore looked like the light was indictating a false negative, due to a break in one of these thinner cables from the alternator to the ECU. I thought it was be a real hassle to find the break, so I wasn't going to bother. But then I noticed it (on the red wire) just one inch from the plug/connector (just inside the split cable cover hose). I soldered the two wires back together and the problem has now gone away (I also put some grease on to prevent corroson).

I have also discovered how the break ocurred. A mechanic had used a Stanley knife to cut off the insulator from the wire to allow him to use a multimeter on it (it was probably the mechanic who replaced the alterantor and battery 12 months ago).

Over time, moisture on the exposed copper strands had corroded it away to nothing, but it takes about 12 months for this to happen.

I also looked closely at the wiring disagram in the Haynes manual, as I was concerned about sometime the guy at Kwik Fit said, that the ECU uses the information from the alternator to vary the charging voltage to the battery. But the diagram shows clearly that this is not the case, as there is a direct connection from the main +VE output of the alternator to the +VE on the battery.

I hope that this helps.

Bruce.
Leeds, UK.