Hi
After a bit of reading it would seem that Ducatos are programmed to raise the idle speed from the normal 800-ish to nearer 1200 if the ECU senses that the battery voltage (as measured at the ECU) is too low. This is presumably to boost the charge rate e.g. if you have a depleted battery in cold weather just after starting. The eventual illumination of the battery warning lamp suggests that this strategy isn't enough to overcome the problem.
This might mean a problem with the alternator or its wiring, preventing proper charging. The best way to check is to monitor the battery voltage (its under the passenger's feet) with a voltmeter, directly at its two terminals. Next best would be one of those plug-in monitors for the cigarette lighter socket. You should measure 14.0 to 14.5 volts under charging conditions with the engine running.
If the battery voltage measures OK it's still possible that the ECU is getting less voltage than it ought to, due to a poor connection, corroded fuse, faulty relay etc. The ECU has more than one 12 volt supply, so you would need to check out all of them.
ECUs internally regulate their 12 volts input down to typically 5 volts, so will still work OK down to about 6 volts input e.g. during cranking in very cold weather. Although this means they are fairly voltage tolerant, they will still "complain" if fed with too little for too long.
