Team-BHP - Tata Indica alternator voltage query

I have observed a peculiar thing.
Just after starting the engine when I put a voltmeter to the battery terminals, I get around 12.5V at idle.
If I immediately switch on the headlights too, it drops to around 11.8 volts for some time. After around 30 seconds or so i hear a distinctive "click" and the voltage jumps to 12.5V with headlights on and 13.2-13.5 V with headlights off.
So my observation is that after 30 seconds or so of idling, there is a click which leads to a boost in alternator voltage.

What is this click? There is a very very slight change in engine note too after the click.

We've heard this sound particularly when the AC is on. The sound is always accompanied by a dip in engine performance. And the engine note changes too.

Apparently Prerana Motors have told my colleagues (the diesel Indica owners) that its something to do with the compressor regulation and is a common trait to all diesel Indicas.

The engine's idling rpm is usually adjusted to be as low as possible, but above the stalling threshold.

When you load the engine, as in shifting into gear and pulling away, you take care to keep the rpm up by pressing the accelerator,

However when the air-conditioning compressor (or in this case the alternator) loads an idling engine, an automatic feedback control mechanism needs to open the throttle to prevent the engine from dropping the rpm below the stall threshold.

That system kicking-in would account for the rise in engine note. Additionally when the battery voltage has dropped below a threshold, the replenishing circuits must hook-up the alternator output (in the old days this was a DC generator aka dynamo) to the battery. A heavy relay (colloquially called the "cut out" in the dynamo days) will snap on, and this is the click that you hear.

Ram

For the indica in case of new alternators also I have seen that the voltage is around 12.8V at idle amd 13.5-14V with revving hard.

As for this click, the AC was not turned on. It was just the headlights.

A month back, the indigo(non-intercooled) in which i was driving to bangalore blew its headlights.I knew it was the regulator.
I went to this Tata authorised service centre and they checked its voltage.It was hovering around 12.3 and hard revving sent it to 12.6-12.7. Once,it crossed 12.8 and the mech said it was the regulator which costs 950 bucks.
So, i was assuming ur Indica works the same way.