Jasu,
I'd reckon it's best to start at the very beginning, in this instance.

"A modern automobile has a 12 volt electrical system.
A fully charged battery will read about 12.5 volts when the engine is not running.
When the engine is running, the charging system takes over so that the voltmeter will read 14 to 14.5 volts and should stay there unless there is a heavy load on the electrical system such as wipers, lights, heater and rear defogger all operating together while the engine is idling at which time the voltage may drop.

If the voltage drops below 12.5, it means that the battery is providing some of the current. You may notice that your dash lights dim at this point. If this happens for an extended period, the battery will run down and may not have enough of a charge to start the car after shutting it off.
This should never happen with a healthy charging system because as soon as you step on the gas, the charging system will recharge the battery.

If the voltage is constantly below 14 volts, you should have the system checked.
If the voltage ever goes above 15 volts, there is a problem with the voltage regulator. Have the system checked as soon as possible as this "overcharging" condition can cause damage to your electrical system."

The link to the beginner's classroom is here...
http://www.carparts.com/classroom/charging.htm

Roger, but unless the charging voltage exceeds 13.8v, the battery will not be charging, so no amps through the battery?

"To charge the battery, the alternator voltage output has to exceed a minimum charging voltage. This minimum charging voltage is 13.8 volts dc across the battery terminals, or at the output of the alternator. A single lead-acid cell starts to charge at anything over 2.25 volts. Since a 12 volt battery has six cells, any 12 volt lead-acid battery needs at least 13.8 volts to start to charge. This voltage will be enough to fully charge or maintain the battery on a trickle charge, but charging time will be very long at 13.8 volts.

To fully charge in reasonable times, alternator output must be 14.2 V to 14.5 V as measured right across the battery posts. Above 14.5 volts charging voltage, batteries have a greatly increased tendency to release excessive acidic vapors, hydrogen gas, and to corrode things around the battery. The battery terminal charging voltage must be less than 14.7 volts to prevent excessive gassing."

Yes Ken

but, its still not just about voltage, but also amps

So, I decide to fit an alternator, rated at 6V and about 100 amps, to my 12v chariot.
How's that going to fare?

Yes, right that...

And no, about Turbofiat's toughts, about having 14,3 volts after car hasn't been driving for a long period. It's about chemistry of battery, the voltage doesn't raise above 13 volts...


About very modern cars, only some years old, I have been told that on some new BMW:s tend to pretend battery not fully charged, because of emission controlling, and how they are related, is the alternator start charge full power when the car is moving and brake pedal is pressed. That claim was told by one BMW mechanic I know... But don't compare that to 2CV, or any "modern car" from earlier than 2000...