[PAOLO137 (OP)]

when I start the engine of my car, clearly some energy (units = current x time) is withdrawn from the battery. Of course the value will depend on the car design but we may guess an average value.
Now, how long will it take to recharge the same amount of energy from the car generator?

This question arises from two reasons :
a) I have got a large car used seldom for vacations or voyages. Then I about every week start the engine and let it go for some time. How long wil be required to keep the battery in good shape?

b) The so called start-stop technology used nowadays on some cars to use less fuel . How does it work if my trip gets into a lot of red traffic lights, or a heavy stop and go traffic?

[alancalverd]

Say the battery runs the starter for n seconds at 100 amps. The alternator can deliver about 50A so it will take a little over 2n seconds to recharge the battery if there is no other current drain on the alternator. Allow maybe 5A for the ignition, radio etc and of course more if you are running any lights, heater fans.....

Fuel saving on start/stop is pretty minimal because the car only consumes about 0.1 gal/hr when idling, and if you were stuck in a major jam you'd switch the engine off anyway. The object is to minimise air pollution at road junctions.

[CliffordK]

If you have a car that sits months at a time, I'd encourage installing a battery isolator switch.  It will knock out your radio channels, but will save the battery.  You could also consider a solar trickle charger if exposed to sunlight.

I'm not sure the current draw of the starter, perhaps 200A for a larger vehicle, with 100A alternators being common, although the alternator may not come up to full power at idle.  There will also be some losses in the charging/discharging circuitry.  So, perhaps a somewhere between a 2:1 and a 4:1 ratio for the starter alone, and a little longer to top off the battery from other losses.  But, if you crank it for less than 10 seconds, then a minute of idling should probably be enough.  I don't like to just sit and idle my engine, but like to at least get it to start coming up to temperature if doing so, which may be about 5 min with zero, or a light load.

A full battery should be about 12.4 to 12.6V after sitting, and perhaps even as high as 13.2V immediately after charging.  When under load, the voltage will drop somewhat, but at rest, anything below 11V or so is damaging to the battery.

[syhprum]

Modern alternators are voltage stabilised at 2.3v per cell i.e  13.8v for a nominal 12v system current limited of course in case the battery has dropped to an abnormally low voltage

[SeanB]

Basically the starter draws around 200-400A, and this will be recharges with about 10 minutes of driving. Car electric systems typically draw around 30-40A in use, just from the engine management and things like DRL and the fan. The green efficiency engines will not stop the engine until it is warm, so they will actually charge the battery fully, and only when the battery is charged and the engine is close to operating temperature will they do the stop start. As well if the battery gets low during stop start driving they will run the engine to keep the battery charged, and as well most actually disable the alternator when the engine starts and when the battery is fully charged to reduce fuel consumption.

[syhprum]

It is rather difficult to measure the current taken by the starter motor, may I suggest the following scheme that I have used.
connect a multi range volt meter to either end of the heavy cable feeding the switch that controls the motor and note the low voltage recorded (as it probably also supplies other loads on the vehicle) then connect a known load from the motor end of the cable (say a 50 watt 4 amp lamp) and note the increase in reading.
You have now calibrated the cable as a meter shunt and when you run the motor you simply read the meter on a higher range and do some simple arithmetic.
Early vehicles pre electronic ignition used to have a 6v ignition coil fed via a resistor that was shorted out when the starter motor was operated to cope with the loss of voltage but its all done by computer today.   

[PAOLO137 (OP)]

Many thanks to all the friends who have taken some of their time to answer my question.
I think that I have a clear picture of the subject now. My best regard to all of you, Paolo.