In other words I could fit a larger one without any effects on the alternator?
I have fitted a lot of extra electronics. 2 rad fans, lambda, msd unit which can suck up to 6 amps also.
Mine is almost 4 years old and it struggles sometimes to crank when cold especially. Thinking the voltage is dropping too much when trying to crank.
When the engine is running the electrical consumers are fed from the alternator,
so the alternator "strain" mostly depends on the consumers' current draw
(unless the battery is low).
Until the current drain stays below the alternator's delivery curve, the charging of the
battery is assured. The delivery curve is highly depend on the RPM, e.g.: the green
labelled Bosch alternator's output voltage will drop significantly when the current demand
exceeds 20A at idle. In this case the battery charge is inadequate.
It may worth to sum the power or the current figures of the consumers.
The attached example shows some power and current figures. The currents could be somewhat
higher in real life, because the rated power of the consumers are valid for
12V supply. (I forgot the front parking lights: 2x5W -> 0.8A plus)
The sum figures of the currents are far beyond of the alternator's current delivery at idle,
when both cooling fans are on. If we remove the fans from the picture 19A draw will
still remain, which is barely enough to charge the battery. So if the car is often used
in city traffic for short trips (or prolonged struggle at idle), the battery may need to be charged from
external source periodically, in order to keep it healthy. It might also need to consider that,
a higher capacity battery needs more time to recharge fully, when the charging "resources" are limited.