Exactly, that 100 amp has to go somewhere, if there is nothing asking
that amount the alternator won't supply it.
All modern cars have smartcharge alternators, no escaping that.
It's needed to lower emissions, it means the alternator will start charging at
full power whenever there is free energy, e.g. when you lift your throttle
going at a trafficlight, the engine on the overrun.
More modern cars need a AGM battery, these batteries can be left in a
partly charged state so that there can be "free" energy stored in it.