RE: Stupid or Just Bad Luck??

"It's 100 amp/hours. Or something like that."

That's exactly what it is. Battery manufacturers specify the number of amps
a battery can theoretically supply in one hour to go from fully charged to
dead so 100 amp/hr is 100 amps for one hour.
As Frank says though this is not practical in real life, An Alternator will
only generate what is demanded of it which should be equal to what is being
used plus what is required to top up the battery. Most cars with Halogen,
Air, radio etc will pull around 40 Amps which everything running.

Duncan