battery covered under factory warranty?

[VTEC V6]

We had the battery replaced within 6 months of buying our 2000 740i in nov 99

it was a defective battery

now im having battery trouble again, it keeps discharging so much that i have to jump it daily

the dealer, diagnosing over the phone, said i have a bad battery and it just needs replacing, which costs $150

i think there is something draining the damn thing, i just dont know what

cause the maint indicator light thing on the battery is usually black when discharged, green when jumped and never orange which indicates a bad battery

he said for 150 he will diagnose it to make sure and install a new battery
i got an appt for tues aft to have it replaced

i assumed since i still have the factory 4yr/50k mi warranty that it would be covered, he said the battery life is only 3 yrs, and the new battery that they replaced under warranty back in 2000 is only covered under warranty for 2 yrs, which is just bs, its bmw not walmart batteries!

he said they have a much higher power demand thats why they dont last as long, ive never heard such a thing!

the battery in my sisters 97civic is the original!