To a very great extent, battery life depends on how the car is driven, and what accessories are used.
If most of those 48k miles were highway miles, then 18 months is a very short life span for the battery.
On the other hand, if the car typically is driven for short drives of 5 miles or less, then 18 months might be realistic.
Also–if the car has heated seats, a lot of use of thiat feature, as well as things like the rear window defroster and side mirror defrosters can lead to shorter battery life if a car is used for a lot of short-trip local driving.