Sony — defective battery

good day sir/madam

all electronics in my home from past 20+ yrs are from sony and are very reliable when compared to others

so thinking that i would get the same reliability from a sony vaio laptop i bought a sony vaio VGN CR313H
in sep 2008 paying Rs 40000 when similarly configured laptops were available for 30000

the system promised 4.5hrs of back up from the battery i used to get around 3 hours still i thought it was ok( note this was normal internet uasge without wifi using wires ethernet) now exactly after 1 year after the warranty expired the battery started to detoriate like anything an before u know it it is giving me appx 1hr o[censored]sage that to with brightness set to half , the new battery pack costs appx 6000 rs why should i pay extra money for a new batery when i have already paid a premium of 10000Rs

Kindly help me in this regard
i Would be very greatful