Bad after sale service and warranty of Nissan

I have a problem which does not make sense.I bought a Nissan Nivara with a 7 year warranty.After less than 2 year the battery have to be replaced.I have to pay 50% of the replacement battery which have a 3 year warranty.After leaving lots of compaints for someone to explain that to me i get people from clientservices who knows nothing.I wanted to know how can the replacement battery have a better warranty than the one the Nivara came out with originally