The warranty on the battery is 12 months, regardless of any extended system warranty. If you've still got 80+% of the battery capacity remaining after two years, it's doing very well and likely won't need replacement for at least another year if not longer than that.  
I have a nearly two years old Inspiron 15 7510, and recently its battery life is getting low. I ran a battery report few days ago and it tells me that my full charge capacity is decreased from 54,891 MWh to 46,318 MWh already. I would like to know if the two years premium support would cover this situation and replace my battery for a new  one?  I purchased the laptop back in China and now I'm currently in Canada so I will be in international support. 
Inspiron 15 7510, battery, Premium Support