You won't get more than a year's guarantee out of any battery you buy (or on one in any new system, short of pre-paying for a battery replacement with a 3-year warranty)  but OEM batteries are usually better than third party batteries.  Also, given the age of the system, if it's never had internal maintenance (thorough cleaning and replacement of the thermal pads), that should be done -- excess heat quickly destroys batteries. OEM battery: [URL]  
Not sure what to do here. Last year I replaced the 97 kwh battery and the SSD on my XPS 15 9560 and maybe a year later the battery is going flat within an hour. I'm debating whether to keep investing money into this machine or get another new ultrabook. If I go the route of replacing the battery on this machine, any suggestions on where to buy a reliable battery that won't go out after a year? My other computer is a painfully slow HP and this XPS is my daily driver. 
XPS 15 9560 potiential 2nd battery replacement