Need a battery for Asus Laptops/Notebooks with the following model numbers: U20, U30, U50, U80? BTI has you covered with our premium 6 cell LiIon battery. This product has been engineered to meet or exceed the original manufacturer's specifications and replaces the original Asus part numbers 70-NA51B2100,90-NA51B2000,90-NA51B2100,90-NA51B2200,90-NA52B2000,90-NA71B1100,A42-A3,A42-A6. All BTI batteries are manufactured and tested under our stringent quality standards and come with an industry leading 18 month manufacturer's warranty. BTI.. True to the Original.