Find Working Batteries for Flashlight

You have a flashlight that takes 2 working batteries. You have 8 batteries but only 4 of them work.

What is the fewest number of pairs you need to test to guarantee you can get the flashlight on?

You need to test at least THESE pairs! 

Selection of Working Batteries for Flashlight

What was the task given?

Divide batteries into 3 groups - 2 of them having 3 batteries each and 1 with 2 batteries. 

Then the working pair of batteries has to be in 1 of these groups and now it's easier test to each group. That is 4 working pairs might be distribute as - 
(2,1,1) or (1,2,1) or (1,1,2).

If A, B and C are name of these groups then possible combinations for testing group A are - 

A1-A2, A1-A3, A2-A3

Similarly, for B group testing pairs are - 

B1-B2, B1-B3, B2-B3

And finally, if we don't find any working pair in above testings then the C1-C2 pair of group C has to be working pair.

You may find in the working pair in testing those 6 pairs from group A or B or can conclude that C1-C2 is the working pair.