Why a battery tester and tip
A battery is fresh goods and should not be left
in the flashlight over long periods of time. When you remove old batteries then check for damages. When replacing with new ones also check them.
Never use batteries from different brands in the flashlight.
All the batteries should be fresh and the same type and brand.
If you have some batteries in a box that you can use then it is a good idea to check them with a battery tester to see. It is not smart
to have an uneven energy supply in the flashlight. I always learned in electronics that this is so. It may not be so dangerous in a modern LED flashlight with the safety measures in them but I feel it is a good rule. So I just use the battery tester and see
if the batteries are even.