Years ago I started testing TV LED Backlights with 9 volt batteries. There are even videos of me doing that on Youtube. Well I fianlly broke down and bought a LED tester. The reason was that I had just received a second identical TV to one where I had replaced all of the LED strips and this time I did not want to buy any, but to reuse the good ones from the last TV instead. But I was in for a big surprise. Only two LEDs were bad in the last TV so I was sure I was all set to fix the new one. However in the new TV there were at least 20 bad LED's. Even with 8 good LED strips out of 10, I was still not able to fix all the lED's!! The best I could do was to leave 2 bad LED's. On top of that, as I reasembled the TV two lenses fell off of the strips, so it has to be taken back apart and repaired again!
The short story is that this tester works great. I did have one strip that tested bad in the TV but tested good once it was removed for some reason.
This are some really burnt out LED's