I see no need to carry the extra weight the whole time. I treat mine like extra AA and 9Vs I carry with my other detectors. You can also hook it up at any time to bring the charge back up, including driving to another spot although you can do that with the cord. The truth is I have never actually needed it although I have used it. I don't like it attached when detecting because of my weenie arm. How long it would last can be figured out by the capacity ratings of the two. The Legends battery is rated at 5050mAh, the backup is rated 5500 so ideally that should run twice as long. I know I charged my Legend up twice from 2 bars down using the backup battery. It might still charge it up more times but I haven't tried. But it is there if I want to go back into remote areas camping.