Battery benchmark on X5

[gnu]

I looked at the "estimated time left" on my X5 and noticed that it was WAY off. Of course I know that it hasn't been optimized yet, but it projected 6 hours with 100% battery...
So I decided to use the battery_benchmark plugin to find out how long my player really runs. I played a 45 min album and started the plugin and then let it run until my X5 shut down (~ 15 hours). Then I wanted to look at the battery_benchmark.txt file for additional information, but it's not there! I searched my whole player, but the only file with the term "battery" in it is the battery_benchmark.rock!
What have I done wrong? I'm sure I started the plugin right!