You can literally do this without an app. This feature is already on your iphone. Go to <Settings <Battery *wait for battery usage to load*
It will tell you the percentage of your battery used on every app for the last 24 hours or for the last week. If you want to know time spent on each app then just click the little clock icon on the far right at the top of the box and it will switch to time spent on each app.
ALSO... I have tried to download this app 8 TIMES. I have never been able to get it to work. When it tells you about the VPN and then how it needs to install new settings THE LINK TO INSTALL THE SETTINGS NEVER WORKS. The page loads, but theres nothing on it. The one time I did get it to work, it told me to wait a few days so I would have data to look at. I waited 48 hours and checked it, APPARENTLY, I hadnt used my phone AT ALL in the last two days because it said I had NO data to speak of. This is false because Im literally a phonaholic Im ALWAYS on my phone, thats why I downloaded this in the first place. I kept the app on my phone FOR MONTHS and checked every so often to see if anything changed... it never collected ANY data AT ALL. So, thats my experience and bit of advice. Just dont. Its not worth it.