To restart bme takes some time.
Check if the app works from the commandline. Should be something like this:

If there is some warning about an incorrect password, you have to adjust the root password in that script.

But indeed, sometimes just nothing happens if you tap on the app icon. But sometimes, the charging percentage jumps back to normal. Just to fall back to 4% in a couple of minutes if you continue using your phone in the same way as before...

Edit: Oh, and to restart bme only brings the charging state back up if you actually experienced a drop to 4%, not if the battery discharged slowly.

I also find it very amusing, how the N9 warns you about your battery during these drops. Everytime I see the notification that my battery is low, I instantly do something to get it back up by connecting to a charger or quickly using this app. Because otherwise, the phone just says: "Battery low", then instantly "You should now connect the charger", then instantly "Battery empty" and then it's over becase the phone just switches off. You get like NO time to react to this Kinda ridiculous somtimes. It actually nearly happened right now while I typed that sentence. Funny!