This app was mentioned in 5 comments, with an average of 0.80 upvotes
A slightly more technical app is https://play.google.com/store/apps/details?id=com.talaviram.overlaypercent which has the option of adding an outline around the numbers as well as some other custimzation options but some of those options require you buy the unlocker to access those feature, still a great app and should work on most android devices 2.3+ I believe
I haven't used this app but I have heard good things from other users.
https://play.google.com/store/apps/details?id=com.talaviram.overlaypercent
https://play.google.com/store/apps/details?id=com.talaviram.overlaypercent
might be what youre thinking of? Im pretty sure it works on android 2.3 and higher
<strong>Battery Overlay Percent</strong> in the Play Store