Paid app ($2.99) — only Google Play can deliver it
Show battery level, temperature & voltage (Overlay)
Battery Monitor Mini Pro — description
This is a mini battery monitor for your phone. It monitor the battery level in %, temperature usage in °C or °F, and voltage. It will always stay in the corner of your phone's screen. You can set the indicator to any corners of the screen, customize the color and transparency of the indicator. Advanced notification support battery usage chart for 12 hours.
Pro version support auto-hide panel and other advanced features, and it is ad-free.