Software
Mobile Apps
personally tested
BatteryBot Battery Indicator
Battery Monitoring Tool
- Introduction
- Developer/Supplier
- Purchase/Installation Date
- Purchase Price
- Rating
- System Requirements
- Languages
- Support
- Features
- Practice
- Pros
- Cons
- Conclusion
- Alternatives
- Text auf Deutsch
Ease of Battery Care
BatteryBot Battery Indicator is the original battery indicator app for Android™, released in 2009 as simply “Battery Indicator”. Both the free and the Pro versions have maintained a 4.5+ star rating in Google Play from the beginning. It offers two modes: the original status bar indicator and now also a desktop widget. You can use either or both, as you please.
BatteryBot monitors and shows your battery charge level (percent) as an icon in your status bar, with how much time you have left, temperature, health, voltage, and time since plugged/unplugged in the notification area. It now also has a 1x1 app widget. The Pro version also has user-configurable alarms, logs, mA current information, and a large (4x1) widget.
Give it a try – it's free and open source! BatteryBot Battery Indicator requires no special permissions whatsoever. It's an excellent and complete solution to your battery needs.
In accordance with developer's business philosophy, the free version is hardly limited. The Pro version offers above all a detailed logbook, which means maximum comfort for mobile phone testers like me. 
Developer/Supplier
Darshan Computing, LLC (on GitHub)
BatteryBot Free (formerly Battery Indicator)
Download at F-Droid, Google Play or GitHub
BatteryBot Pro (formerly Battery Indicator Pro)
Download at F-Droid, Google Play or GitHub
Purchase/Installation Date
13.12.2010: Samsung Galaxy S GT-I9000
13.12.2010: Motorola Milestone 2
07.11.2011: Samsung Galaxy Note GT-N7000
24.01.2014: Fairphone 1
17.09.2021: Fairphone 3+
10.12.2021: Fairphone 4 5G
Purchase Price
13.12.2010: 1.49 USD was equivalent to 1.12 EUR at the time
Rating (1–5 Stars)
***** (5 stars)
System Requirements
Android OS 8.0 or higher.
Languages
Website: English
Mobile App: English, Deutsch (possibly others?)
Support
in English, via E-Mail, GitHub, Stack Overflow
Features
- no ads (why would you want a battery app with ads?)
- exact battery level in the task bar
- or hide the task bar icon, keeping notification in tray – or turn notification off completely and just use widgets
- multiple icon set options
- automatic time-remaining estimates based on recent usage
- small (1x1) circle desktop widget
- Notification (with status bar icon) is now optional – can be used as just a desktop widget
- very lightweight (Developer promises that this app is very power efficient)
- shows 1% increments even on most Motorola devices
- well-tested and supported on all major versions of Android (2.1 through 7.0; Eclair through Nougat) and form factors (small phones, medium phones, large phones, 7-inch tablets, 10-inch tablets)
- Open source
Pro-only Features
- Alarms (fully charged, charge above X, charge below X, temperature above X, or health failure)
- (on devices that allow it) Icon colors (including fully configurable green, amber, red, and black icons)
- (on most devices) Electric current (mA / milliamp /milliampere) reading to/from battery on supported devices
- large (4x1) desktop widgets
- Battery logging. You can log the battery state (charge, status, voltage, temperature) and browse the logs on the phone or export to CSV
- control the notification priority on Android 4.1+ (for example, you can set to “minimum” to hide the icon but keep the notification in the tray)
- lots of configurability of behavior and appearance
- the charging indicator (the bolt under the number that indicates when the device is being charged) for the sets that have it is optional in the Pro version
Practice
BatteryBot Pro has been my longest and favourite consistently reliable app.
I have used this app for almost 12 years. 
Now I am using iPhones again and miss the advantages of this app.
Pros
- the free version also works without advertising
- free version is functionally hardly limited
- informed about current battery status at all times
- easy-to-use, self-explanatory user interface
- does not itself consume a measurable amount of energy, works effectively and saves power
- Alarms can be set for almost any status change (Pro version)
- detailed battery log – can be saved to memory card (Pro version)
- Pro version can be downloaded and tried out free of charge via F-Droid
- sympathetic customer-oriented business philosophy
Cons
- None discovered even after eleven years
Conclusion
Highly recommended! A must have!
Alternatives
I had tested many apps of this kind and none was convincing. 
Most of them were conspicuous by their own energy needs - that' s a no-go!
In addition, there were incorrect measurement results and missing alarm settings as well as missing logging options.
During years of pure iPhone use, I did not find a comparable battery monitor for iOS either.
Since switching to iOS, I had missed BatteryBot Pro very much and installed it as the first app on the Fairphone 3+ as soon as it went into operation. And of course also on the Fairphone 4 5G.
There is still no comparable battery monitor app for iOS.