Are battery monitors accurate ?

Are battery monitors accurate ?

Battery monitors are an approximation and not accurate :

• They are usually based on Voltage measurements




• You need Voltage and Current measurements to get an accurate picture. Most phones have only Voltage sensors. Current sensors ( Coulomb Sensor) are available in some at OEM discretion ( Honor 6 for example). Even if the device has it, the kernel ( since you mentioned flashing ROMs) need to support it. For more details, see this How does the Android estimate the battery level without a current sensor?

I don't know about your device but let's assume that it has only Voltage sensor as is common. Problems with accuracy using voltage sensing ( apart from current sensor bit)

• How often it needs to be updated (and reflect on the status bar), AFAIK is left to OEM implementation. Is the battery icon on your Android phone telling you the truth? says that
  updating battery SoC information is manufacturer dependent and not Android driven (emphasis mine and I haven't found any other source to dispute this)

  
  
  

  
  

  We contacted Google about this, and the indication is that the battery data given in the Settings is the correct one. The updating of the icon happens "every so often", we're told; quite how often is entirely up to the manufacturer, rather than being part of the Android code

  
  




• Further voltage is NOT a true indicator of the amount of charge. This will explain in detail why




• Yet another problem with Voltage based measurement is battery behaviour. Li Ion batteries display a "flattish" voltage drop even though the amount of charge falls relatively steep as seen here at Fig.2

Your app like other apps uses Voltage and inherently plagued with inaccuracies, so at best gives a rough estimate

• Starting from Lollipop Android gives you an estimate of how long your battery is going to last, so the interpretation in other answer is valid




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  Another strange thing: when i look at Android's battery statistics, the sum of percenatges of the battery that goes towards each app(including screen) is only at ~50%.

  
  

Measurement of consumption per app and therefore total can also be misleading. This post, though old sheds light on this How does Android calculate battery consumption by apps?

Now coming to your concern of battery problems - it's hard to say. I would suggest that you revert to stock ROM and then observe to establish ,if, there is a problem ( Could be custom ROM, kernel too)