You want to check the battery graph of your phone as it gives you a clear idea of how much battery has exhausted. There is a built-in app in every Android mobile device to help you monitor the usage.
Go to Settings >Battery (However, the app location may vary slightly depending your Android version). There you can see a history graph of the battery usage of your phone. It gives you a clear idea about how much more you can use your phone before the battery is finally exhausted.
Observing the Android battery graph closely also gives a few more ideas and some other interesting details. It gives you an insight of the pattern of the mobile signal strength.
Android battery graph
The graph is not a rocket science to explain. Those where are breaks means, the device is powered of. Any yellow or red point shows that the charge was critically low during that period.
Tapping into battery history graph, will also display a detailed battery usage and charging time. As given in the screenshot below:
The colored bars reflect the strength of mobile network strength signal. Here are the other interpretations:
The Wi-Fi block indicates the time when the radio was turned on though the device may not be actually connected to a Wi-Fi network. The colored strips for charging indicate the time when your Android phone was connected to the charger. The ascending slopes in the battery graph are also an indicator of the charging time.
Colored bars represent background apps that may be running in the background even when the screen is off. For instance, your phone app may be periodically checking the new mails even when its in sleeping mode.