For charging your mobile phone, you need a phone charger also called wall charger (because you attach them to a wall power socket). Usually these phone chargers come with your new smartphone when you buy it. The phone chargers for smartphones having batteries up to 4000 mAh are usually 1 Ampere charger (they give output of 5 Volts 1 Ampere). Similarly, smartphones with larger batteries have chargers with more output current such as 1.5 Amperes or 2 Amperes.
Moreover, you can buy after market phone chargers that provide multiple charging USB ports or have higher output current. But which of these is the fastest phone charger for your Android phone? You can find it very easily using an app called Charge Meter.
In the Charge Meter app, it displays all the details about the charging and discharging process along with information about the phone battery. It displays the current battery output voltage, temperature, charge percentage, status (charging or discharging), maximum capacity, health, technology (Li-ion or Li-ion Polymer), Android version, smartphone manufacturer, model and build ID etc.
In order to find the fastest charger, you have to attach the charger to the phone and notice the charging speed in the Charge Meter app. It should display the charging speed in terms of %/hour. For example, my stock phone charger is charging the phone battery at the rate of 7% per hour.
Keep trying all the chargers and the one that provides the fastest charging speed is the fastest charger for you. For example, if you have three chargers and they provide 7%/hour, 8%/hour and 8.5%/hour charging speeds, then you can pick the charger that is providing 8.5%/hour charging speed.
This is so easy method to find the fastest charger for your phone. However, you have to keep in the mind that chargers with very large output current like 2 Amps and 3 Amps should be used only with smartphone batteries with larger capacities such as 5000 mAh or 6000 mAh.
You can get the Charge Meter app for Android from https://play.google.com/store/apps/details?id=dev.km.android.chargemeter.