TestBuild/Assets/Plugins/BatteryPlugin.java

package com.example.androidbatteryplugin;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.BatteryManager;
import com.unity3d.player.UnityPlayer;
import com.unity3d.player.UnityPlayerActivity;

public class BatteryPlugin extends UnityPlayerActivity {

    public static int GetBatteryLevel() {
        IntentFilter ifilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
        Intent batteryStatus = UnityPlayer.currentActivity.registerReceiver(null, ifilter);

        int level = batteryStatus.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);
        int scale = batteryStatus.getIntExtra(BatteryManager.EXTRA_SCALE, -1);

        float batteryPct = (level / (float) scale) * 100;
        return (int)batteryPct;
    }
}