代码：

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'dart:async';


class BatteryPage extends StatefulWidget {
  @override
  _BatteryPageState createState() {
    return new _BatteryPageState();
  }
}

class _BatteryPageState extends State<BatteryPage> {
  static const platform = const MethodChannel('samples.flutter.io/battery');

  String _batteryLevel = 'Unknown battery level.';

  Future<Null> _getBatteryLevel() async {
    String batteryLevel;
    try {
      final int result = await platform.invokeMethod('getBatteryLevel');
      batteryLevel = 'Battery level at $result % .';
    } on PlatformException catch (e) {
      batteryLevel = "Failed to get battery level: '${e.message}'.";
    }

    setState(() {
      _batteryLevel = batteryLevel;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Material(
      child: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            new RaisedButton(
              onPressed: _getBatteryLevel,
              child: new Text('Get Battery Level'),
            ),
            new Text(_batteryLevel),
          ],
        ),
      ),
    );
  }
}

iOS 的代码

AppDelegate.m

#include "AppDelegate.h"
#include "GeneratedPluginRegistrant.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  [GeneratedPluginRegistrant registerWithRegistry:self];
  // Override point for customization after application launch.
    
    FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;
    FlutterMethodChannel* batteryChannel = [FlutterMethodChannel methodChannelWithName:@"samples.flutter.io/battery" binaryMessenger:controller];
    
    [batteryChannel setMethodCallHandler:^(FlutterMethodCall * _Nonnull call, FlutterResult  _Nonnull result) {
        if ([@"getBatteryLevel" isEqualToString:call.method]) {
            int batteryLevel = [self getBatteryLabel];
            if (batteryLevel == -1) {
                result( [FlutterError errorWithCode:@"UNAVAILABLE" message:@"Battery info unavailable" details:nil]);
            } else {
                result (@(batteryLevel));
            }
        } else {
            result(FlutterMethodNotImplemented);
        }
    }];
    
    return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

- (int) getBatteryLabel {
    UIDevice* device = [UIDevice currentDevice];
    device.batteryMonitoringEnabled = YES;
    if (device.batteryState == UIDeviceBatteryStateUnknown) {
        return -1;
    } else {
        return (int)(device.batteryLevel * 100);
    }
}

@end

Android的代码

（以后再补充。。。）

本文参考

官方文档