最簡單的

[UIDevice currentDevice].batteryMonitoringEnabled = YES;  
double deviceLevel = [UIDevice currentDevice].batteryLevel;  

方法一：通過蘋果官方文件裡面UIDevice public API來獲取，程式碼如下：

  [UIDevice currentDevice].batteryMonitoringEnabled = YES;
  [[NSNotificationCenter defaultCenter]
 addObserverForName:UIDeviceBatteryLevelDidChangeNotification
 object:nil
 
 queue:[NSOperationQueue mainQueue]
 usingBlock:^(NSNotification *notification) {
     // Level has changed
     NSLog(@"Battery Level Change");
     NSLog(@"電池電量：%.2f", [UIDevice currentDevice].batteryLevel);
 }];
@property(nonatomic,readonly) float     batteryLevel NS_AVAILABLE_IOS(3_0);  
// 0 .. 1.0. -1.0 if UIDeviceBatteryStateUnknown它返回的是0.00-1.00之間的浮點值。
 

但是經過測試發現，在iOS7 上 它是以0.05為單位的，但是在iOS9下測試，它是以0.01為單位的，雖然也是0.01為單位，但是測試多次也會出現偏差1%左右。也就是說, 這個辦法是存在缺陷的, 最起碼, 它不精確。

方法二：找到Mac下IOKit.framework，將IOKit.framework裡面的IOPowerSources.h和IOPSKeys.h拷貝到你的iOS專案中。另外, 還需要把IOKit也匯入到你的工程中去，此方法也會出現偏差，不精確。DEMO 地址：https://github.com/colin1994/batteryLevelTest.git

/** 
*  Calculating the remaining energy 
* 
*  @return
 
 Current batterylevel 
*/  
-(double)getCurrentBatteryLevel  
{  

//Returns a blob of Power Source information in an opaque CFTypeRef.  
CFTypeRef blob = IOPSCopyPowerSourcesInfo();  

//Returns a CFArray of Power Source handles, each of type CFTypeRef.  
CFArrayRef sources = IOPSCopyPowerSourcesList(blob);  

CFDictionaryRef pSource = NULL;  
const void *psValue;  

//Returns the number of values currently in an array.  
int numOfSources = CFArrayGetCount(sources);  

//Error in CFArrayGetCount  
if (numOfSources == 0)  
{  
    NSLog(@"Error in CFArrayGetCount");  
    return -1.0f;  
}  

//Calculating the remaining energy  
for (int i = 0 ; i < numOfSources ; i++)  
{  
    //Returns a CFDictionary with readable information about the specific power source.  
    pSource = IOPSGetPowerSourceDescription(blob, CFArrayGetValueAtIndex(sources, i));  
    if (!pSource)  
    {  
        NSLog(@"Error in IOPSGetPowerSourceDescription");  
        return -1.0f;  
    }  
    psValue = (CFStringRef)CFDictionaryGetValue(pSource, CFSTR(kIOPSNameKey));  

    int curCapacity = 0;  
    int maxCapacity = 0;  
    double percent;  

    psValue = CFDictionaryGetValue(pSource, CFSTR(kIOPSCurrentCapacityKey));  
    CFNumberGetValue((CFNumberRef)psValue, kCFNumberSInt32Type, &curCapacity);  

    psValue = CFDictionaryGetValue(pSource, CFSTR(kIOPSMaxCapacityKey));  
    CFNumberGetValue((CFNumberRef)psValue, kCFNumberSInt32Type, &maxCapacity);  

    percent = ((double)curCapacity/(double)maxCapacity * 100.0f);  

    return percent;  
}  
return -1.0f;  
}

方法三:通過runtime 獲取StatusBar上電池電量控制元件類私有變數的值，此方法可精準獲取iOS6以上電池電量

MRC:
- (int)getCurrentBatteryLevel
{
 if ([UIApplication sharedApplication].applicationState == UIApplicationStateActive||[UIApplication sharedApplication].applicationState==UIApplicationStateInactive) {
        void *result = nil;
        object_getInstanceVariable([UIApplication sharedApplication], "_statusBar", &result);
        id status  = result;
        for (id aview in [status subviews]) {
            for (id bview in [aview subviews]) {
                int batteryLevel = 0;
               if ([NSStringFromClass([bview class]) caseInsensitiveCompare:@"UIStatusBarBatteryItemView"] == NSOrderedSame&&[[[UIDevice currentDevice] systemVersion] floatValue] >= 6.0)
                {
                    object_getInstanceVariable(bview, "_capacity", &result);
                    batteryLevel = (int)result;
                    NSLog(@"電池電量:%d",batteryLevel);
                    if (batteryLevel > 0 && batteryLevel <= 100) {
                        return batteryLevel;

                    } else {
                        return 0;
                    }
                }

        }
    }

    return 0;
}

ARC:
- (int)getCurrentBatteryLevel
{

UIApplication *app = [UIApplication sharedApplication];
if (app.applicationState == UIApplicationStateActive||app.applicationState==UIApplicationStateInactive) {
    Ivar ivar=  class_getInstanceVariable([app class],"_statusBar");
    id status  = object_getIvar(app, ivar);
    for (id aview in [status subviews]) {
        int batteryLevel = 0;
        for (id bview in [aview subviews]) {
            if ([NSStringFromClass([bview class]) caseInsensitiveCompare:@"UIStatusBarBatteryItemView"] == NSOrderedSame&&[[[UIDevice currentDevice] systemVersion] floatValue] >=6.0)
            {

                    Ivar ivar=  class_getInstanceVariable([bview class],"_capacity");
                    if(ivar)
                    {
                        batteryLevel = ((int (*)(id, Ivar))object_getIvar)(bview, ivar);
                        //這種方式也可以
                        /*ptrdiff_t offset = ivar_getOffset(ivar);
                         unsigned char *stuffBytes = (unsigned char *)(__bridge void *)bview;
                         batteryLevel = * ((int *)(stuffBytes + offset));*/
                        NSLog(@"電池電量:%d",batteryLevel);
                        if (batteryLevel > 0 && batteryLevel <= 100) {
                            return batteryLevel;

                        } else {
                            return 0;
                        }
                    }

            }

        }
    }
}

return 0;
}