CIPA ratings between two cameras from the same generation and from the same manufacturer can sometimes be useful. Comparing CIPA ratings across different brands is pretty much meaningless.
CIPA rating - A measurement of the number of images a digital still camera can take on a single battery charge. The procedure for determining this rating comes from the Camera & Imaging Products Association in Japan, and the camera vendor is responsible for the fairness of the reported results.
The above quote is from the PC Mag online encyclopedia entry for CIPA rating. The emphasis was added by me.
So what is the procedure used to arrive at a CIPA rating?
The Camera and Imaging Products Association (CIPA) developed this standardized battery-life test for digital cameras. The test procedure calls for using the camera to take a photo every 30 seconds, alternating with and without flash between each shot. The camera's screen is to be left on continuously between shots and used as the viewfinder device. The lens should be zoomed in or out all the way before every shot. After every 10 shots, the camera is turned off for a while and the cycle is repeated. Most camera makers test their own products which each camera maker self-affirms are done according to the CIPA guide. The CIPA standard states that the procedure specified was selected because it represents the way a typical consumer would use a camera.
Whether most users of the D4, D5, 1D X, and 1D X Mark II use their cameras the way the CIPA standard is written is open for debate. I don't know of anyone with one of those cameras who takes 10 photos, half with flash and half without flash, regularly spaced over a five minute period, then turns the camera off for one hour and then repeats for the 100 to 200 hours that would be needed to complete the test with these cameras. I don't know anyone with one of those cameras who shoots stills with the rear LCD turned on for any appreciable percentage of their still photographs.
(Disclosure: I know quite a few users of the D3s, D4, and 1D X. I'm not aware that any of the 1D X users I know have moved to the 1D X Mark II. Some may have. A couple of those who were shooting the D3s have moved to the D5. Small and medium sized news agencies tend to only replace bodies every other product cycle or they replace only the oldest bodies two models back when a new one is introduced. That is if they haven't eliminated their staff positions entirely and gone to hiring freelancers who buy their own gear and use it until it breaks.)
As you can see, the specification seems to have been written with early compact digital cameras in mind and not larger, professional grade interchangeable lens cameras.
The test procedure seems fairly specific, but there are a lot of variables that are left out when testing an interchangeable lens camera with no built-in flash.
- Most DSLRs, for example, don't have zoom lenses powered by the body. They're generally manually zoomed. So the DSLRs aren't adhering to the the zoom specification.
AF should be used if the camera has AF capability. But the door is left open to the possibility that an non-AF lens might be used with an interchangeable lens camera if one is in the camera maker's current lineup. Since all of the models mentioned in the question only come from Nikon and Canon packaged as "body only" units, they are free to choose whatever currently available compatible lens they want. AF power consumption also varies by lens. Which lens is used for the test will affect the battery performance, perhaps significantly.
Neither the D5 nor the 1D X Mark II have a built in flash. How is each manufacturer testing? Is an external flash attached? Even though the flash itself supplies its own power for the energy of the flash tube, the camera would use more processing energy to perform automatic flash calculations and communicate with the external flash. Or is no flash being used?
- "Leaves the screen on." That is accomplished by using Live View since the spec also requires the LCD screen to be used as the viewfinder function. In which case the sensor is also continually energized and the processor is producing 15 or 30 fps to be viewed on the camera's LCD screen. What is the factory default brightness setting of the LCD screen that the standard specifies must be used? Is it the screen's brightest setting, perhaps a medium setting, or even a very dim setting? Is the screen on one brand of camera brighter at the maximum setting than the screen on another brand?
Is the mirror being cycled up and down between each frame? Or left up in Live View? Even in Live View some AF modes cycle the mirror to use PDAF. If that is the default mode from the factory settings then it is the one that the standard requires be used in the test.
What is the camera pointed at while the test is being run? Is it a solid color scene in bright light with little detail that doesn't demand much processing power and is compressed to a smaller file sizer that means less power is consumed writing to the memory card? Or is it a more detailed scene in lower light shot at high ISO that results in larger file sizes and longer memory card write times? The standard only requires that it is within the range of the camera's AE system.
- Speaking of file sizes, are the files being saved in the largest, finest JPEG setting? Or in smaller, more compressed sizes? Or maybe in much larger raw files?
The spec requires using the camera's AE system, but it doesn't specify which auto exposure mode or if the scene and other settings can be manipulated to leave the aperture wide open and thus saving power.
Power is to be turned off after each 10th shot. How long it is left off is up to the individual manufacturer conducting the test. "It shall be the responsibility of the manufacturer to determine the appropriate length of the off time."
This isn't even a comprehensive list and it is already easy to see how much the manufacturer can alter the results by adapting their the testing methodology to a standard that was designed to be used with early compact digital cameras.
It's also fairly doubtful that either the D5, D4, 1D X, or the 1D X Mark II is being used by most of its users in Live View mode 100% of the time. Yet the CIPA standard requires the largest screen be on and used for the viewfinder function. In other places, though, the standard indicates that the camera should be used with factory default settings. So what do we do if the factory default setting is to shoot through the viewfinder, rather than using Live View?
When using my Canon cameras I almost exclusively shoot through the viewfinder and I can routinely get over twice the number of shots as the official battery rating and still have 30-40% left in the batteries. On my longest shoots I'm usually outdoors all day and into the evening in fall weather that is often cooler than 23°C/73°F fro the greatest part of the day.
Finally, as we already noted above, each camera vendor is free to conduct their own tests in the way they desire and self report the results of such tests. I think most of us can do the math on that one.