Battery diagnostic tool at Apple Genius Bars
It is real.
One of the Genius perform the diagnosis right in front of me. If memory serves, it goes like this:


With your iPhone, or whichever iDevice, connected to the Apple Store's wifi, "Apple Store", open Safari, and connect to URL "diagnosis.apple.com" (or diagnostics)
A page will show up, with a text entry bar for the Genius to enter a verification code.
The genius will pull up a page from his "Genius" iPod touch, (the one with bar code scanner), and possibly clicked a certain icon, which leads him to the verification code. I assume that the verification code is changing over time.
After entering the code into your device, there goes the diagnosis page.


In the brief moment he was looking at my iPhone, I was able to see the maximum capacity (by design) and the capacity left, which is 98% in my case. By this figure he will determine whether your iPhone's battery needs a replacement. If not, he will suggest that you restore the iPhone and the battery issue (if any) will likely go away.

I asked the Genius whether that's a secret page, he just calmly answered that it's just a tool for them to do the diagnosis.

So no, you cannot purchase it, as it is simply an internal webpage which requires Genius verification.



As a side note, informed by the same Genius, the easiest way to tell whether your battery/system has problem with battery life is to go to Settings - General - Usage in your device.

Look at the "Time Since Last Full Charge" section, it tells you how long the device has been used and how long it has been on standby. If the two figure are very close, e.g. 2 hours for usage and 2 hours 10 minutes for standby. But you clearly remember that you haven't been "using" the device for 2 hours, but only let in stay in sleep mode. This means that something running in the background is draining your power, and your device is never sleeping.

In this case, the Genius will suggest that you restore the iPhone (and restore from iCloud or iTunes if you wish) and the problem will most likely go away.

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Update

After visiting the Genius bar again, I saw a genius helping a customer on similar issue and I took a picture.



Instead of connecting to some normal URL in safari, a link in the format of "diags://#####" would be generated and visited by the device in question.