OK, first let’s state the obvious: a phone does not drain its battery while it is plugged into its charger, which is what the question implies.
I have to believe that's not what you intended, but that’s what’s implied by the wording.
What’s also implied is that you think a battery should not drain when it is not plugged into the charger. That doesn’t make sense.
The whole point of plugging a phone into a charger is because it has drained while it was not plugged in an therefore it needs charging.
What I think you might mean is does a phone battery drain when the phone is turned off and not plugged into its charger?
If so, the answer is yes, although it does so very slowly. If you charge up your phone to 100%, turn it off and put it back in it’s box and
turn it on a year later, you will find the battery is no longer at 100%.
It won’t be fully discharged, but it won’t be at 100%. Maybe 80% or so
(I’ve never tried it, so I really don’t know how much — I suspect the amount would depend on the type and size of the battery, too).
Batteries deplete no matter what. They just do so slower when they aren’t being used.
Why is Xiaomi so popular?
A mid-range smartphone only costs around $100 to $150 to be made by a company.
However, the company spend much of the money on marketing and advertisement. That’s why a $100 smartphone cost us $300.
So, what Xiaomi does was they reduced the marketing cost and distributed smartphone by themselves from their own websites.
This made a $100 mobile much cheaper and with good quality.
When people saw the cheap phone, they started buying it.
They loved the quality.
Those who could not afford $300 smartphone started loving MI.
People started spreading the good reviews on MI on internet and everywhere saying how good and cheap the MI phones actually are.
Hence, Xiaomi became popular.
In one line, Xiaomi had the different business model than other phone producer which made them what they are now.
OnePlus (owned by Oppo), Honor (owned by Huawei), Meizu are some other smartphones manufacturers who copied Xiaomi business model after seeing