Isn't it harmful to put a phone that consumes 750 mA to charge from a device that gives 2 A?

Question

Batteries

There is a charge from Lenovo P780 phone. Gives 5V 2A. And there is some other device, say, an e-book, which does not have its own mains charging, but is usually charged from a computer's USB, where, if I'm not mistaken, 5V 0.5A. Is it fraught with the fact that I will charge the book from this charge? Or will the device itself take as much as it needs?

Answer 1

Yes, you can. In terms of replacing power supplies, the requirements are:

• the voltage must match - this is done, you have 5 volts here and there
  
• the power of the power source must be greater than or equal to the power of the consumer.
  

Power is calculated as the product of current (Amps) and voltage (Volts). Since the voltages matched, it is permissible to connect any power source that delivers 0.5A or more.
There is also a third parameter - voltage stabilization. Since in this case we are dealing with USB, the stabilizer is present in both power supplies, so there is no problem with this.
SHVV : I gave the general conditions. For devices with a USB connection (excluding OTG) there are the following rules:
USB can provide at least 0.5A (USB-OTG - 0.15A)
In order to receive more, the device must "negotiate" with the host. In the USB3.0 standard, for example, it must specify its own energy class. In the lower standards, various tricks are used with the help of resistors and the like - so that the charger can tell the consumer that it can give more than the standard 0.5A. Otherwise, the device should not take more than 0.5A.
That is why, if you mix up chargers from smartphones, they may start charging more slowly, or not charge at all, even if the current declared on the chargers matches.

Answer 2

Safely.

Gives 5V 2A.

Gives a maximum of 2A.
When charging the phone will give 0.5A.

Answer 3

No

Answer 4

2 options:
1) it will always be undercharged
2) someday a disaster will definitely happen (lead-free solders, cheaper assembly of b elements ..) and it will start to recharge
Accelerated death of the battery and / or gadget.