I was looking around for a step down converter for a home robotics project, when I bumped into this one (image below). In the description it says

Can not charge iPhone and Samsung cellphones; But can charge other phones.

Although I'm planning on using it to power a Raspberry Pi and not a phone, I was simply wondering: why on earth would those specific phones not be able to be charged by a regular 5V 3A power source? I mean; power is power right?

enter image description here

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    \$\begingroup\$ Some phones may require a certain connection on the data pins to charge or to charge quickly. This charger may lack such connections on its data pins. See for example this question for a discussion on this subject. \$\endgroup\$ – marcelm Mar 4 '17 at 18:59
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    \$\begingroup\$ Or the manufacturer has seen the exploding phones on U-Tube and decided that don't want a piece of that action....... \$\endgroup\$ – Trevor_G Mar 4 '17 at 20:17

Power is power, but handshaking is handshaking. Some devices have prerequisite requirements in order to charge. Including simple shorting if data pins, or specific resistor voltage dividers on those data pins, full usb enumeration, or newer techniques like high frequency data communication over the power lines to enable high speed or high voltage charging. Highly depends on your device.

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