I understand that HDMI's pin 18 carries a +5V charge. Similarly, USB carries 5 volts and this is used to charge many smartphones with a micro/usb connector.
Could it possible to build a smartphone that charges with an HDMI port? Would that be completely impractical currently?
The specification of the +5V charge is 50mA(according to wikipedia) This is even lower as USB before enumeration(100mA) Any device wouldn't be able to charge , or it will be very slow at 50mA (10x slower than normal USB port at full power, 500mA)
I don't think the HDMI connectors are specified for high currents in excess of 0.5A. Furthermore, the +5V is probably used in the monior to power some small circuits for identification of itself to the PC/television/etc. , even if it's turned off.
Yes it can be. But not with just the HDMI standard. There is the MHL http://en.wikipedia.org/wiki/Mobile_High-Definition_Link standard, which uses HDMI connectors. v1.0 has 5V/500mA, V2.0 has 5V/900mA. It's designed to allow a phone or other device to charge from the main device (TV/Projector) while providing a high bandwidth video/audio link.
Your phone and the source device would have to support MHL to take advantage of that, as there is physical routing inside the phone that needs to be in place.
well, for starters +5v 55mA is required to be provided by source device and not receiver - which means your phone would have to act as receiver - for example display what comes from computer on it's screen - etc. No phone I know of acts like that - all of them are source - which means they are ones providing 5v current ;]
On some laptops and tablet PCs if you accidentally short this line to Gnd it can cause strange symptoms, current limit or not and even prevent booting in severe cases. PS3 and PS4 the same applies. I actually found that the Pi Zero V1.1 could actually run on this, charging up an itty bitty LiPo like the ones used in 3D glasses from HDMI power and then activating when triggered from a special remote code or even using a 10F 2.5V supercapacitor. please note, use a single chip 5 pin charger IC or other limiter if you plan to try this as supercaps/LiPos DO NOT LIKE excess voltage. 2.4V seems to be a sensible hard limit for supercaps and 3.92V for LiPo / 3.25V for LiFePO4.
Note: a Pi Zero can be "juiced" to actually run from HDMI parasitic power if you optimize the code for low usage. I found that by replacing the memory card with one hand selected for lowest power usage it will actually run fine. Alas NOOBS won't work as it needs 8GB minimum but ArchOS might work or some other distro. Would someone please release PiDSL already thanks! (<256MB) 52mA average would fit most smart TVs.
Not really possible right now, HDMI spec shows 55mA on that line. Mostly used to connect to I2C devices on the bus to read the identifier of what the host connected two while the device itself is off: http://www.hdmi.org/learningcenter/kb.aspx?c=13#42
If your device can charge from that low, then yeah it would be possible, but its not what the host would be expecting to happen. Most devices need more than 55mA to power/charge a device at 5V. There may be another rev of the spec in the future to allow more current since more and more devices will likely be using HDMI.
You will need an adapter or a connector to connect type-C USB and HDMI ports via an HDMI or adapter cable for charging the laptop. You can also charge the laptop by connecting the type-C port with the type-C port of your mobile phone using your data cable.
