I have a question about powering a 7" LCD. It should be a rather intuitive answer, but I really don't want to risk my screen on the off chance that I'm wrong.
While experimenting with things this year I found that there seem to be 2 kinds of LCD's available at 7". There is this kind, which is similar to the original RASPI.org 7" LCD in that it has a controller board and often appears to draw power from the GPIO pins: Raspberry Pi 7" Touch Screen Display.
Then there is this one, that seems to draw power straight from the standard USB port. It also seems to use that USB port as the data in for the 'touch' part of the touch screen: ELECROW for Raspberry Pi 4 Touchscreen Monitor 5 inch HDMI Monitor Display 800x480 Compatible with Raspberry Pi 4 3B+ 3B 2B BB Black Banana Pi Windows 10 8 7.
This 2nd one is made by Elecrow but I know Waveshare and some others offer it as well. the one huge challenge (for me anyway) of this second type is that if you don't prepare your SD cared in advance, you get nothing.
The back light will probably come on, but that's it.
My question is this, I know that the first screen would be a better option for a 'portable battery driven' setup, but if I have the 2nd one on hand already, would it be more power "efficient" if I used a separate connection from the battery pack into the screen, or should I use the batter pack to power the RPI, and then the USB on the RPI to power the screen?
"Common sense" seems to want to tell me that there would be an overcharging circuit built into the line going through the RPI, but the other side of the coin says that if I'm using a 10 kWh batter on the RPI, it might last longer if the only thing it had to worry about directly is the RPI I could use a separate battery pack for the screen OR if my power pack had a two outputs, I could dedicate one to the RPI and one to the screen?
Anyone tried this?