Instead of splicing the cable to separate the DC draw through the PI, why not put a hub in the middle directly to the same regulated 5v supply that can pwr both the Flir and the PI?
Or if not a hub, maybe a USB splitter or Y dongle.
This way the signal passes and the current draw comes from the battery in the middle.
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That's basically what I am going to do with the usb battery that I bought from the egg.
I did try an external power supply and reduced the voltage until I got the "device disconnected" alert from my laptop, and it dropped connection at 3.5 volts, and reconnected at 3.7 volts.... sound familiar (nominal Li-Ion voltages).
The current draw was less than 1 mA at these voltages, but when I increased to 5 volts, the current was increasing and seemed to top out at about 600 mA, so it looks like the camera was using the external supply to power and charge the battery.
I am going to let the external supply sit at 3.7volts and run the camera for a long time to see if the external battery discharges and the camera runs of the external supply.
It may be the if the external voltage is low, it is just powering the usb interface and will not draw current from the host (laptop).
Then I could drop the voltage from the laptop to something below the charging voltage for portable use.
Thanks Mike for your suggestion and yes, I should have used a couple of diodes instead of a resistor for this test, but it was a quick test and not very good.