Btw: I tried a 1 mF capacitor right across the power input connector, and it was not enough to solve the power-off problem when turning on my external devices.
It turns out that powering the Jetson off a switching power converter means that it’s susceptible to the power step response of that converter, which is generally kind-of crummy for most switching converters. (There’s a definite efficiency/performance trade-off there.)
Using a LiPo battery, with the switching power converter “floating” the LiPo through a diode, ends up working well. I just have to make sure to “float” the LiPo at a lower state of charge, so I don’t wear it too much at top charge. I aim for 3.95V per cell in this case. Once the rover is un-tethered, it will of course run on battery only.
If you’re using the NVIDIA carrier board, or a carrier board that has at least 18V input tolerance, you will probably be better off powering the board directly from your 4S, rather than having a DC/DC converter in between!
However, if you need the ability to turn off the power, in case of battery depletion, then some kind of switch in the middle is good. I use a custom PCB with a small microcontroller for monitoring voltage and some other things, and a big P-channel MOSFET for switching power on/off under micro control.