Thank you again Brett, you help much to improve my understanding.
I like your idea for using battery pack with jack plug and later it will be easy for me to switch to lithium batteries.
My main hesitation for using the jack is if battery voltage can be monitored through the jack.
I wonder if it is possible to read/monitor battery voltage if using the jack plug (like UC battery module can do with pin 3)?
I think monitoring battery voltage will be good idea to warn when they are nearly flat so no unexpected power failures.
A lesser hesitation is that I am thinking about using the black case (forgot name for it) which fits the Cobra II board and in the empty space to put a battery holder (eg UC Battery Module or any one from links you give, or maybe even for lithium batteries) then when the case would be closed the Power Jack, USB, and SD card slots stick out the side and on back I can put a flap cover (like on calculators) so it is easy to change batteries while all wires and connections are hidden inside the box. But then if I use cable to connect battery-to-malejackhead-to-jackpowerplug then the wires would have to stick out of box to reach them.
This is not a big deal, just me thinking ahead for tidy design so always possible to try different design instead.
Otherwise I can try doing like in your first picture and get an extra USB Client DP Module to connect to spare letter plug which I can hide inside case (hopefully there is enough room inside case) and then connection would be battery-to-malejackhead-to-jackpowerplugmodule-to-letterPlugonCobraBoard.
Also, you mention that 4xAA batteries might not be enough for voltage.
Is this because of difference between voltage regulator for UC Battery Module and USB Client DP Module or because cobra board just needs higher voltage?
Could I just replace the 4xAA battery holder on the UC Battery module with a larger one (eg 6xAA) and still use the module okay?
I remember reading somewhere that we should try to keep supply voltages close to 5v for battery power supplies (recommended aim for 6v at lowest to allow leeway for when voltage drops during battery discharge) because for higher battery supply voltages we waste more power when voltage regulator has to convert down to 5v (eg 6v to 5v = small waste power, 9v to 5v = larger waste power).
I think 7v should not waste much more power than 6v (but maybe little bit wasted power can add up and reduce battery life), but would like to hear your opinion for best voltage regulator and best voltage for Cobra board.
One more question, for voltage regulators is their much difference between the one for UC battery module and the one for USB Client DP Module (one with power jack)?
Do they both have about the same efficiency?
Supposing that if the jack couldn’t monitor battery voltage and both modules have same efficiencies, would it be possible to just wire the lithium batteries to the UC module and then use its pin 3 to monitor them?
P.S. I appreciate the pictures you show of connecting lithium batteries with modules in different ways. They make it easier to understand and shows what I want to do with batteries for powering the Cobra II board. Now I must try to decide between UC Battery Module and USB Client DP (Power Jack) Module for which is best overall.
P.S.S It is good to know there is no importance to worry about using D+ and D- for powering Cobra board or monitoring voltage/data.
Thanks and sorry if my explanations are confusing.