TomGeorge said "Definitely not a fritzy...." for a reason.
I could make heads nor tails of the power distribution.
Post#0 seems to be different from post#11.
Why are the 78xx regulators there. Are they for the two? GoPros.
An 78xx regulator has a dropout voltage of 1.5-2volt, so 4AA batteries in (< 6volt) might only be ~4volt out of the ?5volt regulator. Less if batteries go down. Starvation for the GoPros.
I see you use buck converters (bloody Fritzing). They might have a 1volt dropout. Not good to have 5-6volt in for 5volt out. The display also won't help battery life.
470k for the temp sensors? Shouldn't that be 4k7.
The circuit around Q2 doesn't make sense.
130ohm/100ohm and 200ohm/100ohm for measuring battery voltage? I would expect 100x those values.
12volt on a 200ohm/100ohm divider is also dangerous for the analogue pin.
Ratios suggest you measure battery voltage with default Aref. Not good.
Not sure how the Mega gets it's power. It is missing on the diagram.
With 8 batteries on V-in, you will loose half the power of the batteries.
How much current is drawn from the 5volt pin of the Mega. Does the regulator get warm.
How much is drawn from the 3.3volt pin.
Leo..