I took some time this weekend to continue assembling this thing.
At this point there wasn't much left for me to connect, so I prepped the batteries. A quick test showed that they weren't exactly balanced and I figured it would be bad to just... plug them in like this.
So I shorted them together through a power resistor. Now kiss! Also I put them in a frying pan in case I suddenly found myself with a smoldering pile of plastic and lithium.
And now the moment we've all been waiting for!
I plugged in the button and... it doesn't work. As soon as the microcontroller powers up it seems like all the pins go HIGH - the LED's go to max brightness, the relays jump back into "charge mode", and the one pulsating LED in the button never fully dims. I need to figure out if I fried the microcontroller or if there's something else at work here... This is what I get for jumping straight into assembly without testing the subsystems first.