All I want for Christmas is…
… a DC-DC power supply that will take one or two 2s or 3s LiPo batteries (via Deans/XT60 connectors), isolate them from each other, protect them from over-current and under-voltage, and give me 200W of 5V out the other side.
Ok, 100W.
FIFTY ISN’T ENOUGH!
All you want for Christmas is to learn how to build that yourself. 
Then make a million bucks selling it …
Now I’ve piqued your interest Mr Kirchner, this seems to be much more in your wheelhouse than mine. Is it feasible? Would it be particularly expensive to do? I know power supply design is a black art, but there are some integrated 40A DC-DC power supply modules out there (but holy crap, $200-odd? http://tr.mouser.com/ProductDetail/Cincon/CHB200W-24S05/?qs=sGAEpiMZZMvGsmoEFRKS8D3NpBtvpim3CWne1RIJY4k%3D). I’m loath to try this as my first proper EE project, as playing with high current circuitry like this invites some exciting and spectacular failure modes. Also, Lipo go boom!
You’re talking batteries that are low voltage and not enough current and asking them to put out high current. As current goes up, voltage goes down.
I’m talking about 7.4V 2s2p LiPo packs like these: http://www.hobbyking.com/hobbyking/store/__17504__Turnigy_6000mAh_2S2P_7_4v_25C_hardcase_pack.html
25C continuous in a 6Ah battery is 150A, innit? And it’ll be more than that as I’m stepping down the voltage.
So you want to take several of those and have them tied together to provide a much higher current? Why not build your own pack to do that?
One alone would be enough current capacity (that’s 150 Amps continuous at 7.4V they’re capable of—almost enough to give me 200W at 5V), I’d want to connect multiple for capacity. 6-8 hours would be ideal.
I like the idea of buying (safety) tested, cheap, modular, interchangeable packs.
I would seriously question the "continuous"part. Burst sure. Continuous to me would be a gamble to be honest … you’re looking at overheating and cell bloating …
Well, that’s what they say they’re rated for! Even if you derate 50%, two packs in parallel gets you there.
Well huh, looks like I get (the hard part) of my wish. Twenty bucks, according to Mouser: http://us.tdk-lambda.com/ftp/specs/iaf.pdf
Well there you go …
Neat little units, hey? And SMALL! Just need to integrate with a lipo under voltage protection chip (I see a few dedicated ones) and I’m golden 
.
(God I wonder how many lipo fires I can manage to start designing something even that simple…)
Important step: make sure you own at least one, ideally two extinguishers. Make sure they’re rated for electrical fires.
Oh god.
File under: projects I am NOT attempting until I have a shed. With a cement floor.