The discrete electronic device you're looking for is called a choke.
You can buy a black box from solar electric companies that will do all this but they are expensive, if you want to go for a cheap DIY
solution you will probably find all you need in an automobile scrap yard(hint, hint), along with a car battery(s).
The charging system in a car has all the bits you need including the voltage regulator to prevent over voltage charging, its usually built into the alternator these days but used to be a separate box(in a 1980 Datsun) some older GM cars should be the same.
You might be able to extract one from and old alternator but beware that it may be integrated with the bridge rectifier(ac/dc converter).
Then a dc to dc converter will get you the correct voltage to power the raspberry pi(http://en.wikipedia.org/wiki/DC-to-DC_converter).
Its basic magnelectrics, no electronic Engineer required
In answer to your question about the 6v led, is that it has very little internal resistence so the max current available will flow thru it
unless we have a resister in series with it, the same goes for other discrete electronic components that's why there are lots of resisters
on most printed circuit boards. Think of it like this, if you but a wire short across the terminals of a 9v battery the wire has next to no resistance and max current will flow from + to - and the battery will start to get warm and destroy its self(a car battery will boil and probably explode). http://ledcalc.com/