OK, for those of you that don't know:
squint=slang for lighting guy in the entertainment industry, thus, my nick.
RUN SCREAMING from the car headlights. Your looking at a dangerous nightmare waiting to happen if something gets touched at the wrong moment or if it gets wet, or if the wiring is not rated, or the battery overheats, which it will.....I could go on.
Bare bulbs & sockets are also a little scary when exposed to the elements. Room for issues there.
The tripod floods are probably best for portable use. You can buy eave mounted halogens at home depot if you wish to install.
Low voltage LED ropelight is available at home depot for dim, ambient lighting around the deck, just mount it under the top deck rail.
Ikea makes some line voltage (120 volts) LED puck lights & stick lights that are water resistant (not water proof) that you could use for additional lighting.
If you google LED fixtures, you can generally find some form of sealed unit that can be run off of a battery & does not draw too much current.
Squint/aflatt