Hi, I bought a set of 12V DC powered LED's, is it possible to use a lower voltage battery to supply them
You indicate "12v LEDs" - typically, LEDs are not rated for 12v - they are rated for a specific current and the forward drop across them is specified. There are ones out there called 12v LEDs, but they have some sort of built in current limiting - either a resistor or some sort of active limiting like a switcher etc. Without knowing just what you have, it is difficult to say just how you can power them.
Sheesh - talk about a horrid design - anything labeled for use anywhere near 12v should be able to tolerate a range typical in an automotive environment. Not having current regulation on LEDs with a variable voltage to them is just asking to let the magic smoke out.