DD> Al salaam a'alaykum Bob

 And may the Bluebird of Allah fly up your nose.

DD>> This is to run a small DVD player, which came with a mains
DD>> switch-mode PSU. 

BL> Why does it take so much power? 20W is a hell of a lot for a
BL> DVD. It should be closer to 8W... 5V at 1.5A.

DD> Spinning the disc up to speed?

 I had hopes that you might actually measure it... but, yes.

 I suspect that it takes 3A with the motors stalled, and settles
down to bugger all once the disc is loaded and spinning happily. If
that's the case, the battery load would be negligible and any sort of
12-to-6V regulator would do the trick.

 If it were mine, I'd measure the actual load current for a start.

 If it really takes 3A when running, I'd tap into the battery and run
a special 6V lead for the DVD.

 If it only takes 3A on peak and settles down at 1A or so, I'd buy a
small heatsink and an LM350, wire the two resistors on the back, and
use it the way you use your little switch-mode supply at present.
It'll cost you $20 but it'll work first time and as you say, it'll
save you buggering around with extra wiring.

 If it takes 3A continuous and you decide on the LM350 option, you
will need a rather large heatsink (100mm square, with fins). With a
150 AH battery, you don't have to worry about saving a few lousy amps.

Regards,
Bob

--- BQWK Alpha 0.5