how many watts is it supposed to provide? 50W?
Lets just say yes, and do the math. At 13.8 V that gives about 3,6 Amp. Lets leave a margin, say 4.
Then, LM338 - 5A, voltage regulator.
http://www.national.com/pf/LM/LM338.html
can regulate from 1,2 V to 32 V .
Circuits in the PDF (including specs).
Just connect the psu output to the regulator circuit.
done.
drink coffee.