Did you measure this voltage from a single LED with 40 ohm resistor on the 12V battery? If each LED drops 2.4V than 20 of them in series with do 48V. Tell more. The answer to your question might simply be a MOSFET, no need for a relay.
The LED specs say there is a 2.4 voltage drop but I've set up the circuit using all 20 LEDs in series with a 40 ohm 5watt resistor to the 12v battery and it works. Now I simply need a relay go to all 20 LEDs in series and turn them on or off.
What are the power specs I need to figure out for selecting a correct relay in this setup?