HB (high-brightness) LEDs require a large amount of current to operate. When driving HB LEDs from a voltage source, you can set the required current with a suitable series resistor. If the voltage source is a battery, then, as the battery drains, the LED's intensity decreases.
Also, a series resistance has the disadvantage of power loss through the resistor. A better option is to use a suitable dc/dc converter. If the LED's turn-on voltage is lower than the battery voltage, as would be the case with a 6V sealed-lead-acid battery, then you can use a buck converter.
Read the associated Circuit Design Idea - Discrete-component buck converter drives HB LEDs
Source:
http://www.electronicsweekly.com/Articles/2009/02/26/45649/circuit-schematic-driving-high-brightness-led.htm
