A charger typically outputs about 5 volts (mine actually measured 5.16 volts). However, most battery powered LED lights run on 4.5 volts. If you try to run a 4.5 volt LED on 5 volts it would probably work, but it can significantly reduce its lifespan. Fortunately, there are a number of ways that you can make small adjustments to the output voltage of the charger.
The simplest way to reduce the voltage is to add a diode. Most silicon diodes create a voltage drop of about 0.7 volts (0.3 V for Germanium diodes and 0.2 V for Schottky diodes). So by adding a diode, you can drop the 5.16V output of the charger down to 4.46V. This voltage will work fine for the lights. Just make sure that the diode is rated high enough for the current used by your lights. In this case the lights use 720 mA. So I used a diode that was rated for 1A.
Another method is to add a resistor. To determine the value of the resistor that you would need, use these formulas:
Resistance = [(Voltage of the power supply) – (Voltage of the lights)] / (Current of the lights)
Resistor Wattage = [(Voltage of the power supply) – (Voltage of the lights)] * (Current of the lights)
So in this project, the power supply voltage is 5.16V, the voltage of the lights is 4.5V and the current of the lights is 0.720A. This means that I would need a resistor with a value of 0.92 or about 1 ohm and a wattage of 0.48 or about 1/2 a watt.
A third option is to use a variable voltage regulator such as a LM317. This lets you use a pair of resistors to set the output at any voltage that you want. For a simple tutorial on how to use a voltage regulator, you can check out another of my projects where I show how to use a voltage regulator to adjust the output of a DC power supply. http://www.instructables.com/id/Convert-Battery-Powered-Electronics-to-Run-on-AC/