If you use regular white LEDs, they typically have a Vf value of 3.5v and a nominal current of 20mA. From experience, they are still quite bright at 15mA. So here is where the math gets a bit sticky.
I would design it such that the LEDs draw 20mA at the max 16.8v. That way, when the battery voltage gets down to around 14.8v, the current will still be high enough to still be bright. For example; if you use three LEDs in series (10.5v total), and want to draw 20mA @ 16.8v battery, you need a 315ohm resistor. 330ohms is the closest standard value, which will result in 19mA. When the battery voltage drops down to 14.8v, the LEDs will be getting 13mA. At that point, you may start noticing some dimming. The problem is the fluctuating supply voltage on a static resistor and LED.
A better way, albeit more complex, is to use an LM317T regulator configured as a CC source so that the LEDs get the same 20mA no matter what the battery voltage is as long as the battery voltage is at least 2v higher than the total LED voltage drop (10.5v for 3 LEDs). Here is such a circuit (sorry about the crude drawing - it was quick):
A similar circuit can be made using transistors, but a little more complex. RadioShack has the LM317T regulator which is why I suggest it. And configuring it as a CC source is very simple.