That could be a little tricky. The current that flows in an LED is a very sensitive function of the voltage applied. With a little too much voltage, the LED heats up an destroys itself. Low voltage means significantly reduced light output. Furthermore, LEDs rated for 3.2V use can vary slightly, in their voltage tolerance. Usually, LED users put a resistor in series with the LEDs to limit the current flow. The drawback of that method is that the resistor generates heat, throwing away some of the battery's energy.
That said, you may be able to make 4 parallel batches of 4 LEDs each, and power them directly from the battery. When the battery (lead-acid, I presume) is fully charged, its voltage will nicely match the required 12.8V. Of course, that's only 16 LEDs. The other two could be spares.
The best way to do this would be to measure each LED's current vs. applied voltage (I-V) curve. You could then make sets of 4 which match up so that the battery will induce the same current in each set. You could also make sure that the currents won't be too big.
Of course, you could also use some series resistance, with some loss of efficiency.
(published on 03/13/2010)