The RCexl ignition module only needs power from a 4.8v battery pack. The current RCexl modules do now draw all that much power (or current) so a 2000 to 2500 mah 4.8v battery pack is perfect for them. RCexl even states in their instructions that a 4.8 pack is optimum for them.  The 6.0v maximum is really that a maximum voltage. RCexl sort of discourages running the ignition module off of 6v too. I suspect they have some voltage sensitive parts that may fail if supplied with the 6.0v maximum voltage level. The ignition module is pretty much guaranteed to fail if the voltage goes over that.

The voltage regulators, on power up, tend to go over the set max voltage for a moment, then settle back down at the set voltage. So if you use a voltahe regulator you want to set it for 5.0v and leave it there.  A 5v or 6v regulator cannot regulate a 4.8v battery pack, it usually just turns itself off and stops.

So you only need one 4.8v battery pack and you can fly all day and maybe the whole weekend depending on how much you run the engine.
I know some guys come out and fly once or twice during the day others make 5 or 6 flights and a few do more.