Hey, I would just use something like this to do the low-battery warning http://uk.farnell.com/texas-instruments/tps3809k33dbvrg4/ic-supervisor-smd-sot-23-3-3809/dp/1287661 However you seem to be using a Zener diode to drop the voltage down to 3v3, why not use a regulator? What sort of load are you looking to power?
A few more components than your others but it works. D1 is a zener. When the base of Q1 is lower than 1.8V the LED turns on so choose a resistor ratio to make that pin 1.8V at the point your 3V3 rail has hit it's under voltage threshold.
Regulating the voltage using only a Zener and resistor on a circuit that should drive a 3-4A load is nonsense, regulating the voltage using only a Zener and resistor on a battery powered circuit is nonsense. You have a supply voltage of 3,7v (4,2 fully charged), the load is 1Ohm and should have 3,3V, that is about 3,3A or 11W, If you still want to have 3,3V at the load when the voltage of the battery drops the resistor has to be REALLY low resitance. But lets say that it should supply 3,3V with the battery at its nominal value (3,7V) it has to be 108mOhm (milliOhm) at this point the voltage over the resistor is 0,4V and it will dissipate 1,48W of heat, and there will be NO current going throu the Zener, the Rs and load will act like a voltage divider. when the battery is fully charged the voltage drop throu Rs needs to be 4,2V - 3,3V = 0,9V this results in a current draw on the battery of 8,3A, and the Rs needs to dissipate 7,5w, at this point the current thru the Zener will be 8,3A - 3,3A = 5A. Total waste of energy...
Okay there's these guys. You'll need to solder SMDs. http://www.linear.com/product/LTC3785 Which can do you 6A or something simpler but only 2.5A. http://www.linear.com/product/LTC3112 I only did a quick search, search for buck boost converters for li ion and you'll get something close to what you want.
Unless you have tools (oscilloscope, constant current source, etc) and knowledge I would advice to stay away from designing your own SMPS. As for PSU itself - I would advise using two cells in series with a step-down converter. Resons: 1) step-down converters are more efficient and more readily available than buck-boost, sepic or similar SMPS. 2) very few 18650 cells can sustain ~8A draw (worst case). 3) cheaper 4) probably smaller than single cell solution
Another thing to consider is if you need regulator at all. In some situations you can get away with unregulated LiFe cell (3-3.6V working voltage), in others LiPO/LiIon and a simple LDO regulator will be as efficient as SMPS (LiPO cell will be sitting in 3.6-3.8V range at 80% of its total charge). Also, LiPO cell at 3.3V has given away more than 90% of it's chage, you can call it fully discharged at that point, no need for boost. What are you powering that it need 3.3V@6A?
I'd say stick with linear regulator, you'll get 85-90% efficiency on average, about the same as with SMPS at such a low voltage drop.
