Let's see...
A 14.8V pack is 4 cells in series; at max of 4.2V/cell and min of 3.2V/cell, your "14.8V" pack is really a 12.8-16.8V pack. The voltage falls in a kind of linear way with discharge.
Using a buck driver the output can never be more than the input, normally about 0.3-0.5V less than the input. Thus at end of discharge, you are only going to get 12.5V output. By the way, this is the most efficient type, 90% or higher typical, 95+% is possible.
I just tested the Luxeon Rebels I'm using in my light, and at 1% they are 2.67V per LED. At 10% they are 2.83V per LED. At 100% (700mA) they are 3.2V. At 1A they are 3.27V. Note I chose (rather arbitrarily, but with conservativism in mind), 700 mA as 100% output. With good heatsinking they run pretty cool, so I may increase the max output.
This would suggest that at end of battery life your 4-LED series array would get about 12.5/4=3.13V per LED, which would put them well above 10% and well below 100% of current rating. You might consider this pretty acceptable. Or not. Your choice. Given my numbers you would get 100% output as long as the battery can provide 3.2*4+0.3=13.1V. You would cross this line at 85% discharge, meaning you would get 100% output for most of the battery life, then dimming until the battery protection cuts in (don't even think of using Li-Po without protection somewhere). The dimming may not even be noticable before the light just cuts out very suddenly.
You might be pretty happy with this. If so, stop reading HERE.
I can imagine only one way anyone might think this design could be improved: 1. Max power to the bitter end. 2. more warning of iminent shutdown.
I can imagine only two ways anyone might think this design could be improved. 1. Max power to the bitter end. 2. more warning of iminent shutdown. 3. Both.
I can imagine....
Okay, so there are many things that could be tweaked. What are the options?
1. Change to 3 LEDs in series. This gets you lower output voltage for the reg, which in this case means 100% output to the very end. It also probably means less total light output.
2. Change to 3 LEDs in series, 2 strings in parallel This means 6 total LEDs. It reduces the LED voltage allowing 100% output to the end of battery life, but keeps or even increases the total light ouptut (increase comes at the expense of battery life). Lower LED current means the LEDs are slightly more efficient.
3. Change to 2 LEDs in series, 2 strings in parallel This keeps the same 4 total LEDs, but reduces the LED voltage with the benefits mentioned above. The buck driver would need to be selected for twice the output current, and the efficiency may suffer a little for this (depends on the unit selected).
4. Switch to a driver that has low battery warnings. As I built my own, I got nothing for you here. You have to research it yourself.
5. Switch to a buck/boost driver. I'm not keen on this, as the selections seem very limited and the efficiency is typically substantially lower than buck only or boost only drivers. But it does eliminate the problem of the battery voltage being close to or crossing over the LED array voltage.
6. Switch to a boost driver. The efficiency and probably the availability are between those of buck and buck/boost drivers. I chose this for my design, which uses 6 LEDs and 8 Ni-MH cells (8.0-9.6V) because the numbers work out better (it's a long story why 6 LEDs/8 cells). In my professional life I've built some buck drivers with better efficiency of 93%@30W. I'm getting 85-90% for the dive light at 5-14W, with better eff coming at lower power.
So basically, I think your 4-LED, 4-cell (Li-Po) configuration is pretty good, subject to your personal tastes. Choose the best driver you can, and drive on!
I hope this helps. If not please feel free to ask for more, or ignore me.
D
