Going out on a limb, but IMO and as a practical matter, you should plan to use a driver. LED's don't work like an incandescent filament. Incandescent lights you can overvolt (within reason) or undervolt, and run them until the white turns to orange turns to red and finally stops giving off visible light. They're basically a crude device.
LED's want constant current, so as battery voltage perhaps starts too high or too low, the driver can compensate accordingly to maintain the proper current level.
Drivers come in 2 flavors, bucking or boosting. Bucking can reduce battery voltage that would be dangerous to the LED to a level it can work with and maintain the proper current level. Likewise, boosting can take a battery voltage that is theoretically too low to operate the LED, and boost it high enough to make it produce light. The drivers are able to do this because they can switch the LED on and off more rapidly than the eye can detect. A boosting driver raises the voltage, fires the LED, your eye registers light, the LED turns off while the driver recharges, and then it fires again before your eye registers that it went out. The driver will work to maintain the proper current through the LED.
If you don't use a driver, then you need to carefully match the voltage necessary to operate the LED to what the battery will produce. A little too much voltage will drive too much current through the LED. Likewise, as the voltage drops so will the current and the brightness until the voltage goes below what's needed to operate the LED, at which point it goes out. Without a driver, this could be a fairly narrow range.
The one I used is a bucking driver called an nFlex and cost ~$30. There are cheaper ones, but this one has many nice features. You can supply battery voltage up to 30 V and it will control the max current through your LED to 1 amp. It provides for 3 user-settable current levels up to 1 amp that you can access in programming mode. It offers a flash mode, lockout mode (to prevent accidental operation), and limp-home mode in case it sees battery voltage getting close to the point where the LED's go dark.
Of course, you need to be sure the battery has enough "headroom" to keep the LED(s) operational. That is, add up the necessary voltage for each LED and be sure the battery voltage is sufficiently higher than that so as your battery voltage slowly decreases, it is still high enough to operate the LED's. My 4 LED system requires something like 13.6 volts minimum. My old halogen battery pack provided 14.4 volts, which I did not consider to be enough headroom. I replaced it with a 25.9 volt lithium-ion pack. Works like a champ. The driver controls the current to a perfect 1 amp max, or whatever preset levels less than that I program into it.
I have not used a boosting driver, but the idea is the same. It will take a battery voltage that is theoretically too low and boost it high enough to run the LED's. Within limits. Eventually the battery voltage will be too low for the driver to fully compensate for and the LED will begin to dim until it suddenly goes out.
Bucking or boosting, the driver allows the LED to operate over a much wider range of battery voltage than it could otherwise.
Heatsink---LED light output dimishes as the LED heats up. In a sense, in this way it is self-limiting. But a heatsink provides more stable operation. MagLite, for example, makes a LED module that's a drop-in replacement for the halogen bulb. Problem is, the heatsink is minimal and dimming over time is a reported issue. No real formula to figuring out the heatsink that I know of. Get some metal under the star, bond with thermally conductive epoxy, and try to expose the metal to air. I've even seen some homemade designs that incorporate an airscoop! My 4-light unit gets warm to the touch (maybe 105F) after an hour of operation, and the heatsink is nothing more than a 1/4" thick piece of brass. I'd use aluminum next time....used that on my 3-light unit.
Lenses are custom designed according to the LED brand and construction and attach with sticky tape. But at $1 each you can buy a bunch and play around with different shaped packages.
You can see the lenses, brass heatsink, and the end of the driver pcb in the photo below.
Since the light bulb was invented, light has been produced as a byproduct of making heat. LED's change everything. Now, heat is produced as a byproduct of making light.
MT
