Hi Mr5un, Not sure on how much you know about how flashlights work. But I hop this might help.
Basically there are two main ways to power the LED:
-direct drive
-regulated current
Direct Drive simply means the LED is connected directly to battery (in it's simplest form), this results in high voltage at the start on a full battery and a bright light. As time passes with the light on the voltage will drop on the battery and the light will dim. With batteries such as lithium, NiMh and Li-ion this voltage reduction is fairly gradual, so the light won't dim as quickly and it'll be more gradual. Using batteries like alkaline batteries, these drop voltage very quickly so the output of the light will drop a lot quicker.
Regulated current uses a circuit board to control how much current is fed to the LED. There are several types of "driver" often referred to as 'buck' or 'boost'. These either lower the voltage being delivered to the LED or use more mAh to produce more voltage than the battery normally has. The theory is it will offer a stable continuous output of light because the LED is being given a constant current. There are many variations of regulation and some will step down automatically to lower levels to reduce heat or prolong runtime. Others have thermal management systems and others still have low voltage warning system. All in all they make for sophisticated lights.
In terms of run time, a regulated light is likely to offer less overall run time as some of the batteries capacity will go to running the driver and then maintaining a stable output. But what it does offer is a longer duration of constant light output.
When companies rate or claim runtime it's important to study what they mean, is the light regulated, is the light maintaining this output for the duration, if not to what output level is the reading being measure too?
In the case of the Led Lenser P7, this is not a regulated light, it is direct drive. So it will dim as you use it. By default it comes with alkaline cells, these suffer voltage sag during use, so the light will dim quite quickly. I believe Led Lenser don't recommend the use of NiMH batteries, but I believe many use them in P7's. As NiMh has a flatter discharge curve the light will offer up a more consistent output but it is still direct driven, so will dim slightly. NiMh have less capacity than alkaline cells so total runtime will be less. Lithium primary batteries would be the best option for output and runtime, but are quite costly to buy and run.
The M series of Led Lenser lights (Microprocessor) such as the M7 is a regulated light. Although by default Led Lenser opt for the regulation to step down after 'x' mins to prolong run times. Switching the light off and on will reset this and restore full power, although constantly doing this will reduce runtimes compared to letting it step down.
In terms of heat, the P7 is aluminium, so it acts as a heat sink to remove heat from the LED. Normally you'd be holding the light so it'd help dissipate this heat even more. This is good.
The L7 is plastic, this is an isolator. So while the LED is getting hot inside (it probably as an aluminium heat sink too). It can't transfer this heat to the body for additional cooling. Led Lenser know this and that's likely why the L7 is rated at 115 lumens vs 200 lumens for the P7.
And it's this rating that is making the L7 look brighter after a long run time. The lower lumens means it'll deplete the batteries slower and it also means it needs less power to produce it's max output.
If you want long runtime you need to look for lights that use large capacity batteries, having a low mode can be handy too, as this will put less demand on the battery, but evidently it'll offer up less light.
Led Lenser are fine lights, although often retailed at very high prices. However there are other lights which could potentially offer up better run times and more output.
If you want to venture into the world of li-ion batteries then most 18650 lights have very good run times and should be able to offer up good output too.
