How quickly does light output fall as battery loses charge in a LED torch?

How quickly does the light output fall as a battery loses charge in a LED flashlight?

Does light output fall quickly with a small discharge? Or does it stay reasonably good and then suddenly start to get a lot worse?

Has anyone produced a chart of light output against remaining battery charge?



(If the LED makes a difference then I'm thinking of something like a Cree XM-L LED.)

Someone with a lot more expertise than myself will surely chime in, but I believe much of it has to do with the circuitry that is built into the light itself and how the voltage is regulated.

Complicated question, depends on the battery chemistry, voltage curve and state of charge of the particular cell, the driver type, the output current, the forward voltage of the LED in question (they are usually binned at different forward voltage bins for a given LED model as well)

If the light is regulated the output doesn't drop until the battery is unable to supply the power. Some drivers will sense the lower voltage and deliberately drop the output to let you know the battery is low and also to conserve power. Others will just cut the power at a specified voltage and leave you in the dark. Some drivers will keep the light in regulation until the voltage is too low then move into direct drive.
The only type of light that your question applies to is a Direct Drive light, which has no driver and no regulation. There's not many of these around in the quality market, if you find a quality light with DD it's a bit of a niche product.
The output on a DD light drops from the moment the light is turned on, just like an incandescent bulb, very gradually and un noticable until you notice it - haha.
The rate of voltage drop is controlled by the type of cell that you use.
Have a look at HKJ's charts for a specific cell which will give you a good idea of voltage drop over time for a specified current.
I hope that helps even if it's a little wandering.
P

The question should include battery type used.

In the case of the largely used today Li-ion cells, there should be no difference in light power until the cell reaches its official discharge limit. This of course assumes you use original cells that are not degraded. In such a case, the cells will sustain the output with sufficient current until the discharge voltage is reached. At that point the light output will lower until the cells lock or until they over-discharge. If worn-out cells are used, the power output will get lower faster, before reaching official discharge limit.

In the case of NiCd, there's constant light until a sudden drop is observed (when the cell drops from 1.2+ to 1.0 and is considered discharged).

in the case of non-rechargeable batteries, in most cases there is nearly peak output until a certain voltage of the battery is reached and then theres a quick exponential drop in power. The exact voltage when this is observed is very dependent on battery manufacturer and type of load.

Overmind said:

In the case of the largely used today Li-ion cells, there should be no difference in light power until the cell reaches its official discharge limit.

Click to expand...

Only in a regulated-output light. But if the light is using something like a FET driver (very common on high-output lights under $50), then the output drops significantly over the entire run-time.

I measure that a FET driver results in 33% less output by the time the battery is only 50% depleted (down to 3.75v resting, from 4.2v). And it continues to drop until the battery is essentially dead. That first 50% depletion also happens very quickly compared to the final 50% depletion, due to the higher current supplied by the battery when it has higher voltage.