Do LED Flashlights get Dimmer as Battery Gets Weak?

Do LED flashlights get dimmer as the battery weakens or do they stay the same brightness until they blink off?

Depends totally on the light and the driver that controls the LED, but even in output regulated (the light output stays the same regardless of battery voltage) the last few moments are usually dimmer. It also depends on the battery used. Some lights are regulated with a NiMH battery while not necessarily on Li-Ion. Output can also depend on the temperature if there's temp regulation circuitry in the driver. Here's a couple of examples of light output vs. time:

Thanks for your reply.

Everything's more complicated than one thinks!

If you consider an unregulated or Direct Drive Led, the output will drop from startup proportional to the battery voltage. As the voltage drops (battery losing capacity due to use) so does the output.
A Direct Drive Led will have a very high initial output. For an XM L2 say 1200 lumens. A Regulated light will have a lower initial output, say 900 lumens for my Malkoff Hound Dog, and so will be able to maintain the initial output for longer.
That's why we see so many 900/800/1000 lights when the XM L2 is capable of more.
Then of course we get to manufacturer's claims for output which are discussed elsewhere.
With clever regulation, constant current, PWM, etc the light can maintain a constant output for 10 mins to an hour or more depending on the capacity of the cells.
I hope that makes sense, a bit of a stream of consciousness I know.
P

You ask dimmer as battery gets weak...
In a word...yes.
Some high powered multi function light lose 'high' setting if light is turned on and voltage is below a certain point.

Take the Coast HP7 for example.
It is a direct drive light. So it begins to dim as soon as you turn it on.
It runs off alkaline aaa's which don't store a lot of fuel. So with typical alkalines as the amount of fuel is depleted...in this case "volts" the amount of brightness also drops.
Now if you use batteries that hold more fuel the drop off takes longer.

Now take the Malkoff MD2. It is regulated to stay the same brightness until volts of fuel reach a certain low point. At that point the light becomes direct drive and begins to dim.

So the key is to know whether a light you seek is regulated or direct drive in regards to whether it dims or not.
And as stated above temperatures and chemistry can affect if and when it dims.

Some manufacturers use direct drive to have an increase in how long the light stays on. Regulated tends to drain batteries faster.

I'm a simple man with a simple mind. If "needing" a light for a long period of time or before a weather event where the power may go out, I charge all the batteries. If leaving the house, I carry a second flashlight with extra batteries.

Just saying. As to brightness, all our lights are bright lights. All the batteries are protected, high value batteries. The point, I don't understand the question. ???