Electronics, Lighting, & Gadgets - LED lights and low batteries - how to tell?

It occurred to me this morning that I have no idea how LED lights respond when batteries start to run out. Halogens get dim so you can tell what is going on. What happens with an LED? My LED rear has had the same alkaline batteries in it for 6 months of sporadic use, now getting more frequent as the days are getting shorter. What should be my signal to replace them?

I typically go to the dollar store, and get the Panasonic batteries and replace them every two weeks, sure beats being stranded with dead batteries, (well not really stranded)

Mine blink at a slower rate, than when the juice is new

It occurred to me this morning that I have no idea how LED lights respond when batteries start to run out. Halogens get dim so you can tell what is going on. What happens with an LED?

Depends on the light. In the case of rear blinkies, they usually get dimmer, very gradually. As a test, you could turn the light on steady mode, aim it down a dark hallway, gauge the brightness of the beam on the wall, then put in fresh batteries and repeat the test. I'm with chrys9989, just toss the batteries periodically so you're always nearly full... alkalines that are even half-dead are noticably dimmer.

Or, get some low-self-discharge rechargeable batteries such as Sanyo Eneloop, and just top them off monthly.

That depends on the circuit. If it's regulated, it'll stay at a constant brightness until death, although there still might be some dimming before ultimate demise.

If it's not regulated than it'll get dimmer slowly as noted by mechBgon. It can actually be shocking when you go ahead and change the batteries how much brighter fresh ones are.

Edit: I should've also pointed out that it depends on the type of batteries as well. I was assuming alkaline which have a rather linear discharge rate. As noted by tdister below lithiums have a flat discharge curve. mimh have a somewhat flat discharge rate as well. All the batteries have a curve at the end of the discharge cycle at which cell voltage drops fast.

Go lithium if you don't go rechargeable. Lithiums will remain at near full brightness until they are dead, just carry a couple spares with you. More expensive to start, but you aren't running around with half bright lights for so much of the time, then tossing batteries below that point.

I've just been taking out my lithiums when I get paranoid that they are about to go. I use what charge is left over in my LED mini mag lite (for dog walking or whatever). My system has worked well but, when they go dead, they just flat out die with little to no warning. Entirely worth it for the overall brightness increase.

I think talking about batteries dying, is like talking about flats, last night mine suddenly became very dim, with twenty miles to go:notamused:

Batteries in rear blinkies last a long time. I always recommend riding with 2 lights in case one fails. The problem with blinkies, is that if one fails while you are riding you will never know until you get home (or if someone hits you). Hence I ride with 2 of them.

I replaced the batteries in mine after 6 months, even though they were still working with about 4-5 hrs/week usage in those 6 months.

The best lighting systems should include redundant head and tail lights. At any given time, one set should have new or near-new batteries.

For example, I use two Cat-Eye headlights arranged to provide a high and low beam spot. I use the low beam spot at dusk and turn on the high beam after it becomes dark. After several periods of use, when I can "see" that one beam has dimmed, I replace the batteries in that light and reverse the usage pattern.

Same deal with tail lights, only you can wait for one light to nearly fail, because, like I said, you should be running AT LEAST two........

PS - wearing a glove or helmet light is helpful too.

I have AAA NiMh rechargeables in my Superflash blinky. Since they came in packs of four, I have two sets. It's nice not to have to decide if it's dim enough to toss out. I just change them after 15 or 20 riding hours. And I run them during the day, too, since I'm not using up any batteries.

I wondered if NiMh batteries would be dimmer at the start than alkalines, since they have a lower voltage. But I couldn't tell any difference with the original two AA alkalines that came with it.

Eneloops, the rechargeables that don't lose power when just sitting around, would be better if the blinky is only used occasionally.