Rotate your LED flashlight batteries...

[atraildreamer]

My wife asked me to check the batteries in her LED headlamp as it was starting to dim out in use. The battery pack is the cylindrical type common to LED lights and uses 3 AAA batteries.

I tested the batteries and found that 2 were near full charge, while the third battery was totally dead. This seems to be a common problem with this type of light. :confused:

The obvious way to maximize battery life would be to periodically rotate the position of the batteries in the battery pack and allow the draw down of each battery to be equalized thru the life of the battery(s). :-?

Just my 2 cents worth. :)

[The Old Fhart]

That is not correct. The 3 cells are in series so it makes no difference what position they are in. Rotate tires, not batteries.;)

[KG4FAM]

That is not correct. The 3 cells are in series so it makes no difference what position they are in. Rotate tires, not batteries.;)agreed.

[Compass]

In order to have one cell weaker than the rest in series it would have started off weaker, in other word a dudd cell. To maximize effeciency use new sets of batteries that are the same brand and age. With batteries the old addage one bad apple/cell spoils the bunch holds true.

A system that is in parallel could have an unequal discharge due to contact resistance but would tend to balance with time when turned off.

[le loupe]

While I understand the point of the batteries being in series-

When I bought an LED headlamp, I put new batteries in it. One day, when it failed to turn on I tested the batteries and found one cell was dead whereas the others indicated a healthy power level.

Maybe there is something to atrail's point...

[KG4FAM]

While I understand the point of the batteries being in series-

When I bought an LED headlamp, I put new batteries in it. One day, when it failed to turn on I tested the batteries and found one cell was dead whereas the others indicated a healthy power level.

Maybe there is something to atrail's point...
you had one bad battery as well. its a chain is only as strong as its weakest link kind of thing. there is no voodoo going on with these headlamps, they follow the same electrical rules as everything else

[4eyedbuzzard]

For all you electricians and technical types, I'm filing this one in with the "rotating the fuses in a 3 phase circuit because the B phase draws twice the current" file. ;)

[mudcap]

For all you electricians and technical types, I'm filing this one in with the "rotating the fuses in a 3 phase circuit because the B phase draws twice the current" file. ;)

Good choice :)

[Analogman]

That is not correct. The 3 cells are in series so it makes no difference what position they are in. Rotate tires, not batteries.;)

Correct. Batteries in series should drain equally. Sounds like you unknowingly started off with one bum battery.

[aaroniguana]

You should always replace ALL batteries before each outing because you don't really know how long you'll be out.

And please recycle your batteries. Every Lowes and Home Depot have a recycle bin.

[atraildreamer]

While I understand the point of the batteries being in series-

When I bought an LED headlamp, I put new batteries in it. One day, when it failed to turn on I tested the batteries and found one cell was dead whereas the others indicated a healthy power level.

Maybe there is something to atrail's point...

I have seen this happen at least three times with different LED lights and headlamps. In all cases, the installed batteries were fresh out of the package and were tested prior to installation. The charge levels were the same in all the batteries. Every time I tested the used batteries, two were slightly discharged, but still tested in the "Good" range on the tester, while the third battery was near totally dead. :(

I agree that a series circuit should theoretically show an equal discharge across all three batteries, in practice, this has not been the case. :confused:

Could this be due to a design fault with the LED circuitry? I suspect that most, if not all, of these lights are made from the same design, and may all come from the same manufacturer in China, or wherever, and may explain why this is occuring. :-?

[The Old Fhart]

atraildreamer-"I agree that a series circuit should theoretically show an equal discharge across all three batteries, in practice, this has not been the case.:confused:"Theory or not, in the real world batteries in series absolutely have to have the same current flowing through them. Whether the load is an LED, fan motor, or a toy, doesn't affect it one bit-the current will always be the same. Unless there is another path for current to flow from just one battery like corrosion in the battery holder partially shorting out that cell, what you suggest is totally impossible.

When you use the term 'battery', you really mean a 'cell' so you have a 3 cell headlamp. A 12-volt lead-acid auto battery is comprised of six 2-volts cells in series in one case and look at how many charge-discharge cycles they go through before having to be replaced. There will always be some variation in cells due to manufacturing tolerances, etc., but modern batteries are fairly uniform. If you have rechargeable AA or AAA batteries and a 'smart' charger like the LaCrosse BC-900, you can see the cell voltage when you start the charge and the charger will also display the time it takes to fully charge the cells. I have seen one battery in a pair fail prematurely but never question the laws of physics to explain it.

I have a cartoon hanging on my shop wall that says: "Ohm's law is definitive.";)

[KG4FAM]

Could this be due to a design fault with the LED circuitry? I suspect that most, if not all, of these lights are made from the same design, and may all come from the same manufacturer in China, or wherever, and may explain why this is occuring. :-?Headlamps are pretty simple. Voltage source, switch, resistor and LED is all there is to it. Its a simple design that every electronics student learns early on.

[4eyedbuzzard]

There are some higher end LED headlamps with voltage regulation circuitry in them, but even these use series connected cells.

That one battery went "dead" while the other two still tested "good" isn't that surprising given typical battery discharge curves (http://www.powerstream.com/AA-tests.htm)

[The Old Fhart]

Some of the new LED lights use a single 1.5V AA or a single 3.6V 14500 lithium-ion cell and will operate from 0.8-4.2V with fairly uniform brightness. There is a very small circuit board in the head of the flashlight that is similar in principle to the switching-regulator power supplies used in every computer. The circuit takes the input voltage and converts it to the voltage needed for the particular type of LED. The newer 3 watt (or higher) LEDs might require 3.9V at up to 1000ma.. Connecting this LED to a 1.5V battery would produce no light or current flow. The voltage converter circuit converts the 1.5V to the necessary voltage for the LED and may be regulated to give a fairly constant power output. Lights of this design generally have a fairly constant light output until the battery discharges to a point where the light may start to flicker and die suddenly.

The simplier LED lights do use a voltage higher than the necessary LED voltage and then use a resistor to limit the current supplied to the LED to a safe level. Lights of this design start off bright and gradually decrease in brightness until the battery dies.

[Slo-go'en]

The charge levels were the same in all the batteries. Every time I tested the used batteries, two were slightly discharged, but still tested in the "Good" range on the tester, while the third battery was near totally dead. :(


What are you using as a "tester" ? When batteries get near to the end of thier useful life, they start to act differently because of the way the electrodes wear. It is likely that the two "good" batteries are quite close to being "dead" batteries, but one cell wore out just a little faster, which makes the whole string draw less current, which causes the other two batteries not to get into the "dead" zone.

In other words, once one battery goes over the cliff, the other two are spared the same fate, even though they are right on the edge of that cliff.

If the dead battery is found in different locations in the pack, the above hypothisis is true. If the dead battery is found in the same location in the pack each time, then something else is causing it. I belive if you were to number the batteries and thier location and keept track of where the dead cell was found over an extended period of time, you will find the location of the dead cell is random.

[atraildreamer]

What are you using as a "tester"

The tester is a unit that snaps over the end of the battery electrodes. The indicator dial is color coded with green being "Good":D, yellow being marginal:-?, and red indicating weak and/or dead:mad:. The indicator dial spins to the appropriate position when the battery is being tested. I don't know who made the thing, or even remember where I bought it, :confused: but I have used it for years. It can also test 9 volt batteries.

[NICKTHEGREEK]

you had one bad battery as well. its a chain is only as strong as its weakest link kind of thing. there is no voodoo going on with these headlamps, they follow the same electrical rules as everything else
No, many have regulator circuits that change things a bit, but no circuit, no current flow and all 3 batteries should have the same life more or less.

[Froggy]

The discussion reminds me of the difference between theory and practice. In theory, there isn't any difference. In practice, there is.

I suspect that there's a variation in the batteries, maybe a difference in internal resistance or something, that affects battery life when used in these otherwise straightforward devices. But the anomoly might be from the hardware rather than the batteries. For example, one battery might be running slightly warmer than the others, due to it's position, and that might do it.

In any event, it's worth learning what's actually going on rather than making well-meant hypotheses.

[NICKTHEGREEK]

The discussion reminds me of the difference between theory and practice. In theory, there isn't any difference. In practice, there is.

I suspect that there's a variation in the batteries, maybe a difference in internal resistance or something, that affects battery life when used in these otherwise straightforward devices. But the anomoly might be from the hardware rather than the batteries. For example, one battery might be running slightly warmer than the others, due to it's position, and that might do it.

In any event, it's worth learning what's actually going on rather than making well-meant hypotheses.
And right you are. If I open a "my car battery is dead" thread, I'd mention that it has to do with an Optima in a FJ-40. Thank God there's only one type of LED fheadlamp and one brand AAA battery on the market that way we know what the guy is talking about.

So what kind of headlamp and what kind of AAA cells atraildreamer?

[atraildreamer]

So what kind of headlamp and what kind of AAA cells atraildreamer?

The ugly-looking round one with 9 LEDs that go for about $3, and can be found just about everywhere. My wife uses it for her arts and crafts work when she needs extra light. We also have better-quality headlamps for hiking that I picked up at Walmart (the kind with the red LED in the middle - brand name escapes me-"senior moment" :confused:) Haven't seen the battery problem with these units.

Anothe post mentioned heating of a battery. We have noticed that some of the LED flashlights do get warm if left on for an extended period of time. Is it possible that the battery going dead is at the end of the circuit and is being cooked by the current through it?