Why are 9 volt battery flashlights the only ones that keep an LED on for years?

passive101 · Nov 7, 2012

Why can a 9 volt flashlight such as the safe-light keep the small LED on for 1-2 years, when AA and 123A doesn't? I know it uses a very weak LED, but couldn't another manufacturer put a small LED in the side of the light and run it while the light is off?

What makes 9 volt so special in this regard, or is this a feature that others don't want/need, to find the light in the dark?

ampdude · Nov 7, 2012

I guess with a 9 volt battery you don't need to include a step up converter in the light, making the light a lot cheaper. I remember a thread about.. I believe an HDS light that ran for three years or so on one battery on low output. I think it was a single lithium primary, but I'm not sure. An AA alkaline or lithium battery has more potential energy than a 9 volt alkaline or lithium does. The AA has a lot less voltage, but more watt hours.

I have a cheap dollar store 9 volt red led light in front of me right now that has sat there for at least a couple years. It still runs great off of ancient 9 volt batteries that are only putting out about 3-4 volts.

dealgrabber2002 · Nov 7, 2012

ampdude said:
I remember a thread about.. I believe an HDS light that ran for three years or so on one battery on low output. I think it was a single lithium primary, but I'm not sure. .

IIRC, I think it was on beacon mode, not low mode. And it was done by powernoodle(?).

fnj · Nov 8, 2012

I don't know why no manufacturer does this with a single-mode CR123A or CR2 starlight. As has been said, the secret is that you don't need a stepup converter, so there is zero loss except for the voltage drop across the resistor. In fact you don't need any circuitry at all, aside from an LED and a resistor of suitable value. There is no reason these could not be produced very economically. I have made some myself, just breadboards but they work fine. The best is to use a 3.6 volt SAFT LS14500 lithium thionyl chloride primary. These will run an LED for over a year at 0.25 mA with no noticeable dimming at all until the end. The discharge curve is practically a straight horizontal line with an almost vertical dive at the end.

Next best are the common lithiums. A CR123A will run at 0.15 mA for a year, and a CR2 will run at 75 uA for a year. These will progressively dim, but not badly. In fact they will run very much dimmer for a long time AFTER a year.

A 5mm white LED of good quality drops roughly 2-1/2 volts at these kind of current levels.

All of these solutions are much more compact than a 9v.

You could even use 2-3 AAA's in series with decent results not too far from a CR2, but you have to ask yourself why bother with all that extra size and weight and leakage.

fnj · Nov 8, 2012

ampdude said:
I remember a thread about.. I believe an HDS light that ran for three years or so on one battery on low output. I think it was a single lithium primary

Yes. The secret is it has to be the first generation or maybe the second generation. The three years was in beacon mode. They were also good for a month or more on lowest continuous setting, 0.07 lumens. The second generation were not too bad either. The current generation HDS are crap in moonlight mode. They will only run about 5 days on lowest setting.

fnj · Nov 8, 2012

ampdude said:
An AA alkaline or lithium battery has more potential energy than a 9 volt alkaline or lithium does. The AA has a lot less voltage, but more watt hours.

That is actually not correct. A 9v alkaline is 565 mAh * 9 v = 5.1 wh. A 9v lithium is 1200 mAh * 9.6 v = 11.5 wh, though it is pretty expensive and not widely available. An ordinary AA alkaline is 3000 mAh but only 1.5 v, so only 4.5 wh.

So one 9 v battery contains a bit more energy than one AA.

passive101 · Nov 8, 2012

Wow! That is a huge energy difference between alkaline and lithium 9 volt batteries!

kaichu dento · Nov 8, 2012

the.Mtn.Man · Nov 8, 2012

fnj said:
Yes. The secret is it has to be the first generation or maybe the second generation. The three years was in beacon mode. They were also good for a month or more on lowest continuous setting, 0.07 lumens. The second generation were not too bad either. The current generation HDS are crap in moonlight mode. They will only run about 5 days on lowest setting.

It's actually closer to 7-days, but regardless, calling them "crap" is an extreme exaggeration. Henry decided on a trade-off to boost efficiency on high at the cost of reduced runtime on low, meaning you can run the light for roughly twice as long at full brightness compared to the previous models, which is really not too shabby.

But practically speaking, 7-days continuous on low is plenty for even the most extreme emergency situation. If you figure 2-hours of usage a day on low (which I think is rather a lot), you'll have about 3-months of juice (assuming a new battery). As I've said before, if you're ever in a situation where your flashlight is your only source of illumination and you can't get hold of spare cells then you've got much bigger problems to worry about than whether or not your flashlight is a low-runtime champ.

It's all about perspective, folks.

tam17 · Nov 8, 2012

fnj said:
That is actually not correct. A 9v alkaline is 565 mAh * 9 v = 5.1 wh. A 9v lithium is 1200 mAh * 9.6 v = 11.5 wh, though it is pretty expensive and not widely available. An ordinary AA alkaline is 3000 mAh but only 1.5 v, so only 4.5 wh. So one 9 v battery contains a bit more energy than one AA.

Don't forget that all 9V batteries crap out under higher loads (except lithium 9V, which is a different beast anyway). Any continuous load over, say, 50mA is deadly for a 9V battery, so it's pretty much useless for any real flashlight use. AA batteries can handle higher loads like a breeze, especially a good NiMH chemistry (e.g. Eneloops).

Cheers

alpg88 · Nov 8, 2012

i bought 2 9v lithium for my dmms, i was really surprised when they died well before alkalines do, alk. usually lasts me few years, lithium lasted may be half a year. on paper it has more energy stored, but for some reason real world proves otherwise.

passive101 · Nov 9, 2012

Do 9 volt alkaline batteries leak like others? I've never had one leak in a smoke detector.

broadgage · Nov 9, 2012

I made basic lights for long term emergencies that used a good qaulity white LED and 3 D cells, with several brightness settings.
Lowest was about 1ma for a runtime of 2 years continous or about 5 years nightime only use.
Medium was about 3ma for a run time over a year night only use.
High was about 10ma for about 2,000 hours total use.

I dont sell these, but anyone could make their own. Plastic box, holder for 3 D cells, 3 or 4 position rotary switch, good white LED, selection of dropper resistors.
Not exactly EDC but a worthwhile addition to any kit of supplies for a long term emergency in which batteries may be in short supply.

The lowest setting is ample for safe movement with dark adapted eyes, and the highest setting just permits of reading.

Tumbleweed48 · Nov 9, 2012

passive101 said:
Do 9 volt alkaline batteries leak like others? I've never had one leak in a smoke detector.

I suspect that most smoke detector batteries are changed well before they go dead. The warning "chirp" seems to be very voltage sensitive. I just changed our detector batteries last week (I usually wait until one 'chirps')....the 'chirper' tested 8.5 v., with the rest a hair higher.
They'd probably leak if you left them in there long enough, but you'd probably have to suffer warning 'chirps' for a year or so.

On another 9v. note, a few years back Radio Shack had little 'snap-on' LED light toppers for 9v. batteries. I bought a bunch of them on close out for $0.75 apiece. I then had the unhappy occasion to spend a lot of nights in a darkened hospital room, and used one to read by. I don't know how many hours they'd run, but it was an amazingly long time. Not greatly bright, but plenty to read by or to navigate a darkened room. My ex-smoke detector batteries served for a few night each.

Lynx_Arc · Nov 9, 2012

alpg88 said:
i bought 2 9v lithium for my dmms, i was really surprised when they died well before alkalines do, alk. usually lasts me few years, lithium lasted may be half a year. on paper it has more energy stored, but for some reason real world proves otherwise.

It may be they had lower starting voltage than an alkaline battery. Lithium AA primaries don't have much more than alkaline AAs in capacity and their ability to maintain a higher voltage tends to drive LEDs at higher output which decreases runtime. I have cheap DMMs and use heavy duty batteries in them as I keep forgetting to turn them off on occasion and after draining 3 or 4 alkaline batteries and seen the replacement cost of ~$3+ I decided that 50 cent heavy duty give me about half the runtime for less than 20% of the cost.

P_A_S_1 · Nov 9, 2012

the.Mtn.Man said:
It's actually closer to 7-days, but regardless, calling them "crap" is an extreme exaggeration. Henry decided on a trade-off to boost efficiency on high at the cost of reduced runtime on low, meaning you can run the light for roughly twice as long at full brightness compared to the previous models, which is really not too shabby.

Sorry to go off topic but I don't think this is accurate, I too have read that the shorter runtimes at low were done to improve the high out put efficiency but I see no real gain in high out put runtimes and definitely not 'twice' the runtime. Have you had different results? I'm curious.

the.Mtn.Man · Nov 9, 2012

I've not personally tested it. I'm just going by HDS' own claims which state "200 lumens, runtime: 1.5 tactical hours, 2 ANSI hours". I'm guessing that "tactical hours" means no step-down. Previous generation HDS lights were only rated for 1.25 hours after step-down.

Illum · Nov 9, 2012

fnj said:
I don't know why no manufacturer does this with a single-mode CR123A or CR2 starlight. As has been said, the secret is that you don't need a stepup converter, so there is zero loss except for the voltage drop across the resistor. In fact you don't need any circuitry at all, aside from an LED and a resistor of suitable value. There is no reason these could not be produced very economically. I have made some myself, just breadboards but they work fine. The best is to use a 3.6 volt SAFT LS14500 lithium thionyl chloride primary. These will run an LED for over a year at 0.25 mA with no noticeable dimming at all until the end. The discharge curve is practically a straight horizontal line with an almost vertical dive at the end.

Next best are the common lithiums. A CR123A will run at 0.15 mA for a year, and a CR2 will run at 75 uA for a year. These will progressively dim, but not badly. In fact they will run very much dimmer for a long time AFTER a year.

A 5mm white LED of good quality drops roughly 2-1/2 volts at these kind of current levels.

All of these solutions are much more compact than a 9v.

You could even use 2-3 AAA's in series with decent results not too far from a CR2, but you have to ask yourself why bother with all that extra size and weight and leakage.

Most manufacturers want to boast output, you should see the negative reviews about the Fenix E01 on amazon, over half of which complains about the output being dim. It seems the general public doesn't care about runtime, they just want it "bright" when needed.

P_A_S_1 · Nov 9, 2012

the.Mtn.Man said:
I've not personally tested it. I'm just going by HDS' own claims which state "200 lumens, runtime: 1.5 tactical hours, 2 ANSI hours". I'm guessing that "tactical hours" means no step-down. Previous generation HDS lights were only rated for 1.25 hours after step-down.

Tactical runtime should be until it drops to what they consider to be a 'tactical' output, around 70 lumens, and the ANSi when it drops to below 10% of the initial output. Either way the output on high, without step down, from what I've seen has been the same and this is why I don't get where the trade off is.

CarpentryHero · Nov 9, 2012

There's a keychain light that uses watch batteries on ThinkGeek. Claims a 1 year run time, I have one on my diaper bag
(rush12 best tactical diaper bag everrrrrrrrrrrr)

Why are 9 volt battery flashlights the only ones that keep an LED on for years?

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