Why no "efficient" AA lights?

If you're reading this post, I'll ask you to consider your time and effort reading/responding to be charity work for the cause of education of a neophite. Thanks in advance.


Eneloop Pro NiMH cells have a capacity of ~2500 mAh and weigh 30g, a weight efficiency of about 83 mAh per gram. 18650 Li-ion's that I use aren't the most weight efficient for capacity (NCR18650GA), but have about 3500 mAh and weigh 48g, weight efficiency of 73 mAh per gram. By this measure, my AA Eneloops should allow flashlight designers to make lights that are more weight efficient by about 14%. Even if you choose 18650's that are more efficient, AA flashlights should at least be equally efficient at changing Amp-hours into lumen-hours.


But, when trying to find weight efficient AA flashlights, I've seen the exact opposite. A Zebralight SC600 can produce a 600-700 lm spot/flood for 74 minutes with a total weight of 112g. One of the most weight efficient AA lights I could find is the Eagletac GX25A3, which might produce slightly more lumens (a little over 700) for slightly less time (less than 65 minutes). Even granting that the light output from both lights is equally useful, and is a similar light output and duration, the Eagletac weighs 148g...empty! With 3xAA, it weighs 238g, more than DOUBLE the Zebralight weight.


I only give that single comparison as an example. I've tried a few other runtime and lumen combinations, with different types of lights, and seen very similar results--the AA lights are double (or more) the weight of their 18650 counterparts when you try to find equal light output and duration combinations.


Even at low outputs it seems true, even within a single (Zebralight) brand. Zebralight rates their SC63 at 160 lm for 12 hours, with a total weight of 112g. The Zebralight SC5 OP will put out 115 lumens (a bit less) for 3.5 hours (29% of 12 hours). That means, to be equally weight efficient as the SC63, even if we say that 160 lm approximately equals 115 lumens, the SC5 would need to weigh 32g including the battery (29% of the SC63 weight). At 88g, the SC5 OP is a featherweight, but crazily weight inefficient by comparison to the SC63.


So, what gives? I actually can't find a single lumen/time combination where any AA light beats 18650 lights. Why can't a flashlight designer build AA lights to be at least the same weight as their 18650 counterparts for a given lumen/time combination?


The theories I can imagine don't seem to add up to the large difference, but maybe they do. Let me know what I'm missing or underestimating:
1. flashlight structure of AA lights weighs more to hold multiple batteries, while 18650's can have fewer cells and therefore simpler/lighter structures
2. voltages of 18650's are more efficiently matched to today's LED and drivers
3. people are willing to pay big bucks for awesome 18650 lights, and consider AA lights to be "throwaway", so R&D money goes into 18650 lights and not into AA lights
4. consumers don't really care about weight efficiency, so designers just do what they have always done with respect to tube wall thicknesses, materials, heat sinks, design, and layout for AA lights.
5. I haven't found the "sweet spot" for efficient AA output/time

I don't think it's just capacity of the cell. Voltage of cell I think has to do with what ur seeing. But hell you can't knock the AA lights of today. 50-60 lumens for 14-18 hours 😳

Yeah I think your point number 2 is an important one. The voltage of li ion cells match LEDs, but AAs require a boost driver. More parts means more complexity, more inefficiency and more weight. Although personally I fall into point #4 - A few extra grams is meaningless to me. Good discussion topic though, and well thought out.

Your theory 2 is close. More accurately, you're not accounting for voltage, and therefore actual battery energy. Amp-hours on its own isn't energy. You want Watt-hours, which is amp-hours times volts. Eneloops are nominally 1.2 Volts. 18650's are nominally 3.7 Volts.

Therefore:

2500mAh Eneloop Pro = 100 mW-hours per gram
3500mAh 18650 = 270 mW-hours per gram

The 18650's are more energy dense.

There will also be a slight efficiency difference as emarkd noted, but the main thing is to make sure to base output and runtime comparisons on Watt-hours, not Amp-hours. The distinction doesn't overtly matter if you're comparing batteries that run at the same voltage, but definitely does in this case.

I think the proper term you are describing is POWER DENSITY.... or mwh/gram and in that department 18650s are supreme no AA can match them. There is a maximum output in lumens that a single AA can provide, while using a 14500 lithium ion battery you can approach that of 18650s the typical 18650 tends to have a lower internal resistance which allows it more power output with less loss at higher lumen output. Typically single AA lights aren't going to do too well for too long above about 300 lumens output and even at that output nimh AAs are going to be hampered by the higher drain on a single AA by a large boost circuit in which the battery itself starts to add enough resistance into the circuit to dramatically effect runtime.
Basically if you want a light smaller than an 18650 based light you will not be able to have the power density advantage no cell has the same power density in a smaller package.
There are decent lights out there in AA format but you do take a hit on efficiency till you get to the same voltage of an 18650 cell and using boost circuits takes a hit on total output also.
People pay big bucks for 18650 lights because they can put out more lumens than 1AA, 2AA lights and even with 3AA or 4AA the 18650 power density makes those combinations less desirable.
My advice is to decide the lumens you need and the runtime at those lumens you desire and go from there realizing that there are limits to what AAs can do that too often people opt for 18650s.
I've been considering an 18650 headlamp purchase because primarily of the runtime I desire is hard to reach with 1 or even 2AA batteries.

iamlucky13 said:

Your theory 2 is close. More accurately, you're not accounting for voltage, and therefore actual battery energy. Amp-hours on its own isn't energy. You want Watt-hours, which is amp-hours times volts. Eneloops are nominally 1.2 Volts. 18650's are nominally 3.7 Volts.

Therefore:

2500mAh Eneloop Pro = 100 mW-hours per gram
3500mAh 18650 = 270 mW-hours per gram

The 18650's are more energy dense.

There will also be a slight efficiency difference as emarkd noted, but the main thing is to make sure to base output and runtime comparisons on Watt-hours, not Amp-hours. The distinction doesn't overtly matter if you're comparing batteries that run at the same voltage, but definitely does in this case.

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Thanks, iamlucky13--physics isn't just a good idea, its the law!

I was mistakenly stuck on Ah! I'm picking up what you're laying down...

Battery chemistry. 18650s has a much higher energy density and can provide higher voltage & amp in general. While flashaholics do search for batteries with the highest mAh count for runtime's sake, keep in mind that mAh is not the only deciding factor.

Why can't a flashlight designer build AA lights to be at least the same weight as their 18650 counterparts for a given lumen/time combination? Because all flashlights need the exact same components to work, and while 18650 gives much more power, it's not that much bigger than AA.

I'm just saying this in general and it might not answer your questions directly.

Both AA and 18650 based lights have their pros and cons. Brightness wise 18650 is far superior. With current technology a single nimh AA can pump approximately 350~400 lumens without boosting voltage, but a li-ion based 14500 can easily go for double the lumen count, and its big brother 18650 is capable of ~2800 lumens (Nitecore TM03, although the stepdown is crazy) max.

Weight/lumen wise, while it is something to consider, the absolute weight of a flashlight is a more realistic/applicable factor. Fellow CPF member Genzod doesn't even consider 18650 lights because they are way too heavy for his night hiking/running needs (The lightest 18650 light is ~90g with battery included but a light AA light/headlamp weighs about 55 grams) and he values runtime more than crazy brightness.

One should also consider replaceability. Is that even a word? 18650s are all rechargeables which cannot be found in a brick and mortar store, you need to buy them beforehand, charge with a special charger that supports 18650, whereas AA batteries are widely available. Out of juice? Buy a whole pack, tear it apart & stuff one into your light and you are good to go. I'm an 18650 guy myself but I bought an AA light just for this reason alone. This also brings up the issue of cost upfront. One needs to invest into a 18650 charger and at least one 18650, but he/she can probably find AA batteries lying around the house.

Last but not least, battery chemistry dictates that 18650 (li-ion) is more dangerous than AA (nimh). It's not like an 18650 becomes a hand grenade if it touches the ground, but it's something to think about.

thermal guy said:

I don't think it's just capacity of the cell. Voltage of cell I think has to do with what ur seeing. But hell you can't knock the AA lights of today. 50-60 lumens for 14-18 hours 

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No doubt, I love my little AA lights, and wouldn't dream of knocking them. I was just spending the weekend having fun with math...

I haven't seen @reppans posting recently, but these are the types of calculations and related discussion that are in line with his expertise.

Keitho said:

....... Eneloop Pro NiMH cells have a capacity of ~2500 mAh and weigh 30g, a weight efficiency of about 83 mAh per gram.................. AA flashlights should at least be equally efficient at changing Amp-hours into lumen-hours.

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They are, but you have to look at from the standpoint of comparing the power of the batteries. And that is where voltage comes into play since watts = volts * current. Therefore it would be 2.5Ah*1.30V = 3.25 watt hours for the NiMh. The NCR18650GA is 3.5Ah*3.7v = 12.95 watt hours. Therefore that 18650 battery holds 3.98 times the amount of energy as the NiMh. Pretty much its 4 AA batteries equals a 18650 battery.

Thus what you will probably find is that if you have a light putting out 50 lumens on an AA battery and 50 lumens on a 18650 battery, the 18650 light will run 4 times as long.

Electronic LED drivers are pretty efficient for both AA and 18650 driven lights. That's because they utilize a switching power supply technique. I think the efficiency loss of these drivers is around 10 to 15% is all. So it doesn't matter that the voltage of the AA is further from the Vfwd of the LED. It's simply a function of power when it comes to switching power supply type of drivers.

Now taking the weight into consideration we get the energy density of the two battery types: 3.25 watt hours / 30g = 0.108 watt hours per gram for the NiMh and with the 18650 its 12.95 / 48 = .270 watts per gram. Thus the 18650 has more than twice the energy density than the NiMh battery.

So the ball park figure is that a 18650 driven light will run 4 times as long as the AA but might weigh only twice as much if we use a drive level that is compatible with the AA's current sourcing capability which is why I just chose 50 lumens in the above example which would be in line with the 2500mAh capacity.

Conclusion: AA LED flashlights are pretty neat and efficient and their performance is right in line from an efficiency standpoint as is their lithium-ion counterparts. It's just that the Lithium-ion has a higher energy density and greater current sourcing capability. I like both types of lights for different reasons.

NiMH isn't the only chemistry which comes in AA size. Lithium ion 14500, lithium primaries L91, alkaline and zinc chloride primaries. Have we crunched the numbers for other AA options? For example L91s. I think they're .5 oz at 1.7 volts. I think they might be over 3000 mAh but lets say 1.5 nominal volts at 3200 mah is.... 4.8 watt hours at 14 grams is..... guessing better than Eneloop pros for the weight but this is hack math and I dislike math.

http://www.candlepowerforums.com/vb...tery-Question-re-Lithium-AA-AAA-s-vs-Eneloops

post # 10 shows a Fenix LD11 run time charts at high and medium using all the battery types. Conclusion is the L91 will vastly outperform the alkaline, NiMh, and 14500 if you keep the drive level off of the max or high setting.........like on medium or low. And that battery is pretty light so that makes a great backpacking battery if needing light weight. Just have to remember to keep the output off the max level. The rechargeable batteries are better when needing higher current levels.

hiuintahs said:

http://www.candlepowerforums.com/vb...tery-Question-re-Lithium-AA-AAA-s-vs-Eneloops

post # 10 shows a Fenix LD11 run time charts at high and medium using all the battery types. Conclusion is the L91 will vastly outperform the alkaline, NiMh, and 14500 if you keep the drive level off of the max or high setting.........like on medium or low. And that battery is pretty light so that makes a great backpacking battery if needing light weight. Just have to remember to keep the output off the max level. The rechargeable batteries are better when needing higher current levels.

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Some Fenix lights will drop some lower modes as the voltage is higher than 1.5v to start with using L91. I think that happens with my 2014 HL30 headlamp. Once run down a bit it gets the lost modes back. I think you're right about Eneloops Pros handling higher draw than Lithium primaries though L91 beats Alkys in that regard. I mostly use Lithium primaries for ER preps and deep cold winter hiking. More chemistry options is a pro for AAs however 18650 lights with a wide enough voltage range can often take 2X CR123 Primaries.

The higher voltage of a single L91 won't affect single AA lights because the forward voltage of the LED is around 3.0V. Thus a boost circuit has no problem whether the input voltage is 1.3v or 1.9v. The problem with Fenix (LD22 comes to mind and your HL30), along with some other manufactures is in the 2xAA lights when you put L91's in them. Then the voltage is above the Vfwd of the LED when the batteries are new and you lose the low mode because the input voltage is too high for their driver circuit to regulate proper based on the conversion that the driver was designed for.

2xAA lights have always been in the tricky area. When designing for optimum use of NiMh, you want a boost circuit. But that leaves the problem if someone puts L91's in it. Some manufacturers have done an OK job with 2xAA for both L91 and NiMh but many haven't. For that reason, I'm suspicious of 2xAA lights unless they've done it right.

lets compare some specific lights and mfg specs, for illustration purposes:

L11C weighs 54 grams w AA NiMh battery. rated for 160 lumens for 1.5 hours = 240 lumen hours
PT-18 w 18650 weighs 105 grams, rated for 330 lumens for 4 hours = 1320 lumen hours

based on the above assumptions:
The 18650 light is 94% heavier than the AA light
The 18650 light has 450% more lumen hours than AA.

rule of thumb,
if you need 150 lumens, single AA works,
if you need 330 lumens, single 18650 works

hiuintahs said:

The higher voltage of a single L91 won't affect single AA lights because the forward voltage of the LED is around 3.0V. Thus a boost circuit has no problem whether the input voltage is 1.3v or 1.9v. The problem with Fenix (LD22 comes to mind and your HL30), along with some other manufactures is in the 2xAA lights when you put L91's in them. Then the voltage is above the Vfwd of the LED when the batteries are new and you lose the low mode because the input voltage is too high for their driver circuit to regulate proper based on the conversion that the driver was designed for.

2xAA lights have always been in the tricky area. When designing for optimum use of NiMh, you want a boost circuit. But that leaves the problem if someone puts L91's in it. Some manufacturers have done an OK job with 2xAA for both L91 and NiMh but many haven't. For that reason, I'm suspicious of 2xAA lights unless they've done it right.

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Lights with a wider voltage range do well. For example an Armytek A1/A2 has 1 to 4.2 volts just like an old low voltage Quark. So a 1XCR123 old school Quark head runs great with 2XAA or 1XAA body though I am sure there must be a loss compared to lights fine tuned for NiMH. I like to run a 1X18650 in a C2 body with an Armytek A2 head. This way I can also run 1XAA/2XAA/1XAAA (clearly not all modes) 2XAAA, 1XCR123 and all the lithium ion sizes which also apply like 14500, 16340 etc etc but take advantage of the higher energy capacity of 18650. All it takes is an extra body and some spacers/adapters. Then again has anyone run the numbers to compare L91s with 18650 for weight to energy.

A big tank of fuel runs longer than a small tank of fuel.
A big tank of fuel holds more fuel than a small tank of fuel.

If and when the days roll around that the R&D goes into double A's that went into 18650's for laptops, vapers and such.... until then the double a is what it is.

Nice topic to be discussed! 18650s has a much higher energy density which is able to provide higher voltage.
While flashaholics do care for batteries with the highest capacity for runtime's sake, keep in mind that battery capacity is not the only deciding factor.

jon_slider said:

lets compare some specific lights and mfg specs, for illustration purposes:

L11C weighs 54 grams w AA NiMh battery. rated for 160 lumens for 1.5 hours = 240 lumen hours
PT-18 w 18650 weighs 105 grams, rated for 330 lumens for 4 hours = 1320 lumen hours

based on the above assumptions:
The 18650 light is 94% heavier than the AA light
The 18650 light has 450% more lumen hours than AA.

rule of thumb,
if you need 150 lumens, single AA works,
if you need 330 lumens, single 18650 works

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When crap hits the fan 18650 sucks There will be millions of AA primary cells around, but no power and sun (if you are wise to possess a sun battery). For example, in Lithuania it is normal to have a week long rain/overcast weather.

vadimax said:

When crap hits the fan 18650 sucks There will be millions of AA primary cells around, but no power and sun (if you are wise to possess a sun battery). For example, in Lithuania it is normal to have a week long rain/overcast weather.

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Around here most people have a car which with a few gallons of gas in it can supply tons of recharging power for batteries and devices. 10 years ago I recharged about 30 AA nimh batteries in a 4 day outage for use.... I had AA primaries and found I didn't need to use any of them as I had enough extra nimh batteries to have a few sets to charge while using others.
The last few years I've invested in 18650s some as I have got a lot of laptop battery packs to harvest batteries from for less than 10 cents a battery. I still have a few AA lights and devices and like their smaller size but mostly grab the 18650 lights and devices due to longer in use, more powerful runtimes and with this less time taking batteries in/out of lights to recharge them.