Why is the 18650 more powerful than a AA?

alan-31

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I have here a NiMH rechargeable AA battery (brand is Tenergy). It says 2600 mAh and its 1.2 volts. Tenergy most likely isnt that high quality but it works.
I have a single AA flashlight which uses 1amp when its running but I notice the NiMH battery needs frequent charging.

An example of an 18650 battery is this one on Amazon.com:
- 2 Sony US18650VTC4 18650 VTC4 2100mAh 30A Rechargeable High Drain Batteries
People are pretty happy with this battery with most reviews at 5/5 (although the mAh is a lower for example some have 5000mAh).

Its 3.7 volts like any lithium battery. It actually has less capacity than the AA battery (2100 as opposed to 2600mAh) and the 18650 is slightly larger in dimensions but not 3 times as big.

I've seen people say that a 18650 is equivalent to 3 or 4 AA batteries and people here on this forum also said the same and recommended me that battery so I'm thinking of switching from AA to 18650 for the flashlight but first I'd like to understand (1) why its equivalent to 3 or 4 batteries even though the capacity may be lower for some cases (2) what makes a battery more powerful or how we decide if battery X is more powerful than Y. I understand the concept of high-drain, i.e. if a battery can support high current. This is needed in situations like e-cigarettes where there's going to be a lots of current going through the coils.

Is it because... 3.7 volts / 1.2 volts = 3.08? That is, for the same mAh, the battery will be about 3 times more powerful, correct? Just making sure.
I tried Google but I couldn't find an answer. Others who wondered about the same question and observed the similar battery sizes will find this thread helpful.
 

HKJ

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When comparing batteries it is more correct to compare energy (Wh) than capacity (mAh). Energy includes the voltage difference.

The US18650VTC4 cell is not a high capacity cell, you can get 18650 will 3400mAh, i.e. considerable more mAh than a AA battery, but again the mAh is not that interesting.

Lets look at the Wh (The reason for all the "about" is because Wh varies with load):
A 2500mAh eneloop has about 3Wh
A US18650VTC4 has about 7Wh
A 3400mAh 18650 has about 11Wh

Looking at these numbers it is easy to see the why it requires 3 to 4 AA cell to match a 18650 battery.

All Wh numbers are from my website.
 

oKtosiTe

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Also, 5000mAh 18650 cells do not exist at this point in time. The highest capacity one can buy right now is 3400/3600mAh.
 

Particle

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To clarify, it is easy to find 18650 batteries rated at 5000 mAh, but any rating that high in 2014 is a bold faced lie. Batteries, like many types of electronic hardware, tend to be sold with unrealistic ratings solely for the purposes of marketing.

One last point, too, is that of Wh (watt-hours). HKJ told you that Wh are more important than Ah which is absolutely true. I wanted to take a moment to explain why, however, in the hopes that it will help you understand and believe his assertion. A watt measures instantaneous power and adding a time dimension (watt-hour) measures an amount of work performed. To put it in real world terms, a light producing a given amount of luminance might take 10 watts to do it. This is an instantaneous measurement of its power requirement, however, and tells you nothing about how much power it will consume over time. If you leave that light on for two hours, you'll have used 10 watts for 2 hours--20 watt-hours of power. Since it has a time dimension to it, it is a quantity of work performed. If it is powered by a 12V, 5000mAh battery (12*5 = 60 watt-hours) then the battery will have been 1/3 depleted. If it is powered by a 48V, 2500mAh battery (48*2.5 = 120 watt-hours) then the battery will have been only 1/6 depleted despite the smaller mAh rating. You can compare mAh, but this is only useful when comparing batteries of the same characteristic voltage. This is not the case when comparing NiMH to LiCoO2 or LiFePO4 since you're comparing a 1.2V cell to a 3.6V or 3.2V cell respectively. You must include the extra voltage dimension that is dropped when comparing cells of the same characteristic voltage.

Note that with all else being equal in terms of cell construction, design, and quality, a larger cell will allow you to recover more of its capacity at any given current draw than a smaller cell. That is to say, you will see less than the cell's given rating if you draw it at a rate which exceeds the rate at which it was tested. This rate is typically the rate required to deplete the cell over a period of 20 continuous hours. You might only get 85% of the capacity of a 2 Ah NiMH cell before it goes flat at a 1A discharge current and only 70% at 2A. If you had a 3 Ah NiMH cell next to it that was exactly equal in design and construction but just larger, you might get 90% at 1A and 80% at 2A. The smaller your current draw in relation to the size of the cell, the more effective capacity you'll get. The fact that lithium chemistry presents a higher voltage works to the advantage of the 18650 cell since at a higher voltage a lower current will be drawn for any given amount of power consumed.
 

StorminMatt

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The higher voltage not only means higher power for a given current draw. It also means that driver losses are lower. If a light runs off a single NiMH battery, it will require a boost driver to generate sufficient voltage to power the LED. Boost drivers are not particularly efficient - maybe 70%. With the higher voltage of Li-Ion batteries, either a buck driver or linear driver is used. Buck drivers are significantly more efficient than boost drivers, especially if the boost driver must boost from the low voltage of a single NiMH battery. Linear drivers are often inefficient, but can have an efficiency that is as high as (if not higher than) a buck regulator if the output voltage is close to the input voltage. This is typically the case when powering an LED with a single Li-Ion battery.

I should also mention that efficiency is high in 4xAA lights because these lights use buck converters as well.
 

SubLGT

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…………………………..

Lets look at the Wh (The reason for all the "about" is because Wh varies with load):
A 2500mAh eneloop has about 3Wh
A US18650VTC4 has about 7Wh
A 3400mAh 18650 has about 11Wh…………………………..

I read your test of the Keeppower 26650 5200mAh battery, and it has an energy around 18Wh. Which makes me wonder why flashlights powered by a single 26650 are not more available from the major manufacturers? There are certainly a lot of available flashlights powered by a single 18650. The cost of a 26650 battery is similar to a 18650, despite its larger size.
 

HKJ

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I read your test of the Keeppower 26650 5200mAh battery, and it has an energy around 18Wh. Which makes me wonder why flashlights powered by a single 26650 are not more available from the major manufacturers? There are certainly a lot of available flashlights powered by a single 18650. The cost of a 26650 battery is similar to a 18650, despite its larger size.

A 26650 is about double the size (volume) of a 18650, but very few of them has much more energy than 18650.
You can get a 26650 light from foursevens.
 

realista

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excuse my question but what are at the moment the 18650 and 26650 with higher mah ?
for 18650 i think panasoinc ncr18650 3400mah but oktosi said also 3600mah (???wich 18650??)

and for 26650 the max is 5200 mah right?
 

yellow

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depending on You use ... standard single led insert powering? ... both are about the same (at best for 26650) with ~3.000 (at best)
--> slightly over 1 h for XM-L2 insert
(IF it is over 1 h!!)

think it over:
"same" runtime for identical light engine, but size and weight difference of the light?
--> nogo


the real difference seems to be at high drain applications, where the 26650 is said to hold up longer
 

HKJ

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excuse my question but what are at the moment the 18650 and 26650 with higher mah ?
for 18650 i think panasoinc ncr18650 3400mah but oktosi said also 3600mah (???wich 18650??)

and for 26650 the max is 5200 mah right?

The highest capacity for 18650 that is easy to get is 3400mAh. The 3600mAh exist, but is difficult to get and the capacity is not 200mAh more in my test.

For 26650 the highest capacity I have seen is the 5200mAh cells.
 

SubLGT

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What is the most popular original equipment application for 26650 batteries? Is it in building power tool battery packs?
 

StorminMatt

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depending on You use ... standard single led insert powering? ... both are about the same (at best for 26650) with ~3.000 (at best)
--> slightly over 1 h for XM-L2 insert
(IF it is over 1 h!!)

think it over:
"same" runtime for identical light engine, but size and weight difference of the light?
--> nogo


the real difference seems to be at high drain applications, where the 26650 is said to hold up longer

Not true about 26650s being pointless or only 3000mAH. The highest capacity 26650 is 5200mAH. The highest capacity 18650 is 3600mAH, but the highest capacity 18650 that is easily obtainable is only 3400mAH. That's a 50% difference in capacity.
 

HKJ

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printed on, or in real life when being measured?

my 26650 (ICR) has "4000 mAh", the cheapo 26500 has "5500 mAh" printed on,
both are way below any 18650 I have
;)

Looks real to me:
Keeppower%2026650%205200mAh%20(Black)%202014-Capacity.png
 

Andrew LB

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A 3400mAh 18650 has about 11Wh

I recently picked up a couple EagleTac 3400mAh white/green 18650's that say 12.6Wh. Unfortunately I can't use them in my bored out SureFire E2D LED Defender because the battery is massive compared to my AW IMR 18650 cells which fit perfectly. And to add double insult to injury, protected cells don't work in my Vamo E-cig VV mod.

/edit

Scratch that last bit. I just checked and these EagleTac batteries work in my Vamo! I had read they don't, so I didn't try. lol. I guess if I crank the thing up to max power it may trigger the battery protection since it can draw 5a max.
 

HKJ

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I recently picked up a couple EagleTac 3400mAh white/green 18650's that say 12.6Wh. Unfortunately I can't use them in my bored out SureFire E2D LED Defender because the battery is massive compared to my AW IMR 18650 cells which fit perfectly. And to add double insult to injury, protected cells don't work in my Vamo E-cig VV mod.

/edit

Scratch that last bit. I just checked and these EagleTac batteries work in my Vamo! I had read they don't, so I didn't try. lol. I guess if I crank the thing up to max power it may trigger the battery protection since it can draw 5a max.

The 12.6Wh may be correct at low load and if you discharge down to 2.5 volt, but not at a couple of amperes load.
The main size difference is the strip from + down to the protection and maybe an extra layer of wrap, usual the bare cells are very precise in size.
 

mattheww50

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A little basic information first. watts are instantaneous power. For electricity that is instaneous amps times instaneous volts. Integrate watts over time, and you have energy. For large amounts we generally use Kilowatt hours, which is 1000 watts for 1 hour, for lesser amounts, we use watthours, which is 1 watt for 1 hour. For even smaller amounts we usually use Joules (although I have seen Calories used). 1 Joule is 1 watt for 1 second.

So if we look at high capacity NiMh, we have about 2700mAh, or about 2.7 amps for 1 hour (At that discharge rate it will probably be somewhat less, but that isn't really material to the discussion). Typical voltge from a NiMh is about 1.2 vots. so at 2.7 amps and 1.2 volts, the battery is providing about 3.25 watts, or 3.25 joules per second. Over 1 hour that would be 3.25 watt hours, or about 11,700 joules.

If we look at a typical 18650 Li-Ion cell, it is about 3200mAh (at that discharge rate, it will be somewhat less, but as above, it isn't material to dsicsussion). Unlike the NiMh cell, Li-Ion cells decline in voltage as they are discharged. They start at 4.2 volts, and are fully discharged somewhere in the 2.5-2.8 volt range. The average discharge voltage is about 3.7 volts. So at 3.7 volts and 3.2 amps, the battery is deliverying about 11.8 watts, or 11.8 joules per second. That is 11.8 watt hours, or about 42,600 joules. So our 18650 actually has almost 4 times the energy storage as the NiMh AA cell.
 
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