Energy Density: NiMH AA vs. Li-Ion 14500

[EngrPaul]

Lithium Ion batteries have a higher energy density than Nickel Metal Hydride, based on weight.

But it would be a bad assumption that a Lithium-ion battery has more energy capacity than a NiMH battery of the same size.

Example:

1.2V * 2.7A*h = 3.24 W*h for a NiMH AA
3.6V * 0.75A*h = 2.7 W*h for a Li-Ion 14500

Considering this fact, the new Fenix TK40 (8AA) will probably have as good of runtime as a Li-Ion equipped flashlight of the same size. But it would be heavier.

 

[Fallingwater]

There are two fallacies in your theory:
1) LiIon cells operate at a higher voltage than 3.6, and
2) a light of that size would not be powered with 14500s, but 18650s

 

[KowShak]

There are two fallacies in your theory:
1) LiIon cells operate at a higher voltage than 3.6, and
2) a light of that size would not be powered with 14500s, but 18650s

You are right that the li-ion cell operates at a voltage higher than 3.6v for some of its discharge cycle, however, a 750mah li-ion cell would have to operate at 4.32v over the complete cycle to store 3.24 watt hours (joules).

The nimh AA can store 3.24 watt hours but that is under best case circumstances, i.e. no voltage droop because of heavy current draw, no self discharge etc.

Its not really a fair test because AA is the size of cell at the forefront of nimh technology but 14500 is not the most developed size for li-ion.

 

[rantanplan]

more realistic:
8x Eneloop (other NiMHs s*ck ): 8 * 1.2V * 2.0 Ah = 19.2 Wh

comparable setup with LiIon:
2x 18650: 2 * 3.6V (indeed a realistic assumption as an average voltage under a 1.5 amps load) * 2.4 Ah (fresh & cheap Trustfire 2.500mAh) = 17,28 Wh

... not a big difference, but the 2x 18650 setup would have less weight, smaller body diameter, but increased body length . Requirements for charging as well as financial aspects are another things to consider ...

The TK40 has potential ... lets see how it performs against its competitors.

 

[Fallingwater]

You are right that the li-ion cell operates at a voltage higher than 3.6v for some of its discharge cycle, however, a 750mah li-ion cell would have to operate at 4.32v over the complete cycle to store 3.24 watt hours (joules).

Yes, technically NiMH can store more energy than LiIon in AA size, but a 2.7Ah NiMH cell more often than not loses most of its charge due to self-discharge. The 14500 LiIon / 2.7Ah NiMH is only fair if you use the light as soon as you've charged it. Since that's rarely the case, the best comparison is against LSD NiMH, and in that case the 14500 wins.

Its not really a fair test because AA is the size of cell at the forefront of nimh technology but 14500 is not the most developed size for li-ion.

Righty. The most developed LiIon size is 18650, which is why I said that it'd make more sense for a light the size of the TK40 to be powered by those (or by C or D sized LiIon, which aren't very far behind).

To tell you the truth, I think the TK40 is not a good idea. I mean, the light is, it's the power source that I don't like. Why bother with eight AAs when you could power it with LiIons and get rather more energy in it?
Or if they want to completely avoid LiIon (dumb decision IMO, but I'm going to assume they have their motives), why power it with eight AAs when you could power it with two D NiMH? People who want a Fenix TK40 aren't the sort of people who have trouble sourcing NiMH cells of D size (even LSD ones), and it's not like it's meant for old Grandma who only ever buys alkaline AAs from the supermarket.
Or hell, just make it so it can be powered by two Ds or eight AAs in a 4p2s holder configuration.
(note: my knowledge about the TK40 is limited; if it turns out it is, in fact, able to run on two Ds, ignore everything I said. )

Rantamplan: if you're going to compare chemistries, you need to pick ones that'll fit in the same body size. Go ahead and compare eight Eneloops with two 32600 (D-size) LiIon. I'll be waiting.

 

[IMSabbel]

Because nobody even HAS NiMh-D-cells at this point anymore. Hell, i do not even own a charger that could charge them. Plus the fact that its really hard to finr ones that arent just faked sub-Cs. Who wants 2-4 Ah in that big a volume?

I am not saying that i am thrilled by the prospect of an 8-cell light, but at least AA cells are the single cell type in the world where supply and support infrastructure (multi-bay chargers, etc) make is (barely) viable.

 

[Marduke]

But there are already a ton of competitors who have 1x or 2x18650 MC-E or P7 lights. Who makes a stock MC-E or P7 type light that runs on regular cells?

There is your answer.

 

[xpea]

But there are already a ton of competitors who have 1x or 2x18650 MC-E or P7 lights. Who makes a stock MC-E or P7 type light that runs on regular cells?

There is your answer.

I fully agree.
2, or even better, 3pcs of 18650 (like Legion II) is the right choice for this kind of light even more when everybody knows that 18650s are the highest density cells available (up to 3000mah).

 

[metlarules]

With same size cells the li-ion still wins.Fresh off the charger.

18650- 4.2v * 2.2ah=9.24wh
4/3af cell- 1.45v*4.5ah=6.525wh

Here is the two batteries
http://www.lighthound.com/AW-18650-Protected-2200-mAh-Rechargeable-Lithium-Battery_p_105.html

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3240

 

[Fallingwater]

You can't use full charge voltage to calculate energy density. That voltage will go down very quickly under any significant load.

While I agree that the general consumer is far more likely to be able to run a 8AA rechargeable light than a 2D one, I hardly find the TK40 a flashlight aimed at the general consumer. Those who want a TK40 probably have the means to get non-fake D-sized rechargeables and chargers to suit.

And even then, this still doesn't explain why the TK40 can't run on both sizes. The body is certainly wide enough to accommodate two Ds, and I assume the eight AAs are placed in holders...

 

[Marduke]

And even then, this still doesn't explain why the TK40 can't run on both sizes. The body is certainly wide enough to accommodate two Ds, and I assume the eight AAs are placed in holders...

D NiMH or Li-Ion D's? 2xD NiMH/Alk is obvious, you can't boost 2 cells for that much power. We don't know if it will or will not take 2xD Li-Ion yet.

 

[HighlanderNorth]

What is the advantage of LI-Ion over LSD NiMh batteries?

I just started looking into Li Ion batteries about a year ago, although I've owned devices with Li Ion battery packs. But then I started looking into Li Ion single cells, and reading up on the do's and dont's regarding Li Ion batteries. But for some reason the OP question didnt dawn on me til today. I'd always just assumed that Li Ion technology was somehow better than NiMh, because since Li Ion batteries come with more (apparent)risk than NiMh batteries come with, there must be an advantage that makes the trade off worth it, right? I dont see the warnings linked to NiMh batteries that I see with Li Ion batteries. Maybe it's overstated just for absolute safety's sake, but there certainly are available stories about catastrophic failures of some sort with Li Ion batts. Many of them are sold with protective PCB's, etc attached to them, so there must be a reason for that. Nonetheless, I own a bunch of them and have not had any issues.

The said benefit of Li Ion is improved capacity. So I assumed(again) that with Li Ion batts, you would have higher capacity, longer battery life, as well as other electrical current related advantages, which would also justify the substantially increased cost of Li Ion batts.

But when I look at the capacity ratings of Li Ion batts vs. similar sized NiMh batteries, they are either almost identical, or in some cases, the NiMh batts have more capacity. That seems to contradict my assumptions....... For instance, LSD NiMh AA's have capacity of 2000-2100mah, vs. 14500 LiIon(the same size as AA) are only 750-900mah. The Li Ion 14500's have higher voltage, and I dont fully understand how that effects capacity though. **Does a higher voltage battery effect the capacity, so that capacity will always be lower than in a similar sized, lower voltage battery like a 1.2v NiMh AA?

Then, look at 18650's. The new, highest capacity model has 3400mah capacity. Compare that to a 8,000-12,000mah NiMh D battery. If you were to measure the actual internal physical size of 18650 vs. D batteries, the D isnt that much larger, so why does the Li Ion 18650 only have 1/3 - nearly 1/4 the capacity?


Aside from the NiMh comparison, there's the Lithium battery comparison that seems to reflect the same results. CR123's come in 1,400-1,550mah, whereas the same size RCR123 Li Ion battery comes in at around 750-850mah. So, once again, reduced capacity, but this time the voltage is similar. Even the 3v RCR123 has about 1/2 the capacity of a 3v CR123. Why?

So, why are Li Ion batteries of the same physical size as NiMh and Lithium batteries have less capacity? Wasnt that supposed to be one of the main advantages of Li Ion technology, more capacity? What IS the advantage of Li Ion technology that makes the increased potential danger and increased cost justifiable? Is it that Li Ion technology gives us higher voltage in batteries of the same physical size as NiMh? It seems that the new NiMh batteries will last about as long as Li Ion bats will, so why not just go with that technology, or are they not capable of producing 3.6-4.2v in smaller sizes, like CR123 or AA size?

Because I dont have a 100% grasp on electrical theory, I dont know how stupid these question are, but it has me confused at this point..

 

[Shadowww]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

Then, look at 18650's. The new, highest capacity model has 3400mah capacity. Compare that to a 8,000-12,000mah NiMh D battery. If you were to measure the actual internal physical size of 18650 vs. D batteries, the D isnt that much larger, so why does the Li Ion 18650 only have 1/3 - nearly 1/4 the capacity?

A 3400mAh 3.6V Li-Ion has energy equal to 3400/1.2*3.6 = 10200 "virtual" 1.2V mAh. Considering it is half the volume (actually, even less than half) than a D battery, and weights ~3 times less, that makes it 3 times more energy dense per weight / 2 times more energy dense per volume.

 

[mattheww50]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

What you are missing is that Li-Ion cell is nominally about 3.7 volts, while a NiCd or NiMh cell is about 1.25 volts. Power= Volts * Amperes. So a 2000 mAh NiMh cell would have roughly 1.25volts x 2000 mAh or about 2.5 watt hours energy storage. A 2000 mAh Li-Ion would have 3.7 volts x 2000 Mah or 7.4 watt hours. So a Li-Ion packs about 3 times the energy that a similar size NiMh cell would have.

 

[HKJ]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

Take 4 NiMH AA batteries and place next to one 18650 battery, look at the space required for each set or feel the weigh of each set. The 18650 has more energy (At least if it is one of the top batteries) than the 4 NiMH AA!

It is not enough to look at capacity (mAh), you must also look at the voltage and a LiIon has the same voltage as 3 NiMH batteries.
You find the best LiIon in the 18650 size and the best NiMH in the AA size, comparing AA sized LiIon to AA sized NiMH will give the NiMH a large advantage. Comparing 18650 sized NiMH to 18650 sized LiIon will also give the LiIon a large advantage.

 

[Wrend]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

It depends how you're using the cells too, such as not discharging below 20% remaining for the Li-Ions. Energy density then is more comparable, especially with the quality higher capacity NiMH cells like the XX ones.

Li-Ion is the winner in the weight department though. The higher voltage of the Li-Ions can mean more efficient use of space too, in that you don't need as many NiMH cells to reach a higher voltage (proportionally more unused space between and around cells with the NiMH).

 

[lwknight]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

Not that it is worth anything but here is my take on the matter.
In generic and non technical terms, it takes nearly 4 AA nimh cells to equal 1 18650 in total energy availability.
Lets say that X brand flashlight has a 4AA body and a 1X18650 body , both swappable. You want the ability of 500 or so lumens for 1.5 hours if need be so a 2AA is out.
It would be so much easier to fit the 18650 body into your pocket or non intrusive holster.
You could keep the 4AA chassis for desk drawer and night stand use and switch to the 18650 chassis for hiking or camping.

Now if you really want some power and run time for , say search and rescue ops you can stack up more 18650s ans still travel lighter than maybe 8 or more AA cells. The Li-ion cells definitely have a coolness factor that the lackluster AA do not.

 

[tobrien]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

highlander, I like how you actually ask good questions and they're very thoughtful

 

[mvyrmnd]

What you are missing is that Li-Ion cell is nominally about 3.7 volts, while a NiCd or NiMh cell is about 1.25 volts. Power= Volts * Amperes. So a 2000 mAh NiMh cell would have roughly 1.25volts x 2000 mAh or about 2.5 watt hours energy storage. A 2000 mAh Li-Ion would have 3.7 volts x 2000 Mah or 7.4 watt hours. So a Li-Ion packs about 3 times the energy that a similar size NiMh cell would have.

That's it in a nutshell. But we should compare the same size cell.

An eneloop has (nominally) 1.2V and 2000mAH. This equals 2.4WH

A (good) 14500 has 3.7V and 750mAH. This equals 2.77WH.

So the 14500 has more stored power,

Bear in mind for single cell applications, the Li-ion outputs very nearly the same voltage that is required by an LED, making the driver circuit simpler. To boost a single 1.2v NiMH to drive a LED requires a more complicated and less efficient driver setup.

So there's more power in the Li-Ion and less of that power is lost in the driver = a Good Thing™

 

[tickled]

Re: What is the advantage of LI-Ion over LSD NiMh batteries?

What you are missing is that Li-Ion cell is nominally about 3.7 volts, while a NiCd or NiMh cell is about 1.25 volts. Power= Volts * Amperes. So a 2000 mAh NiMh cell would have roughly 1.25volts x 2000 mAh or about 2.5 watt hours energy storage. A 2000 mAh Li-Ion would have 3.7 volts x 2000 Mah or 7.4 watt hours. So a Li-Ion packs about 3 times the energy that a similar size NiMh cell would have.

A 14500 cell isn't 2000mAH though. It's more like 800mAH for just under 3 watt hours. 20% more not 3 times...