26500 vs 18650

[BigHonu]

With these two formats of lithium rechargeables, which would power a light driven at 2.5A longer? How about 1.5A? I remember reading somewhere that the the larger cell may have less voltage sag, but how does that translate into runtime? It seems the newer 18650's have a higher mAh rating than the newer 26500's, so does that mean that the 18650's would run a light longer?

Thanks from a rechargeable battery noob!!!!!

 

[45/70]

Thanks from a rechargeable battery noob!!!!!

What?! A high ranking MR Bulk aficionado a rechargeable battery noob? How can that be?

I suppose what you are seeing is the advancement of 18650 technology possibly, being faster than 26500 technology, although as far as their size is concerned, I'm not so sure that a 26500 is really, all that much bigger than an 18650.

Dave

 

[ShineOnYouCrazyDiamond]

That's a tough question to answer as their are many variables at play here.

The issue of voltage sag comes down to internal resistance of the cell. The battery itself has a resistance - it is not a pure power source. The best visualization I can come up with is imagining the battery a resistor and the LA all in series with each other. The lower the resistance of the cell the less the amount of voltage provided to the circuit is dropped over the cell - this voltage appears in the form of the cell physically heating up as it disipates the energy.

So in your question there are two further things to consider. Battery size and also battery chemistry. For any specific chemistry generally higher capacity cells will have lower resistance, but also for any given size of cell different chemistries will have different resistance values.

The two primary chemistry types you usually see here are LiCO (the most common 3.7v Lithium cell) and the LiMN (a Lithium Manganese cell which is a safer chemistry which can also provide tremendous levels of current with very little voltage drop). LiCO cells are rated to 2C (ie: a discharge current which is 2 times the rated capacity) whereas LiMN cells are rated to 8-10C.

If you search around you will see that in general the higher the load on a cell two things happen - the voltage of the cell drops under load and the capacity of the cell under load is much less than the rated capacity. Usually the capacity of a cell is taken at discharge rates which are 0.2 - 0.5C or less in the case of cheaper cells like Ultrafire. LiCO cells tend to drop more voltage under load than LiMN cells.

So for you question of a the 26500 cells versus 18650 cells lets take the top of each cell in two chemistries right now. The AW LiMN 2.3Ah 26500 cell rated at up to 20Amp discharge and the AW LiCO 2.6Ah 18650 cell rated up to 5.2Amp discharge.

At 2.5 Amps discharge the 26500 will hold better voltage than the 18650 and may last longer. At 1.5 Amps the 18650 will be able to deliver closer to rated capacity and may outlast the 26500 but the 26500 will still probably deliver more voltage even under load.

If you kick the number up to 5 Amps then hands down the 26500 cell will kick serious butt.

In many of the M6 threads people say you can run the WA1185 lamp @ 3.2 Amps on LiCO 17670 cells - literally at the 2C limit. I have tried it and it was pretty scary. The light was dim and the cells gave up after 15 minutes. On LiMN 18500 (1.1Ah) or 18650 (1.6Ah) the voltage holds strong and the light is really bright.

So as you can see it really depends on your purpose. For a high power setup I would always choose the LiMN cells but for lower power (ie: under 1C) setups go for the LiCO cell for longer runtime.

I hope this helps.

 

[BigHonu]

Dave, cut an old timer some slack brother! I can't keep up with all this new fangled technology!

SOYCD,
That was an excellent explanation and put in a way that even I could understand! Thanks for taking the time to type that up!

Brian

 

[45/70]

Yeah, I didn't think about the different chemistries. When comparing LiMn and LiFe to LiCo, there is an apparent rather large capacity difference. I say apparent, because, as SOYCD pointed out, LiMn and most LiFe cells, make up a good part of the difference, when used at high currents.

Dave

 

[MoreGooder]

I think I'll chime in here.

Take a look at this:
http://www.batteryspace.com/limnnirechargeable26650cell37v4000mah10arated148wh.aspx

That is a LiMnNiCO 26650 cell. 150mm longer than a 26500 but the same diameter.

Note: 4000mah and
10A (!!!!! WOW !!!!!) maximum discharge current.

I think this beats the crap out of any 18650 out there right now. The energy density is amazing for something a bit longer than a D size battery.

********** EDIT **********
Some of the other chemistries at Batterspace in the 26650 size have even higher max discharge current. How does 42A continuous sound?!http://www.batteryspace.com/lifepo426650cell32v2600mah42arate832wh.aspx
Geez..... that's a lot of juice.

 

[Mike Painter]

I'm not so sure that a 26500 is really, all that much bigger than an 18650.
Dave

OK, so this is a year old but I'm interested in 26500 Li-ons and can't find what I want.

Since the area varies with the square of the diameter, a 26500 has an area of 531 while the 18650 is at 254. A 26650 would have over twice the volume.

An 18650 has a volume of about 16532 while the 26500 is at 26533.

A standard C cell should be at about 1.6 times the capacity so an 18650 at an honest 2500 would give 4000 honest MaH (And honest can be put in quotes for a lot of these batteries.

Cutter has finally shipped my order so the reason for th C cells should be shown in a couple weeks.

 

[Battery Guy]

For any specific chemistry generally higher capacity cells will have lower resistance,

Dave

I seldom have anything to add after your posts, but is it possible that you meant "higher resistance"? For a given chemistry, the only way to increase capacity in a fixed volume is to increase electrode thickness and decrease electrode surface area, both of which increase internal resistance.

Otherwise, I agree with everything else you said (as usual...except when we dual about "memory" effect )

Cheers,
BG

 

[jasonck08]

Dave

I seldom have anything to add after your posts, but is it possible that you meant "higher resistance"? For a given chemistry, the only way to increase capacity in a fixed volume is to increase electrode thickness and decrease electrode surface area, both of which increase internal resistance.

Otherwise, I agree with everything else you said (as usual...except when we dual about "memory" effect )

Cheers,
BG

I think maybe he meant higher capacity (and larger cells) will have a lower resistance, and this is generally true. Like a 18650 will usually have a lower resistance than a 16340 or 14500 cell. A 26650 will usually have a lower resistance than a 18650 cell of the same chemistry.

 

[45/70]

Since the area varies with the square of the diameter, a 26500 has an area of 531 while the 18650 is at 254. A 26650 would have over twice the volume.

Mike, I'll admit I hadn't actually done the math, so you're right. I think you neglected to figure in the difference in length however. Still, a 26500 is a bit more than half again the volume of an 18650.

If you take into consideration the more rapid advancement of 18650 cells, as I mentioned, that takes a bit more away from the 26500, but you're right, there is a bigger difference as far as volume than I thought, when I originally posted.

Dave
I seldom have anything to add after your posts, but is it possible that you meant "higher resistance"? For a given chemistry, the only way to increase capacity in a fixed volume is to increase electrode thickness and decrease electrode surface area, both of which increase internal resistance.

I'm with ya, BG. Jason accurately described what I was getting at in the above post. "Size matters", for the most part anyway.

Otherwise, I agree with everything else you said (as usual...except when we dual about "memory" effect )

Well, we gotta disagree on something. Otherwise we'd become too complacent!

I've been swamped here lately and not tuning into the Forum much. On top of everything else, my Dad passed away about a week ago. He was 96. That's the 5th close relative to do so in just over two years. My only Aunt and Uncle, my Mom, my oldest brother, and now my Dad.

My cousin, my remaining brother, and myself, are the last ones in our generation left. We're dropping like flies! Anyway, I've been spending a lot of my spare time lately, reflecting rather than "foruming", and haven't been checking in much. It'll pass. It's just that I've had a lot of reminders lately, that I will too!

Dave

 

[Mike Painter]

Mike, I'll admit I hadn't actually done the math, so you're right. I think you neglected to figure in the difference in length however. Still, a 26500 is a bit more than half again the volume of an 18650.
Dave

The 1.6 x volume did take into account the different lengths. A 26 will have about twice the volume of an 18 given the same length.

 

[Mike Painter]

Dave

Otherwise, I agree with everything else you said (as usual...except when we dual about "memory" effect )

BG

Failing that you can have a spelling flame war! "Dual" would sort of imply you have similar views.
You want a duel

 

[Battery Guy]

Failing that you can have a spelling flame war! "Dual" would sort of imply you have similar views.
You want a duel

Good grief...give an old engineer a break!

 

[Mike Painter]

Good grief...give an old engineer a break!

No breaks given, but tehre is always an excuse. It's not spelled wrong, it's the name of the variable."

 

[45/70]

No breaks given, but tehre is always an excuse. It's not spelled wrong, it's the name of the variable."

Mike, it's "there" not "tehre".

Don't worry, BG and I "dual" much more often than we "duel".

Dave

 

[Mike Painter]

Mike, it's "there" not "tehre".

Dave

You know that.
I know that.
My fingers don't.

 

[soulrider]

I think I'll chime in here.

Take a look at this:
http://www.batteryspace.com/limnnirechargeable26650cell37v4000mah10arated148wh.aspx

That is a LiMnNiCO 26650 cell. 150mm longer than a 26500 but the same diameter.

Note: 4000mah and
10A (!!!!! WOW !!!!!) maximum discharge current.

I think this beats the crap out of any 18650 out there right now. The energy density is amazing for something a bit longer than a D size battery.

********** EDIT **********
Some of the other chemistries at Batterspace in the 26650 size have even higher max discharge current. How does 42A continuous sound?!http://www.batteryspace.com/lifepo426650cell32v2600mah42arate832wh.aspx
Geez..... that's a lot of juice.

Ok, I'm still somewhat new regarding some of this stuff and either this is a typo or I missing something? If the one battery is a 26650 and the other is a 26500, wouldn't the 26650 be 15mm longer than the 26500 and not 150mm? It's probably a silly point but I want to make sure I understand the basics, etc.

 

[45/70]

Ok, I'm still somewhat new regarding some of this stuff and either this is a typo or I missing something? If the one battery is a 26650 and the other is a 18650, wouldn't the 26650 be 15mm longer than the 18650 and not 150mm? It's probably a silly point but I want to make sure I understand the basics, etc.

Hi soul. Um, no. In a perfect world where there are no protection circuit boards, or nipples added to cells, a 26650 would be the same length as an 18650 cell.

The first two digits represent the diameter, 26mm and 18mm respectively, in your example. The second two digits represent the length of the cell, 65mm for both cells. The last digit is supposed to represent the shape of the cell, 0 representing a cylindrical cell. Some dealers and distributors utilize the last digit to represent tenths of a millimeter, however. As far as I know, that isn't proper, but you see it once in a while.

Dave

 

[soulrider]

Hi soul. Um, no. In a perfect world where there are no protection circuit boards, or nipples added to cells, a 26650 would be the same length as an 18650 cell.

The first two digits represent the diameter, 26mm and 18mm respectively, in your example. The second two digits represent the length of the cell, 65mm for both cells. The last digit is supposed to represent the shape of the cell, 0 representing a cylindrical cell. Some dealers and distributors utilize the last digit to represent tenths of a millimeter, however. As far as I know, that isn't proper, but you see it once in a while.

Dave

Dave, sorry, I was looking at 18650s so I had that number on the brain. I plugged that number into my post by mistake instead of 26500. I've edited my post to correct my intended thought which was, isn't a 26650 cell only 15mm longer than a 26500 cell instead of 150mm as listed in the original post I quoted? Sorry for the confusion. I should have been sleeping and not posting at that time.

 

[Potato42]

Soul,

yes, as explained above the "650" portion of "18650" would refer to a length of 65mm. Keep in mind that this is not exact! Many of the higher capacity 18650 batteries nowadays are closer to 68mm in length. This is partly due to protection circuitry added to the battery. The diameter of the batteries are also variable. What this means is that sometimes a battery might be a good fit in one host, but may not fit another at all. In general you can expect the manufacturers to make minor adjustments to keep up with the advancing technology, so slightly bigger is expected.