Whenever I think of perhaps grabbing something like the Trustfire TF26650 for use in my Spark SX5 the final thing that puts me off every time is the weight. Also if you look at cost on the 26650, it also seems like the cost per mAh is only marginally better than 18650 format.
For example I compared a scenario based upon my choices of buying a Panasonic 3400 mAH cell, or a Trustfire 26650. I assumed a ballpark capacity of 3800 mAh for the 26650 based on NinjaGuy’s Ninja Guy post.
It looked to me like you’d get about 58% of the power for the amount of weight you have to carry with the 26650 cell if I just divide the capacity by weight and get 70 mAh per gram for the highest capacity Panny, and 41 mAh for the Trustfire. So if I got that right, the Trustfire would only return 58% efficiency in terms of what you’re getting in terms of mAh per gram compared to a Panny. In terms of cost per mAh it looks like you might save about 8% by buying the Trustfire.
So I’m thinking 8% savings at the cost of carrying almost double the amount of weight on my head is probably not worth it for a few more minutes of light. I’m wondering, have I done the seat of the pants calculations more or less right, and I’m also wondering why it seems like they cannot achieve efficiency with the 26650 cells similar to what you can find with 18650 cells (in terms of mAh’s returned per gram)?
At 5A and down to 3V you get 4500mAh out of the 26650 and 2500mAh out of the 18650.
Thats pretty close.. but still not as good.
There is a simple reason for that: 18650 is a widely used industry standard. Almost all development goes into 18650 size cells. 26650s just arent that popular so there is little attraction to spend time and money on developing them.
I don't know why they came up with 26650's, but right now they're a way to get more power for a low price when size doesn't matter. In some cases there are other advantages. Check out a123's 26650. The amount of current that can be drawn from that cell is bonkers.
There are 32650's too. More power and a great price, but the only lights for them are made by a member here.
not quite true re the 32650, there are a couple of commercially available dive lights that use them.
your main issue is your comparing a top of the line 18650 to a mediocre 26650. 26650’s come into their own when you get top end cells to drive a high demand light, think feeding 3a/emitter to a 7*xm-l light.
A side benefit imo, is that 26650 sized lights feel much nicer in hand, its not so noticable in a single 18650 light, but an extreme example is a 3*18650 7*xm-l light I have, the skinny body tube feels ridiculous. One of these days I’ll get an adaptor and 3*26650 tube and tailcap turned up to suit that light, probably at the same time I fit a decent output driver.
That’s a great tip, as I had overlooked that 32650 format and had also overlooked the fact my favourite headlamp, the Spark SX5, can take these as well. I see FT just has the LiFePO4 left in stock, the Li-Ion is out at the moment. Again it looks like you’re not much further ahead in you wallet by buying one of these, but if you’re going to be using your light for an extended time the convenience might make it worth considering.
I did get some of those a123 18650’s a couple of weeks ago and really liked them. Last night while descending from altitude I wanted a lot of light for some avalanche debris so I fired up my DIY light that uses a 8* AMC7135 driver and XM-L emitter on high mode. Although when I previously tested them at home with those cells they did good for about 50 minutes although they got as hot as you would expect. Except last night it just kicked down as soon as I put it in. So as I had another, I also tried that and got the same thing. I assumed the LiFePO4 charger must have had issues so I popped a Spark Li-Ion 2600 in and it was fine for the following hour it took to complete the descent. When I got home I measured them and they were at 3.55v, so in fact they were charged, but I guess that driver needed the higher voltage perhaps because it was –7c where I was and when I tested at home it was much warmer. I also noticed that as I descended even when on high with the light strapped to my headband it just got a bit warm, so the active cooling provided by the wind really does work I guess.
Thanks for the head’s up on those cells! I’ll consider perhaps grabbing one of those 32650 FeiLong Li-Ion cells when they come back in stock perhaps. And I may also consider the LiFePO4 FeiLong because it seems to break the mould in terms of density. If it’s really 5000 mAh then it would be great, but do you think it’s actual capacity could be anywhere near that when the best you can get out of an 18650 LiFePO4 seems to be in the order of 1100mAh? If you juxtapose what the weight of a 5000mAh cell would be if it had the same density as an 18650 a123 cell for example, you’d expect it would have to weigh in the order of 190g, but the FeiLong weighs at 140g. If I was a betting man I’d wager that 32650 FeiLong LiFePO4 would come our around 3800mAh.
From what I’ve seen, the 18650 seems to focus on energy density while the 26650 seems to focus on power. This could be at least one of the reasons why, despite being quite a bit bigger, the 26650 isn’t much higher in capacity than the 18650.
But they are not built to do that (except for IMRs of course) its just how the world works. 2C (usually the maximum discharge rate) for a 3400mAh cell is 6.8A. For a 4500mAh cell its 9A. On smaller cells, its the same. Its just natural that a bigger LiIon cell can deliver more current.