Surefire U2 - how to fit 18650/17670?

[Xe54]

Hi:

I have a U2, bought about 2 years ago. I understand folks run it on 18650 cells, and that even unprotected ones are safe in this light.

However, the end of my U2 has an opening that is only 17.0mm diameter (stepped down only at the end, but inside it seems a larger dia.). An 18650 cell is 18+-0.2mm typically, so there's no way this can fit without modding the light.

17670 cells OTOH are 17mm dia, which depending on tolerances might fit the light. I'm hesitant to try this anyway, since it might not fit if the cell I buy is a little oversized.

Is it the case then that a SF U2 needs either a custom body to fit a 18650, or the step down in diameter on the end needs to be cut away on a lathe?

Oh, one last thing: Isn't it odd that 17670 cells are only 1.8Ah and 18650 are 2.4Ah typically, when the cell volumes differ by only 10%?

Thanks for input.

P.S. Please forgive me for not finding this info in the forums, as everytime I try to run a search, I get search engine errors (out of memory, various badness).

 

[Brozneo]

Does it look as if there is a plastic incert within the tube of your U2?

 

[65535]

18**0 cells fit your U2 if it has a larger diameter above the 3/4" step that is only 17mm, just feed the cells through the head/body joint. Best deal on 18650's or 17670's for the U2 are Aw's unprotected cells.

 

[Echo63]

try feeding it through the head/body join instead of the tailcap/body join
the light should unscrew just behind the adjustment collar

chances are that you have a plastic sleeve U2 and that you can remove the plastic sleeve to fit an 18650 and replace the sleeve to stop primarys rattling
when you use them


i would reccomend AW lions
i use the protected 18650 in my U2
17670 in my L4
R123 in my E1E/Kl1, Novatac and FenixP1 and havent had a problem yet

 

[Grox]

People haven't really stated this properly so I'll have a go.

The earliest U2s were bored out so that an 18650 would fit nicely.
The next U2s were still 18650-sized but had plastic sleeve (removeable in most cases) which gave CR123s a snug fit.
The next U2s now are only bored out to fit 2xCR123 so that the cells don't rattle.

 

[joema]

Very old U2s would accept some 18650s. Later U2s had a plastic insert which could (with difficulty) be removed. The newest U2s don't have a removable insert and won't take 18650s.

I think 17670s will work in the later ones, but exactly as you said: could be a tight fit, some 17670s won't fit, and they have (I think) less capacity than 2 CR123As.

Yet another option is using two RCR123A rechargeable cells in the U2. Battery Station protected 900 mAH cells fit my late 2006-vintage U2, but I'm not sure it's the best choice from a capacity standpoint.

The best way is get an 18650 Leef Body for the U2: http://www.lighthound.com/index.asp?PageAction=VIEWPROD&ProdID=2657

Also 18650 cells and charger:
http://www.lighthound.com/index.asp?PageAction=VIEWPROD&ProdID=1305

The problem is those all cost about $69.

Here's the dilemma -- you can get an entire Fenix P3D Q5 for that price, which is smaller, yet has more output and run time than a U2: http://www.fenixlight.com/flashlight/fenixp3d.htm

I have both: a Leef Body + protected AW 18650 on my U2, plus a Fenix P3D Q5. The U2 is a higher quality light, but time and technology march on. The P3D Q5 has more output, better run time in a smaller size.

Personally I'd say keep the U2 and appreciate it for what it is, and try a Fenix P3D Q5.

Be advised if you want rechargeability, you'll need 2x RCR123As in the Fenix, plus charger. In theory you could use the same cells/charger on the U2. However I don't think 2x RCR123As are a good match for the U2. On my U2 output seems to drop fairly fast on those. By contrast the P3D runs pretty well on those.

 

[65535]

I bought Unprotected 18650's from AW and a Leef body, it's really nice and it goes well with the U2. but if you use the clip alot you will need to find some sort of holster or something.

 

[Grox]

Yet another option is using two RCR123A rechargeable cells in the U2. Battery Station protected 900 mAH cells fit my late 2006-vintage U2, but I'm not sure it's the best choice from a capacity standpoint.

Don't use 2xR123 in the U2. It is too much voltage and you overdrive will and damage the light.

Don't do it.

Repeat: Don't do it.

edit: just one of MANY threads

 

[mdocod]

Oh, one last thing: Isn't it odd that 17670 cells are only 1.8Ah and 18650 are 2.4Ah typically, when the cell volumes differ by only 10%?

well.... I'm not sure where you are finding 1.8AH 17670s, I guess there are probably some off brands that claim that capacity. 1.6AH is about the best you are gona find that is a real specification and not just a marketing attempt...

Most of the quality protected 18650s are 2.2AH, there are some bare cells out there in the 2.4-2.6AH range that are close to those numbers but they do not have protection (which you may or may not need, I'll leave that up to you, I guess on the U2 it will shut down at a reasonable voltage, but I would still recommend protected cells for safety sake while charging)

Anyways, to answer the question why the slightly larger cell has so much more capacity, realize for a moment that some components in a cell are going to be a fixed thickness regardless of what size the cell is. For example, the can and the shrink-wrap. So the actual "cell" within (space dedicated to actual electrical storage) might be about 1 to 1.5mm smaller in diameter than the outside measurements. So the difference in space internally may be a larger proportion.... There are probably more components internally that 18650s and 17670s share that take up the same space, so the 17670 looses out on soem space for capacity...

The other reason that 18650s will generally have the highest energy density of the li-ion cells out there, is that they are the MAINSTREAM size for things like laptops. So they get the most attention from R&D departments to maximize capacity. there are probably 10 18650s made for every 1 of any other size made for consumer electronics.

 

[GreySave]

+1 for the Leef body on the newer U2s. Perfect fit and no change in operation. Output might be a hair lower, but not enough to make any real difference, As one individual already stated you do lose the availability of the pocket clip though. I like mine so much that it has replaced my WE 9 series Raider as my all around go to light, although I still use the raider when I need to punch through rain or for better color rendition over distance.

 

[Xe54]

Does it look as if there is a plastic incert within the tube of your U2?

No. There is a 17.0mm opening on the battery end. Then the diameter increases after about 5mm to a significantly larger opening. Judging by the space for the CR123 to jiggle, it looks like there's another 2mm available in there.

 

[Xe54]

try feeding it through the head/body join instead of the tailcap/body join
the light should unscrew just behind the adjustment collar

That makes sense. No if I could just get that part to unscrew... I'll have to see if I can devise some strap wrench.

 

[Xe54]

Don't use 2xR123 in the U2. It is too much voltage and you overdrive will and damage the light.

Don't do it.

Repeat: Don't do it.

edit: just one of MANY threads

Thanks for the link. I was curious about the situation with RCR123 in U2.

The situation is somewhat complicated now since there appear to be three RCR123 technologies:

1. unprotected RCR123 cells which come off a charger at 4.2V. A definite no-no in U2.

2. protected RCR123 which have a terminal voltage with no load near that of an unprotected cell at 4.2V, but which regulate themselves down to "Initial cell voltage of 3.6V switches to 3-3.2V within 12ms"

3. LiFePO4 RCR123A 3.0V cells which have a peak voltage of 3.6V, rather than 4.2V.

Are the latter two cell types Ok in a U2?

 

[Echo63]

That makes sense. No if I could just get that part to unscrew... I'll have to see if I can devise some strap wrench.

this might help

the bits your U2 unscrews into

U2 with plastic sleeve and 2x 123s

the plastic insert is inside the U2 tube, if you are lucky enough to have an older model with the plastic tube

Lto R, U2 head, U2 body and tailcap, 18650, 2x 123 with plastic sleeve

Thanks for the link. I was curious about the situation with RCR123 in U2.

The situation is somewhat complicated now since there appear to be three RCR123 technologies:

1. unprotected RCR123 cells which come off a charger at 4.2V. A definite no-no in U2.

2. protected RCR123 which have a terminal voltage with no load near that of an unprotected cell at 4.2V, but which regulate themselves down to "Initial cell voltage of 3.6V switches to 3-3.2V within 12ms"

3. LiFePO4 RCR123A 3.0V cells which have a peak voltage of 3.6V, rather than 4.2V.

Are the latter two cell types Ok in a U2?

i wouldnt bother, if an 18650 fits, use it, if not, use 17670 (providing they fit the newer bodys) its not worth blowing up a great torch just trying to use different batterys

 

[65535]

Sounds like you have a U2 without a sleeve, but with the compatibility for 18650 cells, use them and enjoy, if not the Leef body is a great piece. Output will not be reduced at all, the U2 uses regulation on all levels, as long as the U2 is receive more than 3 volts at current, then it will function normally.

 

[Xe54]

Thanks for all the feedback folks!

 

[Echo63]

Thanks for all the feedback folks!

so which model do you have - a new one, or an older one with the plastic insert ?