Surefire U2 Ultra.

After owning this light for many years I just discovered this particular model will accept 18650's without modification.The battery won't load from the tail but it will load from the head.

My question to our members concerns the use of an 18650 3400 mAh protected battery in this light.

Will this cause any harm to my light?

Please explain in layman's terms if a detailed explanation is necessary.

Thanking everyone in advance.

Yes it will be fine. Many other before you have ran it on an 18650..

However I find it interesting yours can fit an 18650.. usually people have to have them bored. Out of curiosity are you the original owner?

Everything is a gamble.

The Surefire supplied 123As are 3V each.

Rechargeable RCR123A are 3.7V each. My expectation, if using RCR123As, fried circuits.

A single rechargeable 3400 mAh 18650, fully charged, 4.2V, not counting bleed off from fully charged. You're good.

Surefire has done an excellent job of boxing you in. You know, like a printer company selling you small print cartridges.

If you go with Surefire 123A batteries, you're good. If you go with rechargeable CR123A, you have too many volts so you're fried. And if you go with a single 18650, you're under-powered and considering you have an anemic 140lm LED light, run off a 4.2V battery, you're seriously under-powered. Well, you can see how weak the light beam is going be.

So it's your call what you choose to do.

Me? I use lights that can use rechargeable batteries and stopped using Surefire 123As that were constantly needing to be replaced. Again, your call.

As a suggestion, my opinion, if you can, upgrade your light of many years.

Search said:

Yes it will be fine. Many other before you have ran it on an 18650..

However I find it interesting yours can fit an 18650.. usually people have to have them bored. Out of curiosity are you the original owner?

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Yes I am the original owner.I bought this light about 10 years ago.

Since I had a large supply of 123's I never had any reason to try any other batteries in it.I recently bought a Surefire Peacekeeper,which uses an 18650 battery and I became curious if these new 18650's would fit any of my other Surefires. I just discovered it did fit my Ultra.

No fair, you did upgrade your light.

Parrot Quack said:

Everything is a gamble.

The Surefire supplied 123As are 3V each.

Rechargeable RCR123A are 3.7V each. My expectation, if using RCR123As, fried circuits.

A single rechargeable 3400 mAh 18650, fully charged, 4.2V, not counting bleed off from fully charged. You're good.

Surefire has done an excellent job of boxing you in. You know, like a printer company selling you small print cartridges.
If you go with Surefire 123A batteries, you're good. If you go with rechargeable CR123A, you have too many volts so you're fried. And if you go with a single 18650, you're under-powered and considering you have an anemic 140lm LED light, run off a 4.2V battery and you're seriously under-powered. Well, you can see how weak the light beam is going be.
So it's your call what you choose to do.
Me? I use lights that can use rechargeable batteries stopped using Surefire 123As that were constantly needing to be replaced. Again, your call.
As a suggestion, my opinion, if you can, upgrade your light of many years.

Click to expand...

My Ultra is an early model and it's top end output is 100 lumens.I do have many lights that output 500 to 1,000 lumens,so I'm not in a dire need for another high output light.I would like to put this old girl back in use but certainly wouldn't want to create any harm to the internals.

It appears that no harm will come to this light,so I'll use the 18650's.

Thanks for everyones response.

My U2 (original luxeon V version) has been running an 18650 for years with no issues.
as yours fits an 18650 i assume yours is one of the early Lux V ones too, unless its been bored
mine needed a plastic sleeve removing to make them fit

i cant comment on the newer U2A which looks the same, but has a different, brighter LED

I spent a lot of time grinding the brass looking ring inside the body near the tailcap, in order to fit the 18650 battery from the tail end with my early U2. I use an adaptor from Oveready to use standard P60 dropins because the U2 body is very nice and do not care for the original rotary ring mode selection. It is great to hear another member enjoying that classic flashlight.

Parrot Quack said:

[...] And if you go with a single 18650, you're under-powered and considering you have an anemic 140lm LED light, run off a 4.2V battery, you're seriously under-powered. Well, you can see how weak the light beam is going be.[...]

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I respectfully disagree with some of the above. This might be at least partially correct if it's the second-generation U2 with a 'buck' circuit, but the OP's light is almost certainly the older boost-circuit LuxV version. This circuit appears to do a pretty good job of getting the U2 to operate at near-spec from a single LiIon rechargeable; testing back in the day had these lights operating at ~90% of max output from a single ~2200 mAh 18650, and I'm guessing that due to the lower voltage drop under load from a modern 3400 mAH 18650, the resulting output would be undistinguishable from a 2xCR123 configuration.

What hasn't been mentioned is that the 'positive' spring in the U2's head might not make contact with a modern 'flat-top' 18650 - my particular U2 has had that spring stretched slightly so that I can do this.

I can't tell the difference in light output between two fresh 123's and an 18650.

I feel like finding another U2 and then having it hot-rodded.LOL

That's an interesting discovery. I might just scout for an ol' U2.

good info, too bad my is Mk2.
using my U2 for hiking(along w/ headlamp) and never need more then 30-50 lu.

I received in todays mail a NIB U2 Ultra.I was hoping it was the first generation,since I had no way of contacting the seller,so I could use an 18650 battery.It has a B in the serial number and the box states that the max output is 140 lumens.When I removed the head it was obvious that an 18650 would not fit in the body.This light is still available for $105.00 shipped.

Now my dilemma.I want to mod one of the two U2's.My question to the informed members of the board is which one?Would it be wise to mod the first generation or the second generation.Any help would greatly appreciated.

Thanking members in advance.

lightsdog said:

I received in todays mail a NIB U2 Ultra.I was hoping it was the first generation,since I had no way of contacting the seller,so I could use an 18650 battery.It has a B in the serial number and the box states that the max output is 140 lumens.When I removed the head it was obvious that an 18650 would not fit in the body.This light is still available for $105.00 shipped.

Now my dilemma.I want to mod one of the two U2's.My question to the informed members of the board is which one?Would it be wise to mod the first generation or the second generation.Any help would greatly appreciated.

Thanking members in advance.

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So the newer version with 140lmn max won't accept an 18650? Thanks.

No, but a 16650 will fit. Seems to put out less than the full output of 140 lumens but my eyes really can't tell much of a difference using two primaries instead.

RobertMM said:

No, but a 16650 will fit. Seems to put out less than the full output of 140 lumens but my eyes really can't tell much of a difference using two primaries instead.

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16650 is also a nice option, I use it on my G2X. What attracts me to the U2 is the 2 lumens low.

lightsdog said:

I received in todays mail a NIB U2 Ultra.I was hoping it was the first generation,since I had no way of contacting the seller,so I could use an 18650 battery.It has a B in the serial number and the box states that the max output is 140 lumens.When I removed the head it was obvious that an 18650 would not fit in the body.This light is still available for $105.00 shipped.

Now my dilemma.I want to mod one of the two U2's.My question to the informed members of the board is which one?Would it be wise to mod the first generation or the second generation.Any help would greatly appreciated.

Thanking members in advance.

Click to expand...

If your original U2 is of the collector version than I'd go with the second one. That way you can keep it original and not kick yourself in a few years. But that's your call. As far as modding one of them you can have jasonh or barry (precisionworks) bore the body of the second one. Both are known for their quality of work. Upgrading the LED could be another upgrade choice. But your going to want to make a decision on what emitter you would like installed. An XML2 totally brings out the full capability of the deep reflector. An XPL is not a bad choice either. But if you want a throw monster an XPG2 turns a U2 into a nice spotlight. Russ does fantastic conversions. He's here on CPF as well.

Is it easy to repair a bent clip on the U2s?

I am thinking about getting one and am curious. Don't have it in hand yet.

Greenbean said:

Is it easy to repair a bent clip on the U2s?

I am thinking about getting one and am curious. Don't have it in hand yet.

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Depends how it has been bent. I have used small bits of wood and pliers to push out a bent clip before but if the metal has been badly deformed then I would contact SF for a new clip. I have a U2 and it takes the 18650 from the head end perfectly (the head was not bonded but greased for easy removal. It is a better fit that the 123's which rattle. Mine gives 113 Lumens on high. Not a modern light, but still very usable.

10-4, thanks man.