Review: Frelux Synergy2 (2x14500/AA, LH351D 5000K, CRI90)

Disclaimer: I received the Frelux Synergy2 free of charge from the manufacturer

The Frelux Synergy2 is the second EDC flashlight from the US manufacturer. Machined and built in the USA, it features a side by side 2x battery arrangement (14500 Li-ion or AA NiMH) and an electronic switch. Immaculate attention to detail with an intuitive ramping user interface drive a high CRI Samsung LH351D LED. It also features some clever additions such as a physical switch lockout and an adjustable force titanium pocket clip.

If you’re here only for the numbers, here they are:

Manufacturer’s specifications
Battery: 1-2x 14500 lithium ion or AA NiMH, not included (accepts button tops only)
LED: Samsung LH351D 5000K CRI92
Water and dustproof: waterproof tested to 8 feet (2.4 meter)
Impact resistance: N/A
Mode memory: yes
Low voltage protection: yes
Thermal regulation: yes, 55°C
Lockout: physical
Tripod socket: no
Tailcap magnet: no
USB charging: no

Manufacturer’s output specs
Eneloop NiMH
Low: 2 lumens, 200 hours
High: 250 lumens, 2 hours

Vapcell ICR14500 Li-Ion
Low: 5 lumens, 90 hours
High: 500 lumens, 40 minutes
Turbo: 700 lumens, 30 minutes

Measured dimensions and weight
Length: 95 mm
Width: 20.8 mm
Height: 41.7 mm
Weight: 115 grams, plus 41g for Li-Ion or 60g for NiMH batteries

Box and contents

Bundled in the box with the light:
Operating intructions
3M lens protector
Frelux sticker

Physical appearance

5000K CRI90+ Samsung LH351D LED with a smooth reflector behind an AR coated lens.

Light is operated via an electronic switch in the front. It can be turned clockwise to engage physical lockout.

Battery orientation instructions in the tail. The batteries are configured in parallel so one or two can be used.

Tailcap comes off for inserting the batteries. The rod in the middle carries the current via low resistance silver coated beryllium copper springs. There’s an o-ring inside the groove in the body. Manufacturer reports the light to be waterproof up to 8 feet or 2.4 meters.

Comfortable pistol grip hold with the index finger resting on the e-switch.

User interface
The user interface is very simple. Click to turn on at the last memorized output level, click to turn off. Hold for ramping output up, release and hold again to ramp down. Lowest output is available with a click and hold from off. Highest output is accessed with a double click from off or on. Another double click returns to memorized mode. Turn the switch clockwise to physically lock out the light. The switch is quite easy to rotate and I sometimes inadvertently engaged the lockout while handling the light.

User manual at:

Beam and tint
The small smooth reflector produces a distinct hotspot with a beam width of 24 degrees. Spill covers 84 degrees. A candela to lumen ratio of 3.5 means the overall beam is quite floody, similar to many triples and quads. However, the hotspot is clearly differentiated unlike on smooth floody multi-emitter lights.

The hotspot is greenish in tint while the spill is neutral. An orange peel reflector would smooth out the tint consistency while sacrificing some throw. D-c-fix film will work too but decrease throw significantly.

Output doesn’t change the tint or CCT very much but as is typical, the light gets a bit cooler on higher modes.

Spectral data and color rendering
For spectral information and CRI calculations I use an X-rite i1Pro spectrophotometer with HCFR, Babelcolor CT&A and ArgyllCMS spotread for the graphs and data. For runtime tests I use spotread with a custom script and an i1Display Pro because it doesn’t require calibration every 30 minutes like the i1Pro.

Explanation of abbreviations

If you have an hour to spare, I recommend watching this presentation on IES TM-30-15 which also shines light into color rendering in general.

Color rendering overview on different output modes measured from the hotspot.

CRI data on turbo with Li-Ion measured from the hotspot at 30 seconds:

Highest mode with NiMH:

CRI data on other modes
Li-Ion low
NiMH low
Li-Ion mid
NiMH mid

When measuring the total output integrated, the duv is a bit more neutral.
Li-Ion high integrated

Output and runtimes

Output is perfectly regulated on all modes. Temperature limit restricts the output on turbo with lithium ion batteries after some time. In my testing at room temperature this stepdown occurred between 6 and 7 minutes when no additional cooling was used. Efficiency is good. On the 25 lumen default level with NiMH batteries average total system efficacy is excellent for a high CRI light at 119 lumens per watt.

A sustained level of 518 lumens on high is a good result for a light in this weight class.

I wish the lowest output was a bit lower when using lithium ion batteries. A sub lumen level often comes in handy.

I didn’t do a runtime at the lowest level, but based on the efficiency, it should run about a week or so on a couple of Vapcell 14500 1000mAh batteries.

Thanks to a buck/boost driver full turbo output is available down to at least 25% (3.5V) battery capacity. At ~12% SOC (3.4V) turbo worked for a few seconds at full power, but promptly dropped down to 127 lumens. Maximum output on NiMH batteries was also tested to be available with almost completely discharged batteries as can be seen from the runtime graph below.

Standby drain
There’s practically no parasitic drain on a lithium ion battery when the light is switched off. 3µA is one of the lowest I’ve ever tested. With a NiMH battery the standby drain is initially only 9µA, but after turning the light on and then off, it is a bit higher at an average of 38µA. This is still an excellent results and it would take years to drain the batteries with the self discharge rate of the batteries being probably higher.

Standby current
Li-Ion: 3µA
NiMH: alternates between 25-50µA

Using a true constant current buck/boost driver, there’s no PWM or visible flicker at all. On lithium ion batteries all of the modes are perfectly stable in output, while on NiMH some lower modes are exhibiting a very slight ripple at high frequencies. There shouldn’t be any problems even with slow motion video capture.

Worst case flicker on the lowest level using NiMH batteries at 15kHz with a low modulation depth

Most modes are perfectly stable:

The Synergy2 comes configured with a temperature limit of 55°C from the factory. Beyond that the output is decreased and at 85°C the light is turned off. I never measured surface temperatures higher than 48°C in room temperature.

Lack of true PID circuitry means that the light will not increase output once it has stepped down from turbo, even after it has cooled down. Turbo has to be reactivated manually. This also explains why the high mode is able to sustain higher output than turbo on its own.

The Frelux Synergy2 is a pleasure to use and handle. It has an excellent fit and finish with a good user interface and tactile feedback of the new electronic switch. The true buck/boost constant current driver is designed well to work with both lithium ion (3.7V) and NiMH (1.2V) batteries. Other than Li-Ion having a higher maximum turbo output, there’s no functional difference. Combined with an efficient high CRI LED, the result is impressive in output and runtimes. I would have like a sub-lumen mode as well.

There’s working low voltage protection for both battery types with the output being perfectly flat throughout the usable voltage range. If you run lithium ions empty so that the LVP activates, you’ll have to wait a couple minutes for the driver to reset before you can use NiMHs as backup. I had trouble fitting some of my Ikea NiMHs inside the light.

My sample had a visible green tint in the hotspot. The effect is exacerbated by the smooth reflector, which isn’t the ideal choice for the LH351D tint wise. While an orange peel reflector would have resulted in a better overall tint and less shift, the tint bin of the emitter itself is on the green side. This was apparent when measuring the beam integrated at a positive duv of 0.005. I hope Frelux gets his hands on some dog farts (SPHWHTL3DA0GF4RTS6)

edit: according to Frelux, the Synergy2 already uses the “dog fart”, so the tint is probably the combined effect of less than ideal sample and/or the tint of the AR lens

+ Flat output regulation

  • Never gets too hot to handle
  • High sustained output in its weight class
  • Highest mode is accessible even with mostly discharged batteries thanks to buck/boost constant current driver
  • Good efficiency for a high CRI light
  • Excellent finish and construction
  • No flicker
  • Intuitive user interface
  • Clever physical lockout
  • Low voltage protection on both li-ion and NiMH
  • Insignificant battery drain on standby

- Greenish tint in the hotspot

- No real PID temperature control

- Some AA NiMH batteries will not fit

- Lockout sometimes engages by accident while using the switch

  • No sub-lumen moonlight mode

How much is the srp?


Thanks !

Good review, thanks Maukka.

Looks like an interesting light. IMO they’ve nailed the UI but the SMO reflector is a negative for me and no firefly is a deal breaker. On their website they say “we want to design and manufacture quality products at affordable prices”, $250 is not really what I’d class as affordable. Looks like all but the bright green is sold out anyway.

maukka no attempt to access the LED pcb? the manufacturer has a nice disassembled view of the parts on their site.

edit; in hindsight with no removable bezel and the price I don’t blame you just curious

Thanks for the review! Very refreshing to see manufacturers doing different things, even though it is way out of my budget and maybe considered homage design.

A good UI and ~100 lm/W efficacy from a high CRI emitter in a custom. It seems the person behind Frelux chose his driver designer well.

I was surprised how floody it is, but then again, the LH351D is an optically large emitter, and that reflector appears to only be about 13mm. It looks like it is very well focused, though, based on the head-on image and the beamshot.

I can’t really disagree with you, as that is certainly higher than what most people can justify paying for a light (myself included). However, for a fairly novel design (especially considering the geometry increases the amount of machining to be done), manufactured in the US in relatively low quantities, that’s a surprisingly low price.

If by homage design, you’re thinking of the Cool Fall Spy series, I’m under the impression, although I can not confirm, that the designer was unaware of Cool Fall when he designed the Synergy 1. He wanted to figure out a design that makes the best use possible of the coin pocket in most jeans, while still taking conventional batteries and having good ergonomics. That goal basically dictated a AAA side-by-side.

Very nice looking light.


Really nice light but at 250USD, it’s more of collectors item.