I have been asked several times about the thermal performance of the Solarforce L2C host. My best answer has always been ‘I don’t know’, so I decided to put together a quick test.
Carbon fiber has the potential to have excellent thermal conductivity (in plane), but perpendicular to the fiber’s plane, the thermal conductivity is just mediocre. Then, flooding the carbon fiber with resin lowers the thermal efficiency even more. My guess has always been that the carbon fiber wraps around the head and body of the L2C would act as a thermal insulator.
The test setup: I have a p60 drop in set up for thermal testing. It consists of an emitter (XML2) on a 16mm Noctigon. I have a thermal probe entering the bottom side of the pill and epoxied to the thermal junction at the slug. This setup does not use a driver, but the emitter is driven by a power supply. For this test, I set the power supply to deliver 5A. The drop in was installed using a copper wrap. The wrap was adjusted to require 15 pounds of insertion force (+/- 1oz).
The test had 2 different runs for each host… the first was air cooled and the second and most important was hand held. Keep in mind that hand holding a flashlight can create some variables due the amount of pressure applied by the hand, but I attempted to maintain a consistent grip on each light.
For the hand held test, the light was pointed upwards while the test was running. The host was cooled to room temperature and then hand held until the Tj reached 27°C from the heat of my hand. When the Tj reached 27C, I turned the power supply on at 5A.
At 2m10s, I turned the power supply off and continued holding the light for another minute. The drop in Tj after the power is removed should be as meaningful as the increase in Tj under power.
For the Air Cooled test, the lights were mounted on my sphere and I used the attached cooling system which approximates a hand being used as a heatsink.
So, here’s the results:
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While handheld, on the rise, the L2C performed slightly better than the L2P, but after power was removed, the L2P performed slightly better dropping the Tj faster. I’m not sure why this was the case… I thought that the best performer under power would also be the best performer after the power was removed. The difference between the 2 was a max of 3.6 degrees, but more typically 2 degrees. Also, the Tj reading nearly converged at the point where the power was removed.
On the test using airflow instead of handheld, the L2P performed better on both the rise and the ‘after power decline’.
My observations and opinions:
The L2C performed better than I had expected and I think it’s because of the smooth finish on the light. The carbon fiber’s smooth finish creates a better contact surface with your hand. This smooth finish also makes the light harder to grip. While hand held, the L2P offers less intimate contact because of the knurling on the battery tube. Although, the hand to metal contact on the L2P is more efficient, because of the knurling, there is much less area in contact. The difference while handheld was low enough, it may have been caused by any variations created by the way I held the light. With such a small difference, I don’t think the L2C is a bad choice…the carbon fiber is more for looks.
The L2P performed much better when cooled with air because the knurling creates more surface area for the air to contact and the aluminum is insulated from the air with a thin layer of anodizing and not the much thicker layer of carbon fiber composite. For biking or any thing else where the light will be air cooled, I wouldn’t recommend the L2C for air cooled applications.
