My night rides are typically ~50 minutes of riding on curvy rocky rooty trails in the woods. I had been using a pair of S2+s at 3A each, which were adequate, but I wanted to boost the brightness some. I wanted to keep the lights self-contained rather than having a separate battery pack just for simplicity. So my goal was to have the brightest 1hr runtime mode in a flashlight small and light enough to use comfortably. I tried some 26650 lights I had (F13 at 260g w/cell, on my helmet) and the extra weight was very noticeable on my head so I wanted to go lighter than that. So I decided to look at 21700 lights and the SD2 satisfied my requirements. It weighs 162g w/M50T cell, only 40g heavier than a loaded S2+. I modded one light with a SST40 for my helmet and one with a XHP70.2 for my handlebars.
The 5000K SST40 in the SD2 reflector keeps approximately the same beam size as the sliced SST40 in the S2+. I used a convoy configurable linear driver with a low resistance low profile spring from mtnelectronics. To fit the driver in the 23.5mm driver cavity I used the 17-to-22mm convoy brass driver adapters. I actually used the threaded retaining ring that comes with the adapter because it is slightly larger diameter than the adapter itself. With some reaming and filing of the retaining ring I got the 17mm driver to fit with its top surface flush. This was important because the battery compartment length is tight. I soldered some small spacers made from folded copper wire on the driver adapter to add even more space to the battery compartment so the cell fits without being crushed.
I run the SST40 at 4.3A and the beam intensity is 20.5kcd. The S2+ with sliced SST40 at 3.04A (8x7135) had 14kcd. The output, measured by integrating sphere, was boosted by 48%. It should be regulated at 4.3A until probably around 3Ah into the discharge (out of ~4.6Ah of the cell). So not perfect regulation, but it won’t drop very drastically for most of the last 1.5Ah. The 2hr mode at 2.3A would be regulated for most all the discharge.
For my handlebar light I decided to use a 5000K XHP70.2 P2 from kaidomain. I have found that for the off-road riding I do I don’t need a very intense beam from my handlebars; the helmet light takes care of spotting the trail far out and around corners. So I decided to maximize output with a wider beam. The driver cavity depth is only 3.5mm which is shorter than most boost drivers so I had to modify the 22mm FX35 boost driver from kaidomain. I replaced the inductor with a shorter inductor of the same inductance. I have some more details of the driver mod here. The inductor height was just right so I could thermally sink the top of the inductor to the bottom of the shelf using some thermal paste. I decreased the output current to 2.2A (6V) using a R056 sense resistor to get the right runtime.
The battery tube just barely made contact with the driver ground ring so I added more copper wire spacers both to improve the contact and to increase the battery compartment length. Again I used a low resistance spring from mtnelectronics. The bottom left picture shows the driver sitting on the battery tube which shows how those spacers are making contact with the tube. This is not ideal for heat transfer from the driver, but without a lathe it’s difficult to find/make adaptors/pieces that fit.
The reflector opening was just large enough to fit the XHP70.2 dome. I did some slight shaving of the dome around the edges so there was no binding. I removed the silicone and phosphor to the side of the dome so the reflector could sit as low as possible, but those exposed metal surfaces are connected to LED positive and negative so I had to insulate those from the reflector. I used an adhesive tough plastic insulating sticker I got from mtnelectronics. I cut the inner hole to just fit over the dome.
In my S2+ I had a sliced 3V XHP50.2. The SD2 with XHP70.2 beam is a bit wider than the S2+ with sliced XHP50.2. I’m very happy with the tint and beam. There is a slight hotspot, but the corona extends all the way out to the edge of the spill and the beam looks very smooth which is what I want for the handlebar light. Throw is about 6.5kcd on high, compared to 5.5kcd of the sliced XHP50.2 in the S2+. Output was boosted by 48%. Power consumption is about 14.2W so this will give me a bit over an hour on high.
