This is a relatively simple LEDs on a board project. The eventual usage may be multiple, in different applications. The board is 100mm x 100mm. Everything was hand soldered.
The 2835 LED chips are Cree JK2835 "X series" and are binned for 18.0-18.5 volt ratings, but I'm running down the curve, in the 15.5 volt area. Specific part number (for this reel) is JK2835AWT-0-6GB-J20-FV-U-02. The LEDs are rated at 1 watt, but I'm running them well below that threshold, closer to 1.5 mW each. Down rating the power results in a heat dissipation that is negligible. The LEDs are believed to contain six emitters (probably 3V) wired in series.
The board is divided into two regions (left and right) with a LM317LZ regulator controlling each half (operating as a 2-terminal constant current device). Total current for both halves is 109-110 mA. That current is consistent until the voltage source drops somewhere below 17 volts, at which point they begin to subtly dim. Using a new fresh Ryobi 18V 2 AHr pack, I was able to run that board for 19 hours at full brightness, then 2 more hours as they began to dim. At the conclusion the measured input voltage was near 16.4 volts. The fuel level on the front of the Ryobi pack, when checked, would flash the left most light (indicating it was quite low).
Each side of the board is divided into 25 sub-circuits, one 43Ω resistor and two JK2835 LEDs. That protects the failure of one LED (either short or open circuit) from taking down the entire side. If both LEDs go open circuit, then slightly more current is available to the remaining sub-circuits. The design is trying to envision most failure scenarios, and to fail gracefully.
The green module, on top of the Ryobi power pack, was found on eBay. It is sold to allow the 18V power packs to run Hot Wheels toy cars. It comes with an on/off switch, pigtails, and fuse holder. The provided fuse is 30A, but I replaced them with a 3A (which was the lowest I can find locally). The connectors are Anderson Power Poles, but I used pink/lavender to differentiate the 18V from the usual 12V red/black.
The image with the power turned on has a bit of paper over the lights, because otherwise it was swamping the camera on my phone.
The first use of this board, once I have a suitable housing, is to mount two of them on the outriggers on my recumbent trike, so I can have better visibility during the pre-dawn hours.
Hope this gives people here, some food for thought, about what is possible.