USB BC1.2 charger for Convoy S2+
Intro
I was looking for the ultimate low budget cycling lamp. Discussed with my cycling mates, what would be good to have in the lamp: long lifetime or fast chargeability was on the top of the list. And one of them said: long lifetime and fast chargeability (charging while lighting), because he is riding long distances! Challenge accepted and started my design. My choice for the host is Convoy S2+, because it’s price, quality and availability. But it is just at the diameter of the 18650 battery, so it can be good for any single cell flashlight.
Looked around for the charging solution, I found a new device from Maxim Integrated, the MAX77751, which is a tiny form factor, but clever and powerful USB BC1.2 charger with up to 3.15A. Solid.
Additionally, I have to find a place in the host, where all the electronics can be inserted. One of my design aspect was to keep the original lamp as intact as possible, which makes this harder. But managed to make a small compromise, as I found another key component, the Hirose CX90 USB type C connector.
Mechanical design
Two PCBs are designed.
PCB A holds most of the components
PCB B is designed to be the contact for the battery:
Manufacturing
Manufacturing was easier than you think. Just using cheap stencil, solderpaste, heatgun, steady hands and voila:
(I burned the Molex PCB connectors a bit on PCB B, but I can live with that 
)
PCB B is sandwitched over PCB A to create a thick, COIN shape object:
Added a thick solder layer to the contact surfaces for the battery (BATT+) and the driver spring (SYS, the C-shape exposed pad on the picture) connections. On the edge, a copper wick is soldered for GND:
Note the cutout on the host tube. Electrical tape and rubber O-ring added to the USB type C connector.
Using drills and files and caliper for exact measurements a cutout on the top part of the lamp is made. The idea here is simple: every time you tight the thread of the tube and the head, their position is the same. First I made the cutout on the tube, relative to the Convoy logo. Then I projected this position to the outer side of the head and drilled and filed the oval shape object:
Here, you can see during charging. The small red/green dot is the indicator, shining through a tiny hole.
Engineering notes for the mechanical parts:
- You have to pair the head and the tube, because of different item has different thread start position, making the alignment of the U-cutout of the tube and the oval cutout of the head wrong.
- It is not water resistant. However, the Hirose connector has O-ring and watertight the mounting surface is not flat, but cylindrical there, having a small gap. The hole for the LED can be made watertight.
Electrical design
The schematic is based on the Maxim datasheet:
Straighforward design, however I have to add some engineering notes here:
- The maximum current is limited to 1500mA by R4. I don’t want to kill the battery, but at the same time, I have to provide some juice for the lamp. As you can get max. 3.15A from the source, the remaining ~1,65A should be enough for that.
- The topoff current is set according to the battery datasheet. Around 200mA we are OK in my case
- Your charger will close the GND and SYS as you pluh the USB connector. Or you will close by your switch on the lamp before and light up. In both case, your driver will be activated, while you are charging. Since I want to keep the rest of the lamp intact (incl. whatever driver you use), I have to use this compromise. I just simply use a low power hungry mode (moonlight) during charging. On the other side, If your lamp is in a power hungry mode, the charging do not turn on. Other negative effect is, that during charging, you cannot control your lamp, since you cannot interrupt the GND line.
- One mistake in the design: I used red for status, however, green would be more logical.
- Important: if your PCB manufacturer cannot make buried vias, doublecheck their position, not to interfere with the SYS expode pad. That’s why I have v1.2
- The Molex PCB connectors are a bit overkill here. You can find cheaper and easier solutions
And one for PCB B:
There are some additional components on the schematic, because an evaluation board is made for exact measurements and for testing other USB connectors. They are marked with DNP (do not place) on the schematic:
I used EasyEDA for designing, JLCPCB for PCB and stencil manufacturing. Here are the sources for v1.3 (other connector, swapped LED), feel free to use them and enjoy.
EASYEDA files
- I think the “massdrop” AAA’s have them and some special lumintops. Wonder if they will sell separate?
I never use them, just the optics! And these fit well in other lights too (DQG Slim TI, Sofirn SF10, SF14 V1, Tool AA…) 
Ubercool Ger
OMG!
