What are /r/18650's favorite Products & Services?

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Yep. You may also be able to pair a usb-c 12v board with a boost converter that has current limiting to charge the batteries fairly quickly. I'm working on a setup to do the same thing with a 4s pack...

https://www.amazon.com/JacobsParts-Voltage-Trigger-Module-Type-C/dp/B08NFKV2LD/
https://www.amazon.com/DROK-5-5-30V-Adjustable-Regulator-Converter/dp/B07VNDGFT6/

Usually the buck/boost boards are current limited on both input and output stages, so, e.g. if it has a 3a limit, you supply it with 5v, and output 12v, you'll be limited to 3a*5v => 15w / 12v => 1.25amp output at 12v. But if you increase the input voltage, you'll be able to supply greater output current.

What if I use power supply that has exactly 16.8V output? There would be no problem? Like this one Adjustable Voltage DC Power Supply, DROK AC 110V-220V to DC Output 0-24V, LED Digital Display 12V 24V 20A 480W Switching Power Supply Regulated Converter Step Down Transformer Built in Cooling Fan https://www.amazon.com/dp/B08GFSVHLS/ref=cm_sw_r_cp_api_glt_fabc_BGQYACKM5CS6YHJCDW8J

100mA is not going to give you very much light. Each LED will consume around 20mA, so you're looking at a light fixture with only 5 LEDs. Also, I wouldn't worry too much about how many years a single cell will last, I would look into cell holders like THIS so you can swap them out like you would an alkaline battery when you need to.

It's not so much how many charges/discharges that usually determine the lifetime of an 18650, it's more about how high you routinely charge it to, how low you discharge it to, and how much heat you subject it to. Don't charge them as high as they can go every time you charge them, and don't discharge them completely until they're completely dead every time. How hot will it get at the hottest time of the year where you live? How cold does it get?

You could put two 18650 cells in parallel to double the stored power, but it would require twice the size solar panel to charge it in the same amount of time.

Your project sounds similar to one I was working on awhile back. I wanted to use LEDs for lighting, and use 18650 cells to power it, and charge them via solar. I just don't know what solar panels to get or where I want to put them yet.

Freight for something that large really wouldn't be that far off. Especially in the size of the picture posted. The pic may not be what your referencing but I can tell you it's what's known as a 4D. They're huge. That I could believe being $400 to ship. Typically a trolling uses either a group 24, 27, or 31... Check this ling. This would most likely work perfect!

https://www.amazon.com/dp/B01NAV9G9X/ref=cm_sw_r_cp_apa_i_ti4vFbFP16QFH

I've been using this box advertised as being for shotgun shells, 18650s fit a bit loosely but it works well. this specific one is designed for 10 or 12 gauge 3.5" long shells. A box for 3" 12ga shells would give an even nicer fit, with the taller box the cells seem like they could almost fall out of the organizing section and get jumbled up, because the "walls" only go about halfway up.

TL;DR look for shotshell boxes on amazon, should give you more options