In a lot of ways, 18650s are just batteries, with a given voltage and capacity. For a huge amount of devices supporting 12V power (or thereabout) you could power them with a back of 3-4x 18650s, such as in a nice chassis with a plug. Would be an advantage to have a low-power cutoff, but it would surprise me if there isn't such as a product out there, and it wouldn't be too hard to make either.
In a similar ways, a lot of the 18650 chargers support 12V og 11-18V input for charging, meaning they' pair well with cars and some solar panels. That makes them pretty suitable for power storage between day (charging) and night (usage for light, radio etc).
Aren't there vape pens that run on 18650s? I seem to recall hearing that the sale of these devices is largely responsible for subsidizing the recent advances in high amp, high draw, IMR type battery technology.
Yeah, 18650s are pretty much the standard battery form factor for vapers. Some of these things are starting to becoming fairly insane, such as packs using 3x Sony VTC4s, supporting up to 200W or even above. In mass-market products I mean.
Not that I'm suggesting you start vaping.
If you're a smoker or otherwise using nicotine, I would.
For the prepper-minded people in here, vaping even makes sense from a long term perspective. For a few hundres dollars, or half a grand or so, you could easily pick up everything you'd need for covering your habit for *years*. Drifting off topic though, so I'll leave it at that.
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