Safe to charge 18350/16340 inside 18650 flashlights?

Is it safe to use in-body charging for 18350 or 16340 cells with 18650 flashlights that supply a short tube, such as the Sofirn SP40/SC31B and the Wurkkos FC11? I saw a review that mentions the charging current on the SP40 would be “way too high” for the smaller cells but can’t find any other corroborating information.

As I’m sure this varies from cell to cell, I’d appreciate guidance on how to compare specs between the light and the cell if that’s the way to confirm.


The lower the charge current, the more charge cycles it can take (=longer lifetime). That being said, charging a cell once, twice… a few times with a current higher than usual, will not significantly impact its lifetime.
You can read the datasheet for your cells to see what is the recommended charge current.
As a general rule, it’s better to charge li-ion cells at 0,5C, 1C at the most, where C=cell capacity. Most 18350 have around 1000mAh, that means you should charge at 0,5-1 A.
I don’t know the charging current of the SP40 but I highly doubt it will be higher than 1A, so yes you can charge any 18350 without fear of damaging it. If at 1A, it will be a bit too high for most 16340s but you can still use it.

Thanks, that’s fantastic information. I was wondering what the C meant…

Plug it into an anæmic usb port like on a laptop, and that might limit the current, vs plugging it into a 2A-capable port.

the charger will not know what size it is, and you can shorten a 16340 life a lot by overcharging, and high-but-safe charge current for 18650 is about 5x what a 16340 can stand

because the capacity C is about 5x less

MOST internal charging is pretty slow though, because the internals of a light do not usually have space or heat sinking for generating high currents to charge with

like the Mankers, it would probably be ok there, charging took like 8 hours, so it was probably about 200 ma

If one were to use a charging current that was above the recommended level it would be only a performance/longevity issue and not a safety one, correct?

Only one way to find out for your particular case, but why would you want to take that risk?

Lots of good information on the "Battery University" web site and in this particular link:


For lights with on-board charging, a "USB-A / USB-C Digital Meter" is a useful tool for measuring the charging current (A) and how much charge the battery receives (mAh and Wh but this will not be the actual battery capacity which will be less due to the unavoidable losses during charging).

There are many types of USB meters available with various capabilities at different price points.

I have been using the "ET920" USB meter from "Klein Tools" since it was first released in late 2018. It is not the best or least expensive but it is simple to use and I also use it regularly with other types of USB-A and USB-C devices besides flashlights.

The Klein Tools ET920 cost $30 at Amazon US:

Where did you find 18350s with 5x the capacity of a 16340? The highest capacity 16340 have up to 800mAh while the highest capacity widely available 18350s have up to 1200-1300mAh. So it’s not even 2 times, let alone 5 times the capacity.

No safety concern if using the on-board charging of a flashlight. Even if it charged at 2Amps it is still not enough to blow up a battery, not even a 18350. Maybe avoid charging a 500mAh 16340 in it, but even then, I highly doubt it can be dangerous. Just keep in mind that most multi-cell chargers can give up to 1A (sometimes 2A) to each channel, and most are stated as being compatible with all li-ions all the way down to 10440! If there was any safety concern here, they would make it impossible for you to charge the smallest sizes in then don’t you think?
If it can make you feel safer, later I’ll try to charge one of my Vapcell 16340 at 2A.
It’s more dangerous to overcharge the cell (going above 4,2V) but the onboard charging circuit can’t do that.