I hope this post is in line with the forums rules, please bear with me.
I just received my Sofirn 35 flashlight. I have bought some rechargeable 21700 batteries to go with them (Keeppower 21700 5000mAh, protected 8A USB).
However, the flashlight won’t turn on with these batteries. It powers on just fine when I insert the USB cable into the flashlight. But it won’t run off the batteries.
I’ve fully charged the batteries and verified with a multimeter that they both have around 4.16V. I can’t see any sort of isolating strip or paper on either the battery or the battery compartment in the light. Does anyone understand what is going on?
My Sofirn SP35 makes contact with the battery using a spring in the tail cap and a pad on the driver. It also came with an unprotected 21700 Sofirn branded flat top battery that measures 70.8 mm long.
I'm guessing that your protected 21700 batteries are too long to allow the tail cap to completely screw onto the body of the light which basically creates an "open circuit" between the tail cap and the body of the flashlight and will not allow electrical current to flow to the driver.
When my SP35 is ON, if I unscrew the tail cap even a few degrees, the light will turn OFF.
Without a battery installed, note the rotational orientation of the installed tail cap to the body. Then re-install the battery and determine if you can still screw the tail cap on to the same rotational orientation (while checking that the overall length of the flashlight is the same as before and not any longer).
EDIT: Do not measure the length of your batteries with anything that conducts electricity. I use plastic non-conductive digital calipers.
EDIT #2: Also, make sure the flashlight's body is firmly screwed into the head of the flashlight.
And to test if the problem is from the tail cap not being able to be completely screwed onto the flashlight's body, you could remove the tail cap and use something electrically conductive (like a piece of wire or a paper clip) to "short circuit" and create an electrical path between the negative end of the battery and the shiny non-anodized bare aluminum end (on the cylindrical edge) of the flashlight's body to verify that your SP35 can turn ON using your protected battery without the tail cap.
Member @Lumeniac mentioned in his REVIEW that at the plus-side of the SP35, there is a small contact pad. Could it be that the (sometimes: recessed) plus-side of a flat-top battery just can’t reach that contact pad? Try putting a solder blob on the plus-side of the battery. Or even better: use a small magnet.
BTW: the battery shown in your link of the seller appears to be a button-top.
the battery you have there has built-in charging which makes it longer then normal button top batteries. So this is not a problem regarding protection. You are not able to screw the tail cap down all the way.
Let me make very provocative comment. If you have created an account on BLF, then you are smart enough to figure out when a brand new light does not light up by using logical thinking about how a typical flashlight completes its circuit.
I didn’t originally think that the batteries were too large since it seems like the tailcap still goes on all the way. But maybe I’m just looking wrong. It would be the most logical cause, although intuitively I would have guessed the circuit to close with even a loose fit. Well TIL.
I bought protected button tops since sofirn advised me that I should specifically use protected button tops. The other batteries in the store that I found were only slightly shorter, or even longer. Maybe that was a misunderstanding between me and them.
I have emailed sofirn again to clarify whether these batteries’ size is really the problem, and if these batteries aren’t right, which others I need.
Hoping the store won’t mope about me returning the batteries. They came in a cool plastic case that unfortunately splintered during transport.
That battery is all marked up on the bottom and possibly top. Most places will not accept returned batteries unless they are faulty. What you bought was a button top with protection and an additional charging circuit that adds a whole lot of extra length to that battery. Zero Air did a review on it and sofirn shipped it with a flat top battery.