My Sofirn SP35 won't power on with my 21700 batteries, why?

Dear forum,

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?

I’m attaching some pictures for clarification.

Sorry I’ve had to put this in a second post as I couldn’t get the images to embed in the first.

It might require unprotected batteries. Some lights wont operate with protected. It also could be that you have flat top and it needs button tops or the other way

Good luck!.

Oh that would suck, I specifically asked Sofirn whether this one needed protected or unprotected ones. I’ll check that with them.

Look down the hole at the driver.

If the driver has a spring, it can take flattops or buttontops.

If the driver has just a pad, it needs a buttontop.

Sofirn by default only sells/includes cells with buttontops.

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.

Oh yeh, with the tailcap off, short the cell to the edge of the tube. If it lights, it’s the length of the cells, etc.

Just checked, and yeh, my SF47’s cells are “flattops” but protrude enough (like buttontops) that they make contact even stacked in series.

So if the pictured cells’ positives don’t protrude enough, that’d be a problem.

Judging by the photo, there ain’t much guessing involved. Cell is waaaaay too long to work in that light.

It’s even longer than a normal protected battery due to the integrated charger.

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.

This ^ is what I was thinking. You are not able to screw the tail cap down all the way.

what will happen if you put the aluminum foil between cell and the spring and let that foil complete the circuit? Trim the foil after putting the tail-cap on.

As has been said, the battery is to long for the light.
This picture shows the problem perfectly.

That’s what I used to do to temporarily test/charge 14500s in my MH20. Take a ball of Al-foil, squoosh it tight and roll it into a cylinder, then fit everything together.

I wouldn’t recommend it to the faint of heart, because, well, it’s still a ball of Al foil, but it worked.

Also to fit those shorty 2×AAA holders in lights that had the longer holders with the pogo-stick on top (to also fit 18650s).

Just be careful, use common sense, and if anything bursts into flames, we don’t know each other, mmmkay?

Probably make it a lot less water resistant since the o-ring won’t be fully engaged if the tail cap isn’t all the way down.

did usolve problem? use unprotected yours are too long and protected u dont want that

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.

Hi all,

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.