Spring bypass question

Admittedly, I’ve only been here a short while and I have a lot to learn, but something is gnawing at me…

You folks are bridging negative-side springs with heavy wire, and that seems silly to me. What’s gauge wire is a spring turned from? Maybe 22AWG or so. Next: pull the spring and measure it’s length. How long is the spring wire? Two or three inches maybe? Consider this- 1000 feet of #22 AWG has a resistance of 16.14 ohms (copper conductor. Alloys will have more resistance, but not enough more to make a difference). So how much resistance would 3 inches or so of #22 AWG coated copper or alloy wire have? Not very much. Measuring voltage drop across the spring would probably require a 6 1/2 digit DMM.

I’ve seen springs bypassed with really thick wire which seems silly. That coiled wire (the spring) is somehow detrimental doesn’t seem to fly either, as there is only DC across the spring. No AC component to supply voltage means that even if the spring capacitive or inductive properties it means nothing whatsoever.

Someone please explain to me the performance benefits I’ll see by bypassing the springs in my high-current lights.

Cheers

There is some interesting reading and testing here from djozz which may help to answer your question.

I was an Unbeliever, too (well, just a bit skeptical), ’til it was pointed out that typical springs are steel-based with a fairly high unit-resistance. So at high currents, voltage drop across a spring can be a fractional-volt, vs bypassed where nice conductive Cu reduces that to mV instead.

The thing you need to look at is the voltage drop. As mentioned above djozz spring measurements are a good place to start.

Your battery will immediately sag to maybe 4.1 volts if it’s a good one under a 5 amp load. The internal resistance of all the components in your light will contribute more drop in voltage. Why is this important? If you look at either djozz or texas ace charts for specific leds you will see that they need a certain voltage to allow maximum current flow for that led. This translates into the lumens/lux you will get.

So a spring bypass will effectively reduce the voltage drop and give you increased current and output in your light.

One way to get around this is to use a buck driver with two cells in series. Now you have either a long tube light or lower mah capacity with 18350’s or 26350’s.

Copper alloy springs usually have resistance in range 10-25mOhm, steel based springs 40-80mOhm, bypassed steel springs 1-2mOhm.

I’ll piggy back a question on this thread.

Why do I not see copper braid used for bypasses on the driver spring? Is it to avoid potential grounding? Anyone try it?

I always do braid on tail and wire on driver. But that’s because I see that’s what all the cool kids are doing. I’ve been tempted to use braid both ends.

This pretty much answers the question. It doesn’t matter even if it is the negative current flow in a circuit. Most of us measure AMP draw via the tail cap afterall. It’s all comes down to reducing electrical resistance for better conductivity, whether or not the improved conductivity gives any noticeable improvement in light output.

If your flashlight is pulling 20 amps, 10mOhms is a .2v loss, which is 4 watts of additional inefficiency.

A .2v loss could also mean much lower output from fet/direct drive flashlights since the available voltage under load could be 3.8 instead of 4v (for example). Or shorter regulated output time.

And also springs are not made of pure copper like a wire is, since it needs to have a high elasticity. The alloys they are made from is often much worse than copper conductivity. Using a wire as the conductor and spring as the pressure medium solves the problem easily.

Another solution is to use a special alloy of copper spring such as the one in the BLF high current spring group buy that’s currently happening. Few flashlights come stock with springs made from those alloys.

I would think the most relevant thing regarding spring resistance is the fit. If the springs are 90% crushed by the battery, then it doesn’t much matter how long or thin they are, the current is going to be going sideways through a cone of crushed metal. Under those circumstances only experimentation with careful measurements is going to determine if a bypass will make any difference or not.

I wonder If it might be better sometimes to push a small amount of something metallic into the centre of the springs so that they crush harder together, instead of soldering a wire. I’m thinking of something similar to wire wool, but not as prone to breakage, wouldn’t want those small particles floating around and shorting things out.

This thread made me a believer. I plan to do a spring bypass soon to try it out!

I don’t use Cu braid anywhere anymore, it always fatigues and breaks after not very many battery changes at all compared to wire.

I use 24 or 26AWG Teflon coated, stranded wire at the head and tail in most cases, some super high current builds get 22 or even 20AWG.

If you are talking for single 18650 lights than you may be right…

You can do such modd but then you’ll loose some wire flexibility which means protected 18650 cells won’t fit any more?

Most drivers have spring on the driver and on switch.

So make tail switch spring direct or stiffer with something(make 18AWG snake inside that spring) and then it will push unprotected 18650 battery harder or more directly on driver spring making better + “crushed” contact as you said.

I found out that Amperage readings are greater when I savagely press 18650 cell to drivers + contact. So yes I agree if you make more solid plus contact it will be better imho.

I used to braid both ends, but the results weren’t great. The braid would tend to fray and then the connection would break.

I get much better results using 22 AWG copper with with silicone wrap. I bend the top end of the wire so it can hook over the top of the spring. The bottom wire goes below the spring or is tucked into the bottom-most coil. I then solder the wire on.

I typically do both head and tail springs the same way: with the wire inside the spring.

To prevent the solder at the top of the spring from wearing out and breaking off, I now always solder a small copper disk to the top of the spring on top of the wire. This greatly extends the lifespan of the bypass and lasts much longer than solder braid.

I have some 18 AWG wire but have never used it. It’s so thick, I don’t think I could get it inside the small EDC lights I tend to mod.

For drivers with a particularly small or hard to access contact pad, like the H17F, I use the same technique as above, but might install the spring inverted so it’s wider at the top. I still have a copper disk for where it touches the battery though.

I do spring bypasses, but I hate them. They will break, it’s just a matter of when.

Which one is better and in what condition: spring bypass or beryllium copper spring?

Spring bypass is lower resistance so your lights should be slightly brighter. However, as a previous poster mentioned, they do tend to break after awhile and need to be redone.

In contrast, a beryllium copper spring should last for the life of the light.

Thanks. Guess I’ll try with beryllium copper spring. The reason I ask about condition is because I remember reading someone (forgot who it was) said he didn’t want to put too much stress to the LEDs so he didn’t do spring bypass.

I use lead free solder exclusively for >2years, mechanically it's much stronger and harder than lead based solder, so I recommend it for bypassing, reliability should be better than with lead PbSn solder which is very soft and more prone to break.

Gentlemen,

I didn’t abandon my own thread, that would be rude. I was out doing photography in an area that doesn’t have cell service… No texts, no internet, no phone. God, it was wonderful looking up and observing Perseid meteorites…

I understand and appreciate everything you’ve said, but I would imagine that contact resistance would be more of an issue that spring resistance. Only the smallest part of the spring actually touches the battery. I would rather see a pair of concentric springs than a single bypassed spring. Concentric springs would be less resistive, but the real benefit of concentric springs would be more spring in contact with the battery.

Bypassed spring may be more harmful than helpful. Consider that the bypass wire may cause axial misalignment between the spring and the battery. Less of the spring contacts the battery = increased contact resistance = more heat = loss of efficiency.

My profession requires that I use a good LCR meter. When I get a chance I’ll measure the resistance of several springs taking a 4-wire bridged measurement.

I appreciate the information and comments presented here, but I’m still a non-believer, until I prove that spring resistance results in a meaningful loss of efficiency.

Cheers

I’m just wondering why small brass pills aren’t soldered directly to the driver vs a spring (get ’em by the dozens at FT and the like). Have a big-ass bypassed spring in the tail, but the solid flat brass pill soldered to the driver should be seriously low-resistance.