Discussion where do I have big voltage drops in a Flashlight

It is not that hard to measure the voltage drops in a light
All you need is a short circiutalble powersupply witch can deliver a constant current

So if you got measurements done please share it

The biggest drops people discovered are from long and thin steel springs
Up to several 100mV @3A can come from a cheap steel springs
Bronce alloy springs are better, but best is always a short copper wire bypass the spring

As well known the thinner copper wires and lanes on PCBs are the more drop you get as well

Finally there is a drop in the current regulation
AMC7135 based drivers have a fixed voltage drop
DD FETs have a very low resistance

A voltage drop can be also seen on PCB of protected cells

A big issue is the battery cell itself, depending on manufactor and chemistry the internal resistance varies a lot

There was a question if the battery to the spring contact is also an issue

In a tight light there is quite some force pushing the battery to the spring, so I made a test setup to determine how big it is

The spring is gold plated and the batteries are also plated so with a good spring there should be a very small voltage drop, that I can measure in an external setup

I measured a AWG20 bypassed springs at 5A with 17.5mV using my banana plugs also measuring the contact and plug resistance, pushing only slightly on the spring I got about 18-20mV

I measured today the spring again, but using 4 seperate probes to eliminate the voltage drop I had with 2 probes and 4 wires

I measured the spring with 5.5mV drop, so for 5A I get 12mV drop for losses in the plugs + contact drop

The 5.5mV is measured with the voltmeter directly holding on the tip of my babana plug and on the base of the spring
To separate the measurement from the banana plugs that are delivering 5A

If I stack the banana plugs so I handle only 2 probes I get additional 12mV drop within the plugs measured as well, leading me to 17.5V with the spring
If I shortcut that setup I measure 10.5mV

So I get 3 voltage measurements
17.5V with plug, spring and contact drop
10.5mV shortcut probes
5.5mV 2 probes on spring with seperate current tips

So to get the real contact voltage drop I need to calculate
full drop -spring -plug

That leaves 1.5mV @5A drop for two contact surfaces as you have within a flashlight

I know there is a possible error or very small force pushing the spring on a short cell so that there is maybe 2 or 2.5mV loss @5A
That leads to 1mOhm resistance for contacts inside the light

While I measured on my bronce alloy 6 and 12mm double springs in my Klarus XT11GT 15 and 36mOhm

Conclusion contact drop on gold plated springs can be ignored as it is less than the AWG20 bypassed spring
The voltage drop on switch with PCB, FET and driver lanes is a lot more