I thought I read here once that a clamp meter around the body of the flashlight was a good way to measure current flow from the battery. Something like the Uni-T 210e
Anyone have any experience with using a clamp meter before for this purpose?
There are various methods depending on the design of the flashlight.
Measuring the current of some lights is more complicated than for others and you also need to consider how you will select the brightness level you want to measure the current for.
A video demonstrating using a clamp meter to measure “Sofirn SP10 Pro” current complements of forum member SammysHP:
A video demonstrating using a clamp meter to measure “Lumintop FW3A” current complements of forum member M4D_M4X:
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I recently received the 210‐E, it’s my first clamp meter so can’t compare it to others, and I’m still figuring out how to get the best out of it. It appears clamp meters need current flowing in one direction, or (AC) be a single conductor. You might need to bridge the tailcap with a short length of thick wire.
Not going to work, the current flows through the body one way, back through the cell and the overall magnetic field is null. You’d need to create a cable loop to pass through the clamp. Also beware that DC clamp meters are highly sensitive to permanent magnets that are also commonly found in torches.
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Same goes for instruments, appliances, and powertools connected to an electrical outlet. The ingoing and the outcoming flows cancel each other out.
I wish it was that easy
Well I’ll scratch that idea for now. I foolishly thought I could just clamp it around the light body.
I have a technically illegal extension cord made of single core cables for this purpose. It’s super versatile that way, not only can you measure the live current but also for example make several loops of live+neutral combined, you get an amplified earth leakage current readout.
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Extension cords can be illegal? 
Im going to jail
Well in my region they should be made with double-insulated cables. If I used double insulated single cores and a proper strain relief in the plug and socket I’d be fine. But at least they’re proper 2.5sqmm conductors, not like the ones in stores that are 3G1 H03VV at best, 3G1.5 H05RR for allegedly heavy duty work use.
ETA It would be illegal for me to sell it or build one for a lay person to use. As a licensed electrical engineer I’m allowed to have and use it for work but not make it part of an installation for example.
If only my flashlight could run on a zipped extension cord…
It’s not a problem for most lights. You just need a small piece of wire to jump between the battery neg and body that you’d put the clamp on. It’s only a problem, or at least tricky, if the tailcap isn’t removable or it’s a tail eswitch light.
Make sure you use thick enough wire that it doesn’t create abnormal resistance in the circuit.
There is a device on AmazonUS that does something similar except it does not include your intriguing “amplified earth leakage current readout” detecting method.
The device plugs inline between the 120 VAC receptacle and the load’s power plug and has a X1 hole for normal clamp meter current measurements and a X10 hole for magnified 10:1 current measurements.
It also has two small holes for inserting meter probes to measure AC voltage and includes some LEDs to warn of open ground, open neutral, open hot, hot/ground reverse, or hot/neutral reverse faults. There is also a button that when pressed creates a ground-fault condition to test the functioning of the GFCI (ground-fault circuit interrupter) if there is one on that circuit.
I bought two of them when they were $7 each a few years ago, but they are now a different brand and cost $20:
Triplett ET200 AC Line Splitter and Receptacle Tester
Below is the device I originally purchased:
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If its your only option they can but you must isolate the wires. you are going to be limited to mA though I dont think muA clamps even exist-- though that may not be important for you. Also note that the clamps use a Hall-effect sensor to detect the electromagnetic field and must be zeroed when on DC mode, I say that because It is very easy to get erroneous readings from other noise in the area or a bad zero.
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Oh yeah, I’ve seen something similar though without the built in socket tester and RCD tripificator, for that I just connect my meter in low impedance voltage mode between L and PE. I’ve been thinking about designing and 3D printing something similar with IEC plug and socket at either end to avoid the long cantilever plugged into wall, with multiple holes and internal bobbins for several different measurements but I don’t do mains current measurements all that often to be bothered 
So I tried using my DVM Amp setting (max 20A) on my convoy s6 with W1 and 5 amp linear driver set to 100%.
Max reading I got was 3.2 amps. I expected approx 5 amps.
Is there a high resistance in the DVN circuit impeding the current flow? Can I get an accurate reading this way?
At such low voltage and high-ish current the burden voltage caused by the internal shunt resistor might be considerable, along with internal resistance of the battery and all leads. Also do keep in mind that the output of a linear driver is highly dependent on the state of charge of the cell.
What Size Wires are your leads ?
Yes, there is a current shunt inside for current measurement. It adds enough resistance to the low voltage circuit to significantly affect the current measurement / brightness.
No, you need a clamp meter.
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Correct. Even if you modify it for lower resistance, like I replaced the fuse on one with a chunk of solid copper and used 10awg grounding wire for leads, but it still wouldn’t really measure above 7A very well. And you could only use it for a few seconds then it had to cool for awhile before it was accurate again. Resistance in traces on the board or the components or something.
Clamp meters are awesome, and those UNI-T UT210E’s are great. Just gotta zero it and not touch it after it’s zeroed, and make little wire loops with 12awg or w/e
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