A $10 USB Multimeter for testing cellphones and powerbanks

Originally published at: https://boingboing.net/2018/01/16/a-10-usb-multimeter-for-testi.html

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This meter is not compatible with standards like Qualcomm Quick Charge which use voltages above its 7 V maximum.

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I really doubt the accuracy of that mAh reading. That’s the current capacity of the battery, but is useless without knowing the voltage. The battery-to-5v converter that all battery banks have makes it impossible to determine the actual current and voltage at the battery – your reading will be affected by the efficiency of that converter (which varies drastically with load) and assumptions about the battery (what the meter thinks its discharge curve is vs. the actual curve). So, 80% isn’t too bad… but it still irks me that mAh is a useless unit and batteries should be rated in wHr.

I thought the same thing at first. But, it appears that you can either use it on the 5V output of the battery, to test voltage and current output, or connect it between the power source and the battery input, where a current sense resistor can tell you how much total charge reaches the battery. Pretty nifty!

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I believe those KMASHI batteries are lithium ion. I don’t know much about battery chemistry, but according to Tech Republic you shouldn’t discharge them completely because you are unlikely to be able to charge them again.

Also, they are designed to play dead when the individual cell voltage drops below some specified voltage. I think this means you probably can’t completely discharge them anyway.

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Does anyone know of a unit that works in the other direction? Basically discharges the battery and detects how much juice it was able to give off.

Last time I needed to do that, I seem to recall I used a hundred watt incandescent light bulb, a clock, and some math.


You are correct. They are also $1-$4 on aliexpress.

There are more accurate models that have a lot more information and are still under $20


I use this for liPO & sealed lead, apparently there’s counterfeits out there, so caveat emptor.


Plug in the unit in the other direction.


I actually have this model and it works properly with my QC at 9V, though I doubt that the mAh reading is correct given that the voltage isn’t 5V.

The internal circuitry in the battery pack should (all being well) protect them from complete discharge.

AKA in the UK as https://www.amazon.co.uk/Muker-KCX17-Voltage-Multimeter-chargers-capacity/dp/B01BTRLVYQ/ref=sr_1_6?ie=UTF8&qid=1516181577&sr=8-6&keywords=usb+multimeter

The resistance of a bulb filament is highly variable as a function of current. Unless you’re using some very advanced math, your results won’t be accurate.

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