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I wanted to measure how much current a usb-c device (specifically, a reMarkable 2 Tablet, but this could apply to any device) draws when in use, so I bought one of these:

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I hooked it up and got a value (about 480 mA). But then I started thinking, the device isn't at 100% charge, so would this be the current required to charge AND use the device, or can I not make that assumption, as I have no way to know how much is going to each function?

Even if I charge this to 100%, I would presume it would still be charging to keep it at 100%, no? Or again, it depends on the circuit design.

Would there be a better way to measure current usage? My goal is to determine how much current different "modes" of the device draw. For example, when wifi is on or off, or when it's in airplane mode, etc. without tearing it apart and putting an ammeter inline with the battery.

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  • \$\begingroup\$ What value did you get once at 100 % charge? \$\endgroup\$
    – winny
    Commented Mar 22 at 15:59
  • \$\begingroup\$ I need to get the device there first. \$\endgroup\$
    – LarryBud
    Commented Mar 22 at 16:11

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A battery at 100% is not still charging - that is what "100%" means; it's at fully charged capacity. For the rest of your questions, the answer is, yes, that device will measure the TOTAL current consumed by the device at a given point in time. NOT the individual device circuits. That information will have to be inferred by taking differences of current consumed in different modes as this device has no way to "probe" consumption by peripheral devices such as Bluetooth radios or speaker amplifiers. For those tech specs, you will need to contact the manufacturer.

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  • \$\begingroup\$ Thanks. I was assuming that at 100% charge, there's still a certain amount of current draw to keep it pegged at100%. Understood about specific features. The plan is to turn everything "on" then turn one off at a time and record the difference in current draw. \$\endgroup\$
    – LarryBud
    Commented Mar 22 at 16:11
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    \$\begingroup\$ @LarryBud to keep the battery "pegged" at 100% charge that you are referring to is the leakage (self-discharge) current of the battery which will be on the order of single digits of uA's (probably even less) for a typical Li-Ion battery. The point is you won't be able to measure it with this device and the salient point is the device operating current will dominate that effect by 5 to 7 orders of magnitude. In fact, the device in sleep mode still dominates that albeit a much lower degree. Your approach will give a reasonable estimation, however, a running processor is a very erratic load. \$\endgroup\$
    – MOSFET
    Commented Mar 22 at 16:18
  • \$\begingroup\$ Thanks. Looks like this simple device has some sort of data logging, (not sure how much, however) so I may be able to get average current draw over time. \$\endgroup\$
    – LarryBud
    Commented Mar 22 at 16:50

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