Bidirectional current measurement

I use the Joulescope for measuring the current of a rechargeable battery. Most interesting ist discharge current, but the battery is charged also (when the device is on mains) and I was wondering if Joulescope could do this as well. I found no other specification in the manual except for the recommended operating conditions which are -1.5 to 3 A - which would be fine.
Somewhere I found a statement that the Joulescope is one quadrant only.
What’s the status on this?

  • Martin

Hi @Martin

Yes, your Joulescope is intended as a 1 quadrant (+I, +V) device, but it does measure some -I and -V. The fast autoranging only works on +I. Autoranging on -I is measured in 10 µs.

Here are the designed ranges (not the guaranteed specified range) for each range selection:

*** VOLTAGE ***   
Select     Range      Range    Range   Accuracy   Accuracy   Resolution   Resolution
         typical        min      max    typical        max      typical          max
     0      15 V    -1.16 V   16.8 V    5.10 mV    10.2 mV       567 µV      1.13 mV
     1       5 V    -513 mV   5.04 V    1.58 mV    3.15 mV       175 µV       350 µV

*** CURRENT ***
Select     Range      Range     Range   Accuracy   Accuracy   Resolution   Resolution
         typical        min       max    typical        max      typical          max
     0    10.4 A    -1.80 A    10.4 A    3.52 mA    7.00 mA       429 µA       816 µA
     1    2.00 A    -470 mA    2.17 A     741 µA    1.49 mA      75.2 µA       158 µA
     2    182 mA   -42.7 mA    197 mA    67.4 µA     135 µA      6.84 µA      14.4 µA
     3   18.0 mA   -4.23 mA   19.5 mA    6.68 µA    13.4 µA       678 nA      1.43 µA
     4   1.80 mA    -423 µA   1.95 mA     667 nA    1.34 µA      67.7 nA       143 nA
     5    180 µA   -42.3 µA    195 µA    66.7 nA     134 nA      6.77 nA      14.3 nA
     6   18.0 µA   -4.23 µA   19.5 µA    6.67 nA    13.4 nA       677 pA      1.43 nA

As an aside, one of the earlier Joulescope prototypes was fully bipolar with four quadrant operation. However, I decided to go to (mostly) single quadrant operation to save cost and double the effective performance.

So it should measure the current correctly when I charge the battery with 700 mA. Strange, I thought I had tried that and the current simply read zero. I’ll have to try again, maybe the charger was not enabled.

Should work:

Note that I am shorting out OUT+ to OUT- and using my power supply’s current limiting to deliver -700 mA.

Yes, just tried it out, it works! Maybe you should put this in the manual?

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I have been hesitatant to specify Joulescope’s 4-quadrant operation since it does not meet the fast autoranging times of +current operation. I probably should describe how to to use Joulescope to measure battery discharge and charge, though. Thanks for the suggestion!

Conveniently, the “slow” autoranging for negative current operation is not that bad for battery measurement. Batteries are charged with a fairly high (more or less constant, depending on battery type) current and a little extra delay doesn’t matter.