I measured the power consumption for an HP-41CX under various operating conditions. This CX is an early non-halfnut version (serial number 2404Sxxxxx).
OFF - 8.5 uA (0.0085 mA)
On, but Idle - 1.3 mA (this varied between .95 and 1.9)
Running a looping program, and Catalog 3 - 7.0 mA
Running a program of countinous beeps - 7.0 mA
The CX manual indicates power consumption as follows
Off - 0.01 - 0.05 mA Idle - .5 - 2.0 mA Running - 5.0 - 20 mA
Possibly you could reach the max. of 20 mA with card reader operations.
I wouldn't check this with a meter. I'd take a series resistor and a scope (if available). The meter integrates over the measurement with a rather long time constant, so you'll possibly never reach the extreme values.
I have most of the HP-41 accessories, and plan on measuring power requirements of each, indirectly through the HP-41. I expect that the card reader will pull much more than 20mA, probably over 100mA, when it is pulling a card. I believe that there was a technical article in the HP Journal about the card reader in 1979 or 80, that described the peak current draw close to an amp, essentially a spike when the card reader motor first turned on. I have an auto-ranging meter, but I will have to lock it down on the appropriate scale to get a proper measurement. It provides practically instant approximate measurement, (through a bar-graph), and updates the precise displayed value about every 1/2 second, so I should be able to capture peak current draw, as well as estimate the average.