HP 41CL - Power Consumption

[Hans Brueggemann]

in a chain of cells, the weakest one (i.e., the one that drops to zero volts first) experiences reverse charging from the remaining cells, hence the reversed polarity on the dead cell.
when chosing cells, specifically when it comes to NiMH, the "mission profile" of the calculator becomes a factor of cell "mileage", too: when used on a regular basis, cell self-discharge is less important than total available charge (mAh), whereas in the case of sporadic usage, a cell with minimum self-discharge is preferable over those with max available charge (completely disregarding use of the cardreader, of course). so, you may actually get better "mileage between charges" by using cells that have less than the maximum available charge for a particular brand.

edit:
my HP-41CL (first gen., cardreader attached) draws approx.
115 µA when OFF, and
3.5 mA when IDLE.