HP 41CL - Power Consumption

[Hans Brueggemann]

(08-25-2021 05:53 PM)Michael Fehlhammer Wrote:  The NiMH lady cells are specified to provide 500 mAh.
Divided by 135 µA (or 390 µA, respectively), I would assume a set to last 154 days or five months (53 days or almost two months for V.5/ time module clone).
The actual life cylce is around two weeks only. That is too short by a factor of 4 to 10!
Even if there is some strange behaviour at the end of the discharging cycle which reduces the ideal life span, I can't believe that such effects (alone) are responsible for the observed ridiculous durability.

as has been pointed out before, in a failing battery pack there's always that one cell that capsizes first, while the others are still delivering. you may be able to identify that cell and replace it with a known good one to find out whether that brings your entire battery pack up to the numbers that you calculated.
one more point to note here: the mainboard's power supply (PS) is a regulated switch-mode converter, which will pull proportionally more current with decreasing battery voltage (which is to be expected: the load of the converter is constant, hence the input power has to be maintained constant despite decreasing input voltage). that said, once a cell goes down in a battery pack, the PS will pull more current in order to stay in regulation, thus bringing down the remaining cells even faster.
for example, a HP-41CX w/ CLONIX attached and RUNning a program consumes about 14 mA when the battery pack is fully charged at 6 VDC, but 25 mA when the battery pack is almost flat (i.e., 3.7 V, just before the LOW BAT warning kicks in).
so, your math is correct, but the actual value of the measured current you are using for runtime calculations depends heavily on the battery voltage.