HP 50g battery voltage: How accurately can it be measured?

[SammysHP]

I just tried BatStatus for my 50g and the readings it gave me are way off (by about 0.2 V less). Well, that is expected because the 50g has no precise and consistent voltage reference. I checked it first with a multimeter and later with an oscilloscope just to verify that the error is not a result of the higher drain while the calculator is working.

To fix this I would need to take several measurements and create my own calibration. The question is how accurate the result would be or how stable the voltage reference / ADC is. Especially with the flat discharge curve of LSD batteries like Eneloop every mV matters.

Do you have any experience with the internal voltage measurement? Do you use it for checking the batteries, do you simply replace/charge them once the battery warning pops up or do you charge them anyway prior of know periods of usage?