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?

[TravisE]

Interesting. I never really questioned the voltage reading in BatStatus because the reported values seemed fairly reasonable. I use it mainly to get a relative idea of battery level, but I ignore the state-of-charge percentage it gives and instead use my own formula to estimate SoC by reported voltage, based on actual previously-measured values during discharge of the particular cells I use. Although this varies widely by battery type, brand, usage conditions, and even age / cycle count, it's enough to get an approximate idea. I use Eneloops and in most cases wait until the system low-battery message appears to charge them. Until then, BatStatus gives me a rough idea of where I am in the discharge curve and when I'll need to have a freshly charged set ready.

One problem I have with BatStatus is that when the calculator has been on for some time, the reading it returns often “sticks” and stops updating until the next power cycle, and no matter how much the battery voltage drops it will keep returning a stale value. I have no idea why. This seems especially prone to happen after the low battery annunciator has activated. In that case, I frequently have to power cycle in order to be sure I'm getting an up-to-date reading.

[Sukiari]

Well the thing uses more power when it’s loaded with work for the processor, and still more with serial or IR going. I seem to recall readings were taken and posted here years ago using a multimeter.

On-board voltmeters are notoriously dodgy and even the DM42 with a much newer SOC has iffy real time voltage readings.