Devo 7e battery voltage calibration issue

Hi,

On my Devo 7e the voltage reported by the firmware is never the correct voltage (its always a good bit lower than the actual voltage that I measure with my trusty volt meter).

Due to this it keep thinking it's running out of juice after like 10min of use. Is there a way to calibrate it, or should i just find the "right" low voltage alarm that corresponding to real-life low voltage that I measure as a work-around?
Is it also possible that I need to supply more voltage (I use 1.2v eneloops so its lower voltage than "stock"), or use/solder an additional regulator?

thx

when you say a 'good bit lower' how much are we talking?

about 0.5v (it jumps around a bit)
it doesn't seem to be linear which bothers me most...
real voltage - devo voltage
4.8v - 4.4v
4.6v - 3.9v (starts complaining about low voltage)

This somehow seem to be more of a problem when using the FrSkyX protocol (has telemetry/consumes more probably) rather than DSM2 though I'm not sure if that makes a diff.