Maybe I am not seeing this correct, but the MXT my wife has runs on 8 -1.5 volts which is the 12 volts alkaline and is fine. Now you are showing 8 rechargeable in the pack which should be !.2 volts each with 8 of them run 9.6 volts. Yours show only 9 volts when it is turned on, but you run 10 of them in the other pack and it shows 13 volts, now that should read at 15 volts if these are !.5 volts each or if they are 1.2 volts they will read 12 volts.
If these were fresh off the charger, they will read slightly higher and if these had been used for a while they will show lower voltage. my wifes MXT shows 11.2 if I remember right after charging when we do check it it reads around 11 volts.
The one detector I seen people add 8 rechargeable in the 8 pack is the Sovereign as it needs 10 of the 1.2 rechargeable to work correctly as many have tried it seen it works for a while, but lower the time it will run.
Now I also found the Uni-probe if you use a 9 volt rechargeable battery will start chattering almost right away and also the Tesoros went into low battery alert unless they use a higher voltage battery which was too long to put in the battery compartment.
I was always told many detectors have a voltage regulator that will only let so much voltage through, so if it is slightly higher voltage battery it only regulates 12 volts through. It can't make the voltage higher and that then will trigger a low battery alert.
This is the way I have always understood it.
Rick