I wanted to give an update on the excellent advice I received here. I got the Krisdonia 25000 mAh Power Bank thanks to SOS and the power cable thanks to jbell.
Tested both today with my MixPre-10 II. Two mics P48 on two tracks (no mix tracks) at 24/192 and got almost exactly 5 hours recording time.
I may have gotten more time if I'd removed the USB backup drive for the test. I don't know how much drain the additional writing mght be.
I had one question for SOS about his Krisdonia. Does the Display ever turn off? After I first charged the unit, the bright green display came on and has stayed lit ever since.
I've tried all ways of pushing and holding the power button but it won't turn off.
Anyway, thanks again for your help.
2023-07-20 UPDATE: It turned out my first Krisdonia unit was faulty. After replacement, I got just over 9 hours recording time with the same setup as above, but without the USB backup drive.