I tried taking it apart to look for a CMOS battery since it displays even after I disconnect and reconnect power, but couldn't find the stupid thing.
HP use (Dallas or equivelent ) NVRAM what includes inside Li batteries. You can not measure batt voltage.
I do not know this model NVRAM (I have not anymore these scopes) but I have fixed lot of 5450x series and this model keep cal data in NVRAM. But It can recovery just driving full selfcal routines after first defaulting cal data.
Only what I know about 54600 serie is that it have also NVRAM and typical battery life is (depending many factors) 10 - 20 years. It is HP designed lifetime limit for this product.
If it tell in selfcheck nvram ok, it still do not know battery status.
But first, reset to decfalut (factory cal) and run full selfcal routines.