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.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
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.