I was recently given a TI-55 calculator. One of its interesting features was a dc-to-dc converter module in the battery pack that converted the 2.4V from the two NiCD cells to 9V to run the calculator. The previous owner had replaced the battery pack with an ordinary 9V battery.
I can't recall ever seeing an HP calculator with a dc-to-dc converter in the battery pack. HP's battery packs always appeared to be simple assemblies of NiCD cells. The dc-to-dc converter, if any, was located on the main printed circuit board.
Not having opened many of the calculators that I've owned over the years, I was curious about which HP models ran directly off the batteries and which ones needed a dc-to-dc converter for a voltage boost and possibly voltage regulation.
I've noticed that HP's graphing calculators do not seem to be afflicted with a problem that I've noticed in TI's graphing calculators, an LCD display that fades as the batteries are losing their charge, requiring the LCD contrast to be adjusted. Which suggests that HP uses a voltage booster/regulator and TI doesn't.