The original battery tech uses some sort of legendary toxic battery that is no longer permitted due to hazardous chemical regulations.
Alkaline button batteries used to contain mercury, which has been phased out in the last 20-25 years. The original Voyager line used 3 x LR44 (alkaline), but even then you could have replaced them with their less toxic silver-oxide equivalents. Today's LR44 cells are mercury-free.
Later iterations of HP12C use 2 x CR2032, not because of any concerns about toxicity -- the 2032s were equally toxic -- but, I suspect, because the original battery controller was all custom HP silicon in some funky semi-analog process, which couldn't survive when HP, along with the rest of the industry, fully succumbed to cost-cutting disease. Everything possible in the new calculators if off-the-shelf.
The original Voyager line used silicon-on-sapphire chips, which are extremely energy efficient. My 1987 vintage HP-15C is only on its third set of batteries. When they changed to a conventional CMOS process and emulated the original CPU on top of an ARM microcontroller the power drain went up.
I still use my two vintage 15Cs for simple calculations on a regular basis, and change the batteries every decade or so.
To be fair, lack of self-discharge in the battery chemistry has a lot to do with the long battery life; you can actually drain the batteries fairly quickly with long-running calculations.
I've had other HPs (28C, 48SX, 48GX, 33S, 35S, 49G, 50G, Prime) but the 15C is the one I keep returning to, as anything more powerful really should be handled on a computer CAS like Mathics or Matlab-like environment like Octave instead.
All those cells have a zinc anode, which used to be amalgamated with a small amount of mercury to prevent corrosion and leakage[1]. (Mercuric oxide batteries certainly contain even more.)
Alkaline button batteries used to contain mercury, which has been phased out in the last 20-25 years. The original Voyager line used 3 x LR44 (alkaline), but even then you could have replaced them with their less toxic silver-oxide equivalents. Today's LR44 cells are mercury-free.
Later iterations of HP12C use 2 x CR2032, not because of any concerns about toxicity -- the 2032s were equally toxic -- but, I suspect, because the original battery controller was all custom HP silicon in some funky semi-analog process, which couldn't survive when HP, along with the rest of the industry, fully succumbed to cost-cutting disease. Everything possible in the new calculators if off-the-shelf.