Please see my new project page at github.com/calcpsu/cphp82001
A Li-Po battery replacement design for HP35/HP45/HP55/HP65/HP67 calculators. The original batteries had ~ 500mAh capacity, while modern Ni-MH replacements are typically ~1500mAh.
While a higher capacity is not necessarily feasible, Li-Po has the advantage of far reduced self-discharge in comparison to Ni-MH, which is an advantage for an intermittently-used device.
Ni-MH / Ni-CD batteries are notorious for leaking, which is somewhat traded-off against the notoriety of Li-Pos to self-immolate.