HP-41CL and 82120D Power Bank.

[Monte Dalrymple]

6.3V from the battery is fine. As Diego noted, the D2 polarity protection diode drops the battery voltage by about 0.3V immediately, and everything from that point is rated at least 6.3V. I wanted to use higher voltage-rated capacitors for C1 and C2, but a higher voltage rating means physically larger, which won't fit. And I didn't want to go the "cut off the corner of the PC board and lay the component sideways" approach used with most of the original HP designs.