Summary

A 3D printable HP-41 battery replacement.

You can stick in 4 sub AAA NiMH cells or go for the pro version, which powers a HP-41 from either

2 AAA NiMH cells or
1 old Li-Ion mobile phone battery, USB chargeable

http://www.askrprojects.net/hardware/hp41bat/index.html

Instructions

WARNING: In case you decide to use the Li-Ion version:  I intentionally wrote and mean "old Li-Ion phone battery" and nothing else!  These types of Li-Ion batteries come with an integrated short-circuit as well as undervoltage protection!

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I prepared three battery cases:

a standard, two part version
a single part, open top version
a single part, open top version with charger hole

The open top version comes in handy for NiMH operation, whereas the closed, two part version was developed for old phone batteries.

All parts are already in their printing position.
Please notice that the the single part case, as well as the bottom part of the standard, two part version require support structures for an overhanging edge and the two snap-in detents.

For a (little more complete) description, just follow the the link above.

HP-41 Battery Replacement