Never need to charge:
You'd need a special display where the black is actually transparency to the solar panel and text is white. See "The Mission's Talent".
You'd need about 1/10th of current power consumption.
You could add motion and vibration charging. After all, mechanical self-winding watches are over 200 years old!
Quote:
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The Geneva Society of Arts, reporting on this watch in 1777, stated that 15 minutes walking was necessary to fully wind the watch.
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The Eneloop or other NiMH cells are much heavier than Lithium for the same capacity (though curiously close to same volumetric capacity) and higher self discharged. Though they don't degrade if stored discharged. OTOH, bad stuff happens if you try to charge a flat Lithium cell, so if it's left too long the internal charger won't charge it at all.