> Sounds good, I'll probably use a large capacitor.

I don't know how good a rule-of-thumb this is, but "they"
reckon 1000uF per A, for eg a PSU. However, a PSU has
a fairly stable impedance. A battery's internal impedance
increases as it discharges, IOW the current has to get out
through an ever-increasing resistor. So you'll see terminal
voltage that drops with load

Purpose of the reservoir cap is to hold charge and deliver it
with more of a force than a weak battery could. A low ESR
(low internal resistance) type would be preferable, as noted.
You'll see them specified in SMPS for example for that charge-
delivering reason

A low-leakage type could mean a power switch is not needed,
which is not just a design extravagance. It means that no need to
remember to turn it off because an insignificant amount of power
is used

I've several projects with nW PICs which simply sleep at less
than battery self-discharge. One here is 6 years old and the
capacity of the original 3 AA alkalines is still very good. The
terminal voltage has barely dropped
--=20
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