Hoping that some techie stuff here can be of value, I have perused Cornell
Dubilier's Aluminum Electrolytic Capacitor Application Guide (i.e., the
computer grade items). Some tidbits:
Use any voltage rating you want, but at least add expected surges, spikes,
and transients to the working voltage, and further derate it for tolerance
and degradation in service as per usual (thus my original balking at the
suggestion of 16v cap in a 14v system).
The life of these caps is about 1,000 hours at max ambient of 85C. For every
10C decrease in core temperature, life can be doubled (they suffer from heat
evaporation of the electrolyte) . This suggests that the engine room is not
the place to install it, noting that the cap itself heats up in service
depending upon ripple current. I have read on the Kitfox mail list where
builders blamed failures of Rotax regulators on too much ambient heat (as
trigear builder, with oodles of room in tunnel, I'll probably put cap and reg
there). Also, Cornell Dubilier says that heat is most effectively conducted
out the bottom of the cap (and top too, saying that direct connection to bus
bars is best), rather than sides -- i.e., mounting considerations. Mount
vertically, top up, for best venting.
The guide says also that infant mortalities are possible, but minimized if
the cap is given an extended burn-in. They don't say how, but elsewhere they
say DC voltage for 30 minutes at rated voltage, through a 1k resistor, cures
DC current leakage caused by sitting on the shelf too long.
Regarding capacitance rating in excess of 22,000uf, and filtering of
alternator whine in general, it's been my experience that larger caps have
little effect, especially at higher freqs. And Mr. Nuckolls states that the
battery alone accomplishes significant filtering. Also, in the audio panel I
designed and installed in the my other plane, a faint but annoying alternator
whine in the headphones was eliminated only with a small toroid-core choke in
its power supply, not any size/type cap I tried. Auto stores sell a similar
items to eliminate whine in automotive applications.
Sooo. I guess, consensus now might be that a new, computer-grade 22,000/25
should work fine, and keep it cool...no?
Regards,
Fred Fillinger, A063
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