8KCAB What is the mercury switch between voltage regulator and alternator for?

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Hi, slogging through some issues in my mid-90s-but-new-to-me aircraft. Working an electrical problem, my mechanic and I were puzzling over this. Does anyone know the purpose of the mercury switch listed in schematics between the voltage regulator and alternator in Super Decathlons, at least older years? Haven't traced wires, but I think it's the small blue cylinder on firewall next to Christen inverted oil unit. Thanks!
 

Bob Turner

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I think I took mine out when I went to Plane Power. No idea what it was supposed to do. Yeah, it was a blue cylinder.
 

Bartman

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Isn't a mercury switch supposed to make or open contact relative to attitude? Would there be a reason to not charge the battery when inverted?
 

Sully

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Learn something new every day. From POH

The 60 amp alternator provides charging current and has sufficient capacity to operate all electrical equipment wiithout battery drain.
During inverted flight, the charging circuit is disconnected by a mercury switch.
 
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Thanks, interesting. Those two sentences are not in my airplane-specific Airplane Flight Manual, or the Service Manual. But when I dug out the more complete but generic ACA-reprinted "Bellanca Pilot's Operating Manual" issued 1979, purchased from the flight school I did my first Decathlon training from in 90's, there it was. I don't think aircraft that spend a lot more time inverted than Decathlons have these, but guessing and will confirm that later. Maybe a holdover from older battery technology?
 
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To tie this off for posterity, the recent EAA Decathlon webinar had a viewer question about this switch. It was confirmed that it's for old non-sealed batteries that apparently shouldn't be charged upside down, and the switch can be removed when using modern batteries.
 

aftCG

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To tie this off for posterity, the recent EAA Decathlon webinar had a viewer question about this switch. It was confirmed that it's for old non-sealed batteries that apparently shouldn't be charged upside down, and the switch can be removed when using modern batteries.
Uh, "if" you have one of them new fangled modern batteries.
 

Bartman

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To tie this off for posterity, the recent EAA Decathlon webinar had a viewer question about this switch. It was confirmed that it's for old non-sealed batteries that apparently shouldn't be charged upside down, and the switch can be removed when using modern batteries.
Thanks for the follow-up post!