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Why, on a Cessna 172S, there is a positive voltage reading on the Main bus when only the standby battery switch is on? In the checklist it says

verify M BUS Volts are 1.5volts or less.

I thought that the standby battery was supposed to power only the essential bus.

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    $\begingroup$ Measuring volts doesn't mean you can actually power anything with it (necessary but not sufficient), and it's quite possible to measure a potential difference between two non-powered points. I'm guessing the buses are separated using diodes, and I can't speak to their electrical characteristics off the top of my head, but I'm guessing they're saying anything less than 1.5 means it's not actually shorted and no or minimal current will flow through it. $\endgroup$
    – falstro
    May 19, 2016 at 11:48
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    $\begingroup$ Capacitance can provide small voltages on an otherwise dead bus. Ever unplug something and see it stay on for a few seconds? $\endgroup$
    – Ron Beyer
    May 19, 2016 at 13:05
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    $\begingroup$ @RonBeyer: Likely the noise filter. $\endgroup$
    – mins
    May 21, 2016 at 23:08

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It would appear that there is a standby battery controller PCB. It's important to note that M BUS VOLTS are read off the crossfeed bus as there is no main bus. The label is a bit misleading in my opinion. It's been awhile since I have flown a G1000 C172 but if I remember correctly you should not see 1.5volts on the M BUS VOLTS but zero.

The checklist is verifying the PCB box doesn't have a fault that will allow the standby battery to power more than just the essential bus. Unfortunately, the POH is silent as to why this check is done.

C172 Standby Battery Electrical System

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  • $\begingroup$ Well, if there is voltage where it shouldn't be it means there is a short somewhere and it should be rather obvious a short is bad. $\endgroup$
    – Jan Hudec
    Jun 21, 2016 at 7:28

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