Why is there a positive charge without engine running?

Skid

Skid

Pre-takeoff checklist
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Skid
See the pic below:

pr8kaA2.jpg


Why is there a positive charge on the main battery when only the standby battery is on (and engine off)? Follow up question, why is the M bus volts showing .3 if the master is off too? Shouldn't it be 0?

Thanks!
 
That's the Ammeter. It displays the main and standby battery load in amperes.

Sounds like you may need some ground instruction with your CFI So that you understand what the various indications mean and have an idea of what is happening.

I'm going to guess this is N6186T from KTPF w/ Atlas Aviation. I can't recommend their flight school. If you feel you are not getting adequate instruction from them then I would say seek another flight school.
 
I know it's the ammeter, and a negative value indicates the current being drawn. So that makes sense that 3 amps are being drawn from the standby battery since it's powering the items on the essential bus. However, why is there a positive charge on the main battery if the alternator isn't turning?

A positive rate of +5 amps indicates a charge, but where is it coming from?
 
Skid said:
I know it's the ammeter, and a negative value indicates the current being drawn. So that makes sense that 3 amps are being drawn from the standby battery since it's powering the items on the essential bus. However, why is there a positive charge on the main battery if the alternator isn't turning?

A positive rate of +5 amps indicates a charge, but where is it coming from?
Click to expand...

There are diodes between the main and essential buses. A bad diode could crossfeed standby battery power into the main, discharging the stanby battery and charging the main, and the ammeter might not register the extra draw on the standby while doing that.

I retired almost a year ago and don't have those schematics handy anymore. There's an annual diode check that's supposed to check those things for correct function. It's a place I'd start, anyway.
 
All of this makes me wonder if the current shunt is high side or low side.

I believe there are MOSFETs between the main and essentials bus. It's a one way street, main can charge essentials but not vice versa.

Was there a load applied to the battery and then disconnected before you observed the indicated charge?
 
Thanks for the responses. JonH to answer your question, I had the master on initially to check the lights, flaps, avionics, etc, so there was a significant draw on the main battery. Then it was shutoff and only the standby battery on (pic).

http://media.avit.und.edu/c172sElectrical/index.html

This is a nice interactive schematic that shows diodes prior to the ess bus which, with my basic understanding, should prevent any current from going in the opposite direction.
 
Thanks for posting that diagram. I was digging around and coming up empty handed, I'm on 600 kbps hotel wifi.

I was thinking if there was a load on the battery that was disconnected, and the battery voltage was going up (due to the load being disconnected) it's possible the current sensor registered it as a charge. I don't know if that is the case here, just throwing it out there.
 
Did you try turning it off and back on again?
 
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