I'm trying to configure (and wire up) my primary electrical power system.

I'm used to the ammeters in Cessnas, which tell you how much current the battery is providing / using.  A positive number means the battery is charging, and a negative number means the battery is discharging. 

Like most of you, I have a VMS instrumentation system, although I'm using the EPI 800 displays.  There is a Hall effect current sensor which I have mounted to my firewall (the cold side).  The VMS installation instructions tell me to take the alternator output wire and run it thru the sensor, and (presumably) then connect it to the main power bus.  The battery would be connected to the main power bus without passing thru the current sensor.  This would, I presume, measure how much current is being provided by the alternator.  (I don't know how you get a negative number in this case...)  It would tell us nothing about what the battery is doing.

How would I use this device to determine whether the battery is charging or discharging?  To accomplish this goal, the current sensor would have to be on the wire from the battery (actually the downstream side of the battery contactor) to the main bus, and the alternator output wire would be connected to the main bus without passing through the current sensor.

Sound good so far?

Okay, now I look at the EPI 800 gauge that clearly shows positive numbers (up to 25 amps) to be "in the green", with yellow bands for less than zero and greater than 25 amps.  That would suggest to me that this meter is intended to be in the line from the battery to the main bus.

Now let's talk about sensor polarity.  The manual tells me to run the wire from the alternator thru the back side of the sensor (the PC board) and out the front.  So presumably positive is when current is flowing from the PC board side to the sensor side.  If I put this in the line from the battery, I want a positive number to indicate battery charging, don't I?  That would mean the wire should go thru the sensor in the other direction.

Obviously I'm confused.  Can anyone help out?

- Rob Wolf