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And then I did some wiring!

The first two pictures show the 6AWG wires connecting the ground power to the battery.  It takes a circuitous route. From ground power + it goes to the ground power contactor which keeps the ground power isolated from the plane until I switch it on. From here it continues to the positive side of the starter contactor and then to the positive terminal of the battery.  The negative side goes straight to the firewall ground buss and then continues to the battery minus terminal.  This setup will directly charge the battery if the ground power contactor is closed and will provide a minimum 6AWG path all the way for cranking the engine from ground power.

      The first picture below shows some temp wiring added for testing. There is a wire from the third terminal on the ground power receptacle.  This is the sense pin. It is tied to the positive pin on the external plug. Without the plug, there is no contact between the sense pin and the positive pin. This pin will supply the power to turn the contactor on. The circuit is set up to provide failsafe protection from two ground power dangers.

      One, is that the external battery cart may have gotten the polarity wrong while changing batteries for example. There is  diode in line that will protect from this. You can see the diode dangling from the contactor terminal. The second danger is from hooking up a 28v ground power unit to my 14v system.  Most aircraft use 28v so this is a common mistake on the flight line.  I have a component that will sense voltage over 16v and hard short this line to ground. It will have a small in-line fuse that will blow every time this happens, but it is only a fuse, not everything else.

      The other line coming from the contactor goes to ground allowing the contactor to close if the power is clean. It will of course have a switch and fuse itself in the final wiring.  I hooked them up so I could system test the ground power and the battery. The last picture shows the plug in place with power on. With the polarity right, as soon as external power came on, the contactor closed and the unit began recharging the battery! I then reversed polarity of pins on the remote unit and the diode did its job. The contactor would not close. There is one other item that can be operated, the starter contactor. I connected a lead from it to the battery and closed it as well.  More wiring tomorrow.