Roee Kalinsky's RV-7A Project

2010.07.27: (0.0) I'm planning to install an external power (i.e. ground power) receptacle that can be used to:
1. charge the main battery
2. provide power to onboard systems for prolonged ground ops without discharging the battery
3. start the engine from an external power source (e.g. weak battery or cold temp ops)

The external power receptacle I'm installing is an AN2552-3A (a.k.a MS3506-1), which is the standard 3-pin receptacle used on Cessna, Beech, Mooney, Grumman, etc. (practically all GA aircraft except for Piper).  It receives an AN2551 power plug.  It will be installed somewhere inside the cowling, preferably on the left side, and be accessible through a hatch that is hinged on one side and latched on the other (like the oil door).

2011.01.01: (0.0) Working out the details of the installation:

Electrical System Architecture

The external power will first be routed through a relay which will be activated only when a good voltage is detected on the short contact of the connector, and possiby on the main contacts as well.  This is to protect against accidental application of the wrong voltage (i.e. 28V) or reverse voltage.

I will use a Cole Hersee relay p/n 24115, same as Van's standard master relay.  Note that this relay is a 3-terminal device, where the coil shares one contact (labeled "BAT" on the case) with the switched circuit.  In my application, the shared "BAT" terminal will actually connect to the positive terminal of the external power connector, whereas the other terminal of the switched circuit will connect to the battery's positive terminal.  So in order to close the circuit, positive voltage must be applied to the external power connector, and the 3rd terminal of the relay must connect to ground, which will only happen when my supervisor electronics are satisfied with the incoming voltage.

Note that from the relay, there are two major possibilities for where to route the external power:
1. to the battery (my choice, Cirrus's choice...)
2. to the main bus, i.e. downstream of the master relay (Cessna, other old school spam cans...)
Both can work, obviously, but there are some trade-offs.  The biggest advantage I can see for option 1 is the ability to charge the battery or leave it on a trickle charger with the master relay off.  The biggest advantage I can see for option 2 is that the starting current from an external source only has to flow through two relays, whereas with option 1 it flows through two relays.

Physical Architecture

The external power relay will be mounted on the forward side of the firewall below and to the right of the master relay.  In this geometry, contacts of the external power and master relays can be connected to each other conveniently and efficiently with copper bar, just like the connection between the master and starter relays.

The actual external power receptacle will be mounted on the forward side of the firewall, on the right side, a few inches off the bottom.  This position makes for a very short and direct wire run from the receptacle to the relay.

Note that my original thinking (as mentioned in the 2010.07.27 entry) was to mount the receptacle on the left side of the aircraft.  My reasoning was that twofold:
1. From the left seat, the pilot could then see the GPU or battery cart and visually ensure that it is cleared away before taxiing after an external power start.
2. Walking around the ramp, it looks like most spam can have it on the left side.  I'm not aware of a "rule", but this appears to be a de facto standard (likely for reason number 1).
But in this case I decided to trade that off for the sake of keeping the firewall neat and uncluttered and keeping high-current wiring runs short.