Electrical Stuff    Return to home page

My B&C order.  Now if only I knew what to do with it.

Okay, I admit to being very ignorant about aircraft electrical systems, so don't laugh too much as you look at my electrical plan.  I, like many others, have read Bob Nuckolls' Aeroelectric Connection and I have gained much knowledge from it.

I made some philosophical decisions regarding the electrical system and they differ somewhat from Bob's teachings.  I've incorporated ideas from other builders, too.

Electrical System Design Philosophy

Design an all-electric aircraft.  Flight and navigation instruments are all electric - no vacuum.

Design as Hands-on system.  With electrical system malfunctions, pilot intervention is desired.  I believe that having adequate indications to the pilot is critical and that the pilot must then get involved in configuring (or confirming the correct configuration of) the electrical system for the situation.

Provide adequate power for all systems.  My concept is to provide maximum power and use what I need.  For me, there is no decision between a 40 Amp alternator and a 60 Amp - the 60 Amp wins hands-down.  Weight and cost differences are not significant.  If I don't use the extra amps - well, that's okay.

Provide sufficient power to continue to destination. Design an electrical system to continue providing power in all situations until landing is assured.  The engine will not be electrically dependent.  It will have self-powered P Mags.

Provide cold airplane comm capability.  I want the ability to use the comm radio for obtaining clearance and to use some aircraft lighting without powering up the entire aircraft system.

Incorporate the latest electrical technology.  I intend to use LEDs, HIDs, recombinant gas batteries, solid state devices, EFIS, and anything else that brings the most innovative products to my aircraft.

Use HTOS.  When it makes sense, put system activation switches on the control stick, throttle, or in close proximity to make operation user-friendly.

Use Circuit Breakers.  I understand all of the reasons for using fuses.  I prefer circuit breakers.  I've flown with them since I began and I'm comfortable with them.

The following is my current electrical plan.  I will use one alternator and two batteries.  One battery (PC680) will be aft of the rear baggage (for reasons of CG) and the other will be in the lower front baggage (for reasons of short cable lengths).  The front one is what I will call the Main Battery and the aft one is the Standby Battery.  In Windows, you can print the electric schematic by pointing to the image below and selecting the printer in the top left corner.  Select "landscape" for the printing layout.

I'm using a manual battery switch to connect the main battery to the bus.  I'm also using a manual switch as the crossfeed or bus tie.  They are 200A continuous and 500A intermittent capable switches.  The standby battery will be connected using a contactor.

The Standby Battery will be replaced annually with a fresh battery and the Standby will be moved to the Main position.  In this way, my Standby (emergency) battery is always the freshest.

I have three buses - the Main, Standby, and Avionics.  I wanted an separate Avionics bus so I could power up the avionics without powering up the entire aircraft.  Also, I like the convenience of turning on most of the radios with a single switch.  I've put my primary EFIS on the main bus and the backup EFIS Lite on the Standby (Avionics) bus.  Should either bus fail the other can provide an IFR-capable instrument platform.

The largest threat to bring down the entire system would be for the alternator to fail and the Standby Battery (less than one year old) to short internally and fully drain the main battery.  I haven't tried to calculate the odds of that happening during a rare IMC flight, but I am confident it is not worth worrying about.

Should there be a simultaneous alternator and main battery failure, the main bus could be powered, if desired, through the Crossfeed (or bus tie).

Following the diagram above during a normal power up sequence, I would first close the standby battery switch thus powering the standby and avionics (through a P&B W-31 CB Switch) buses.  Once normal bus operation is confirmed, I would close the crossfeed switch to ensure I could power the main bus.  Then after opening the crossfeed switch I would close the main battery switch to power the main bus.  I'll have a hidden starter enable switch that must be closed for the starter contactor to engage.  Once running, the internally regulated alternator would power all buses via the main bus to the standby bus to the avionics bus.  The Power Schottky diode would allow power to flow from the main bus to the standby, but not the reverse.  I've planned the minimum instruments needed for landing to be on the standby bus and, if needed, additional load shedding on the standby bus can be done.

I'll provide an external power connection for charging both batteries, for powering the aircraft systems without engine power, or for jump starting.  I'm planning at least three hall effect devices and/or shunts to sample current flow at the alternator B lead and each of the batteries.

I ordered a Powerschotty diode and Wig-Wag module from http://www.periheliondesign.com and I found the quality to be excellent.  Here's a shot of the items received.