Below is my main power back bone that is based on one of Bob Nucholl’s “Z” diagrams. When looking at this note that:

  • The main alternator will be completely isolated if an overvoltage condition is detected. This is accomplished by having a Perhelion Design LOVP module control a solenoid which in turn controls both the field and B lead of the main alternator.
  • There is a dedicated engine bus which powers only engine related devices. It can be powered via the main bus and/or directly from the main battery. The Schottkey diode that connects the main bus to the engine buss prevents power flow from the engine bus to the main bus. This preserves the battery power for the engine in the event of a main alternator / backup alternator failure.

Battery / Alternator Considerations

I installed a 60 amp Denso 101211-2471 alternator. The alternator had the internal voltage regulator replaced with an OEM regulator which I consider to be more reliable than the original Chinese regulator. I chose a Denso alternator over an “aviation alternator” for a couple of reasons:

  • It is several hundred dollars cheaper
  • It if fails it can be replaced within hours by visiting an automotive supply house, aviation alternators are not as readily and easily available

My single battery is an Odyssey PC-925 dry cell sealed battery. I chose this battery over a lithium/ion (LiFePO4) battery as:

  • It is cheaper than the LiFePO4 alternative
  • If it fails, it will still buffer the alternator while many LiFePO4 batteries will go offline if there is a problem
  • If it fails, replacements are easily and readily available. This may not be the case for LiFePO4 batteries
  • If a LiFePO4 has an albeit rare but serious failure, it may experience a thermal runaway resulting a fire with noxious fumes. Located over / near the elevator bell crank, such a fire could be catastrophic
  • The chief advantage of a LiFePO4 battery is its light weight. However, removing 20+ lbs from the tail cone would mean that I would require ballast in the baggage area to compensate for the CofG impact. As I use a light weight MT prop, I don’t normally need ballast to maintain CofG.
  • Note: Builders using a heavier metal prop would likely need more ballast to maintain CoG when lightly loaded.

I do use a B&C 35-60 amp BC462-3H backup alternator. This alternator is mounted on the starboard magneto pad. The port magneto pad is blanked off.

Guide to the Power Backbone

The power backbone uses two buses – an Avionics Bus and an Engine Bus. The buses are normally electrically connected. However, in the event of an emergency, the avionics bus can be taken offline and the battery power reserved for the engine bus. The only time this would be required would be if both the primary and backup alternators failed.

In normal operations, both buses are electrically connected.

The following describes the logic behind the power backbone shown below.

  • The avionics bus is only powered when the battery contactor is closed. The avionics bus will also power the engine bus as the two are connected using Schotkey diodes. The diodes prevent power flowing from the engine bus to the avionics bus.
  • The main alternator is connected to the avionics bus. With main contactor closed, it charges the battery as well as the engine bus (through the Schotkey diodes).
  • There is a second connection from the battery to the engine bus via a separate contactor. The availability of this connector allows the avionics bus to be powered off while still maintain power to engine bus.
  • A Perihelion Designs Linear Over Voltage Protection (LOVP) module is used to monitor the avionics bus for an over voltage condition. In normal operations, this module powers the main alternator field. It also powers a contactor used to connect the main alternator B lead to the avionics bus. In the even of an over voltage situation, this module will remove power to both the alternator field and from the contactor. The result will be the main alternator be completely isolated from the power backbone.
  • The standby alternator (BC462-3H) which is connected to the engine buss, is controlled by a B&C SB1B-14 standby alternator controller. This controller will energize the standby alternator whenever the engine bus voltage drops below 13.0 volts. In normal operations, if the main alternator fails, the standby alternator will automatically come online. Although it has sufficient capacity to meet all ships power requirements, extended single alternator operation is not recommended.