Designing a Redundant Electrical System

I will be running the SDS EFI system, which uses a Dual Electronic Control Unit (ECU) to control the coils and spark pugs, as well as the fuel injection. This means that i need electrical power at all times, or the engine will stop! 

There is a never ending debate on electrical system design - is 2 batterys or 2 alternators better? 

In my case, i have decided that BOTH is better! So how do i design an electrical system to make the best use of all these power sources - there's no point having power sources if they can't get to the electrical boxes that relay on them! 

SDS gives a loose design diagram:

The First Conundrum - Single Source Injector Power

The SDS has a few components - the brains behind this is a dual ECU module containing two redundant ECU computers, each with separate power and grounds. There are 2 coil packs, and 2 sets of spark plugs - each with seperate power and grounds, and each run from a seperate ECU.  

Since everything is seemingly redundant, you would think that a split bus arrangement would make sense. One battery and one power source (alternator) would drive a bus which powers half of each component of the SDS system... but...

Each cylinder only has one injector. This injector has a single power and a single ground. It gets its signals on when to inject fuel from the primary ECU in normal operations. If this ECU should fail, there is a switch and a couple of relays which power up and switch control of the injectors from the primary ECU to the secondary ECU. 

So this makes it difficult to achieve a truly split bus redundant system. 

As such i have elected to use a single "Engine Bus" which will power the SDS system in it's entirely. 

The "Engine Bus" will physically be a Bussman RTA fuse block and will be located on the angled 'Power Module' below the panel in the centre tunnel area. The 20 position fuse block was 'bussed' in a 12 and an 8 configuration - i used some brass bar and rivets to make this a single bus unit, in the same way i did for the main bus. 

Aeroelectric Z-14 - the classic reduntant electrical system

My main electrical system will be based on the Aeroelectric Z-14 system, which is a dual battery / dual alternator split bus system. This is essentially 2 seperate busses, connected with a manual 'cross-tie' contactor. Each of these busses is fed by a battery passing through a contactor to the bus, as well as an alternator direct to the bus.

My system design will be similar to this with one main difference - some direct feeds to the 'Engine Bus' to support the SDS EFI system. 

The design will consist of 2 identical batteries (likely EarthX ETX680 or ETX900). 

Electrical System Diagram

The main electrical system diagram is can be viewed in PDF form, or as an image below. Note that this is the current diagram as at Feb 2026 (some minor changes are likely as this gets built). 

The main electrical system switches and their labelling.

Main Bus

The main bus will be another (larger) Bussman RTA fuse block and be located on the left hand subpanel, so you can access the fuses with the canopy open. It will be fed from the right hand battery (green line) through the 'R BATT' switch / R Batt Contactor. The main 6awg feed is protected by a 100amp Midi fuse. 

The bus will also recieve power from the Main Alternator (blue line), through another 100amp Midi fuse (which protects the b-lead wire from being fed by the battery in the event of a short). This allows the alternator to feed the right battery for recharging. 

The starter will get it's power from the 'R-Bat' contactor, and the start switch will be fed from the Main Bus. The primary feeds for the avionics will be from the Essential Bus, so this ensures that the bus volts are not drawn down during the start. This means to start, the Main Bus needs to be powered.

The main bus will support all the avionics seondary power feeds, as well as all of the non-essential equipment, such as the smoke system, USB port, strobe / position / taxi lights. 

Essential Bus

The 'Essential Bus' will consist of traditional circuit breakers and copper bus bars. These will be located on the angled power module plate, and sit underneath the engine bus fuse block. 

The 'Essential Bus' is fed from the left hand battery, through the 'L BATT' switch and L Batt Contactor (green line). The 10AWG feed is protected by a 14AWG fusible link. 

The 'Essential Bus' will have a small number of items, and is limited to those which represent the minimum equipment required for IFR flight. Other items which may need quick interuption of power in a runaway scenerio such as Trim and Flaps are included. 

Cross-Tie Contactor

The cross-tie contactor will join the outputs of the 2 main battery contactors, allowing for the 'cross-feed' of power between the Main Bus and the Essential Bus. An example of it's use would be the failure of the Monkworz MZ30 (which is located on the Essential bus side of the system - see more below). 

If this failed, then the Essential Bus would only be recieving power from the L Battery (and the L Battery would have a limited life). 

By closing (activating) the Cross-Tie contactor (blue line), power from the Main Alternator can feed the Essential Bus (and charge the L Batt). The same would be true of a Main Alternator failure, however the MZ30 can output a maximum of 30 amps - and the loss of the Main Bus is not that big of a deal. 

Supporting the Engine Bus 

Since the engine bus is so critical to the continued running of the engine, i wanted to keep the feed to this bus as simple as possible. Since i will have 2 batteries, i decided to have a dedicated feed off each off the two batteries, through a modern relay (rather than a contactor), through a diode, direct to the bus. 

The relays are Gigavac P195BDA relays rated at 80amps contunuous at 25c, or 60mps continuous at 65c. They will do up to 200 amps for a few seconds. Well over-rated for the expected 20-30amp draw on the engine bus. 

The diodes are necessary, as each battery is feeding a common source ('Engine Bus'). If no diodes were present, then the split bus would not actually be split - it would share a common connection at the engine bus. A fault on one side of the system, would affect both batteries. 

The diodes selected are STPS24045 in an ISO package, which will get its heatsink from the mounting plate. These are rated 120 amps with a 45 volt reverse voltage, and rated at 150c. 

You can see the diodes mounted above the engine bus below:

Each Battery will feed the 'Engine Bus' through a decicated switch located on the left hand side of the panel, along with the other Engine related controls. 

The 'Engine Bus' is fed from the "ENG BUS PWR" switches.

Each battery can connect to the 'Engine Bus' through the ENG PRI Master and the ENG SEC Master relays, using then 'ENG BUS PWR' switches. These feeds are 10AWG using a 14 AWG Fusible Link. The diodes on each feed line isolate the left and right sides of the system. 

Monkworkz MZ30 Generator

In most electrical system designs, the alternator (or generator) will directly feed into the system on the bus side of the contactor. This is the case for the 'Main bus' side - the alternator connects on the bus side of the R Batt contactor (green line). This means that in a Smoke in the Cockpit scenerio, both Batt Masters AND the Alternator switch need to be turned off, to depower the bus. 

In my case, for a smoke in the cockpit event, all three switches in between the switch guards will kill power to both the Main and Essential busses, in one nice easy motion. Like turning off the split master in a Cessna - KISS.

Turning off the three switches above, will remove power from both the Main adn Essential Busses.

If we place the Monkworkz in the traditional location, i.e. in between the master contactor and the Essential Bus, this causes a couple of issues:

1. We would need an additional switch on the panel to turn the Monkworkz off in smoke in the cockpit scenerio. 
2. The Engine Bus would be left with only the L and R Batteries for power - limiting endurance to battery capacity before the engine stopped! 

I really didn't like this idea! So the solution was to use the same concept of the generator being on the bus side of the master contactor, however instead of placing beweeen the 'L Batt' contactor and the Essential Bus, i placed the feed beween the 'Eng Pri Master' and the Engine Bus (green line). 

This means that once the engine is running, the Monkworkz will be producing power and will be feeding this power to the 'Engine Bus'. It should output 15amps down to 800-1000rpm (and 30 amps above 1800rpm). Every battery contactor in the aircraft could fail open, or be turned off (by a silly pilot), but the MZ30 would continue to power the 'Engine Bus' (blue line), with nothing in between except a couple of fusible links and the diode. 

In normal operations, the MZ30 will feed the 'Engine Bus' direcly (blue line), and also feed the L Batt, and therefore the 'Essential Bus', through the 'ENG Pri Master' and the 'L Batt Master' (pink line). 

Note: at the moment, the MZ30 does not have it's own enable switch. It will be enabled anytime the engine is running. The 'enable' circuit will run down to the 'Essential Bus' panel and be connected through a 2A circuit breaker. This allows me to disable the MZ30 for maintenance or trouble shooting reasons by opening the CB, opening the enable circuit. If i run out of CB's, i will install a switch on that panel, or soemwhere else! 

Smoke in the Cockpit

As mentioned above, with smoke present in the cockpit i can turn off the 3 main switches and remove power from the 'Main Bus' and the 'Essential Bus'. This completely removes power from the whole cockpit, with the exception of the G5 Standby instrument (powered off the Engine Bus), and the SDS EFI cockpit controller (also powered from the 'Engine Bus').

This now shows the reasons for the design choice of using Circuit Breakers on the 'Essential Bus'. With the master off, i can pull all the 'Essential Bus' breakers, then put the 'L BATT' Master back on, to provide power to the 'Essential Bus'. If no smoke appears, i can begin to re-power my IFR critical items one by one, and hopefully the smoke doesn't come back! 

In this scenerio, the MZ30 would be powering the whole aeroplane, so load-shedding might be a consideration. 

An overiew of Power Sources during Normal Ops

For normal operations, the 'Right Side' of the system will be powered by the Main Alternator (green line). I am as yet undecided on the alternaor, but it will either be the B&C LX60, or the B&C Silverflite SF601 (60 amps). This alternator would power the 'Main Bus', and send volts back to the R Battery for charging. From there, it would pass power through the 'ENG Sec Master' to the Engine Bus. 

The 'Left Side' of the system will be powered by the Monkworkz MZ30 set in primary mode (blue line). This would send power directly to the 'Engine Bus', then through the 'ENG Pri Master' ('Eng Bus Pwr' L Batt switch) to the L Battery. From there it would power the 'Essential Bus' through the 'L Bat Master' ('L Batt' Switch).

Diodes and Voltage Drop 

The STPS24045 diodes are published as having a Forward Voltage (Vf) drop of 0.52v. 

The LX60 and SF601 Alternators both have normal voltage setpoints at 14.4 volts. The voltage is sensed at the Main Bus, so we can expect that the feed to the 'Eng Sec Diode' on the 'Right Side' will be 14.4 volts. Taking away the voltage drop, we will see 13.9 volts approx at the 'Engine Bus'. 

The Monkworkz MZ30, running in primary mode, is set to 14.6 volts output, so would see approx 14.1 volts at the 'Engine Bus'. 

This means that the primary provider for the 'Engine Bus' would end up being the Monkworkz MZ30. At this stage i am unsure if this is a problem or not - a brief analysis of the loads shows potentially over 30 amps on the 'Essential Bus' and 'Engine Bus' combined, when the pitot heat is running, both fuel pumps are running and the flaps are moving. 

If I went with the SF601 alternator (around $595 USD Cheaper), this does not have an adjustable voltage output. If this output is 14.4 volts, then i can expect that the MZ30 will do all the heavy lifting on the 'Engine Bus'. 

If i went with the LX60 Alternator, with the LR3D-14 regulator, the the voltage is adjustable from 11v - 16v. I would be able to adjust the voltage upward, so the 'Engine Bus' would see more than 14.1, and the Main Alternator would carry the load on the 'Engine Bus'. 

I am unsure at this stage is the extra $595 is worth it in this case! (TBA!). 

FMEA Analysis

Being far from an expert at all of this, i thought i better approach the analysis of this system design from a somewhat formal standpoint. To do this, i created a spreadsheet where i analysed each component and each wire for various conditions, including a short or a component failure etc. The risk of engine stoppage was calculated. I was also able to analyse switching SOP to create a good run-up testing plan, to make sure component was working as desinged throughout the life of the aircraft - this will inform the eventual 'checklist' procedures.

The FMEA document can be accessed online for those who really have trouble sleeping. This is based on a slightly different system drawing layout, but is functionally the same - I jsut redrew it to match the locations of the physical objects on the firewall. 

The main risk which remained, was the risk of a 'cross contaminated' fault. I.e. there is a short in one side of the aircraft system, and that battery is taken off-line by the BMS. If i open the Cross-Tie without first considering the risk, the short could then be exposed to the other battery. So careful use of the 'Cross-Tie' is recommended. 

Summary 

As at Feb 2026, i have not yet built this system, however i have made a lot of inroads. I have already mounted by master contactors, mounted the Main Bus Fuse Block, mounted the 'Engine Bus' fuse block as well as ordered all my wires from Steinair. 

So if there are any comments from those in the know, especially if you think i have missed anything, please send me a message below.