Avionics: Electronic ignition backup battery and controller

Since I installed dual SureFly ignition in my airplane, having a fully electronic ignition means being thoughtful and deliberate about how those magnetos get their power, i.e., one needs a backup battery and a means to control how that battery is connected to the ignition and the main bus.  The SureFly's STC points to the guidance in the install instructions on how the backup system should be designed.  Below I outline my approach which closely follows that guidance.

In selecting the backup battery,  I wanted something very light, in front of the firewall and that could hold enough energy for at least an hour of flight under non-ideal conditions (i.e., a tired or somewhat discharged battery). I didn't want to use the TCW backup battery because it's very expensive, large, seems difficult (and costly) to replace when the time comes and doesn't give me flexibility in controlling its behavior.

In my case, I decided that the EarthX ETZ5G battery would be a good fit, at 3.4 Ah and 1.15 pounds.  This battery would ideally provide 3.4 hours of run time for a single SureFly, so factoring in non-ideal conditions, it seems more than adequate.

There were two reasonable locations to place the battery.  One was above the main Odyssey battery on the ride side of the firewall.  The issue with that location was that I'd need to drill into the firewall to mount the EarthX battery.  I didn't want to add more holes, so I decided to mount the battery below my GPS antenna shelf on the left upper firewall by using a bolt on that shelf and two bolts that hold the AHRS tray to the backside of the firewall.  

Below shows the EarthX battery situated in the proposed location.  I anticipate replacing this battery every 5 years.  Having the battery located here means it makes sense to have the backup battery connected to the left SureFly and the main battery connected to the right SureFly.

With my newly developed 3D CAD skills resulting from my larger oil cooler modification, I designed a mounting tray for the battery using OnShape (EarthX didn't offer a tray for this specific battery).  The design includes provisions for riveting it together, holding tabs, mounting plate and mounting holes for a circuit board whose design I'll explain later in this post.  Below is an isometric view and various images of the design.  My design was fabricated by SendCutSend for $57.06 shipped.

 

The design includes a plate that attaches to the outboard hole of the GPS antenna shelf (which is really the upper left outboard hole of the AHRS tray) and the bottom two holes of the AHRS tray (shown on left image below)  SendCutSend doesn't do joggles, so I did that one myself.  Then I used clay to locate and drill the upper bolt hole.  That's the left image below.

After the parts were primed, the battery tray was riveted together then riveted to that plate as shown in the right image below.  Then the battery was mounted as shown on the lower image below.  Note, you can also see the initial revision of a circuit board I designed, the final revision of which I'll explain next.

Note:  In the above image, you can see how I grounded the battery.  I used a 1/16" piece of aluminum to bolt to the negative terminal and then a screw into the battery tray.  Since the tray is riveted to the mounting plate, which itself is bolted to the firewall, this seemed a great approach rather than using a wire.

Having a backup battery requires one to have some means to control how its energy is accessed and replenished and having a means to isolate that battery from the main bus so that it doesn't power the main bus and only powers what it's intended to power.   

My requirements for the backup battery controller circuit board that I would design included:

• Can't provide energy to the main bus.
• Backup battery charging available by the main bus.
• Pilot can control when the backup battery can be charged.
• Very low energy loss through the circuit when being recharged and when being used (i.e., no diodes as a primary means of current control).
  
• The left SureFly should be connected to the battery with the highest voltage at all times.
• If the main bus fails to a short or open, the backup battery can still feed the left SureFly.
• If the backup battery fails to a short or open, the primary battery can still feed the left SureFly.
  
• Provision for providing auxiliary power directly from the backup battery.
  
• Allows for power up of the EFIS and AHRS before engine start.
• Allows for monitoring the backup battery voltage.
  
• All primary circuit paths must be fused.
• Redundancy in case of discrete device failure. 
• No jumper wires on circuit board required to accommodate high current paths.
• All copper traces must be of adequate dimensions for carrying expected current.
  
• LEDs to indicate circuit state (useful only for testing on the ground).
• Moisture/oil impervious.
  

Here is the circuit board I eventually designed to fit all my criteria (this was my third revision of the design and is my final design).  I used KiCAD for the layout and OSH Park for board fabrication. 

 

The left image shows the board after soldering it up, whilst the left shows it during testing.  The green LED shows when an alternator's field is enabled and thus the backup battery can receive current from the main bus for charging.  The red LED shows when the main bus is above about 13.7 V (this threshold ensures that the backup battery is isolated from the main bus when the main bus voltage is too low, thus preventing the backup battery from powering the main bus). 

The schematic is shown below.  Paired PFETs are used for redundancy and load balancing (though the latter isn't needed given the specs of the FETs).  The PFETs are connected such that, when not enabled, their body diodes always allow the left SureFly access to the battery with the highest voltage and prevents the backup battery from powering the main bus and the main bus from recharging the backup battery.  Thus the left SureFly is always energized regardless of the board's state (i.e., no matter the alternator field state or main bus voltage).

When the main bus exceeds about 13.7 V, the upper PFETs turn on, bypassing their lossy body diodes.  When an alternator is selected, the lower PFETs turn on allowing the backup battery to be recharged, but only if the main bus is above 13.7 V (thus the backup battery can never power the main bus).

Here's the board installed on the side of the backup battery tray.  This is shown before I designed a case and before final wire routing was completed.  I'll add the case design to this post when the former is finished.

The board is encased in acrylic to help protect it from moisture/oil.  Two additional wires were pulled through an existing firewall pass-through.  These wires were for the alternator (field) select line and  connection directly to the backup battery (to power the left EFIS and AHRS if needed and to monitor the backup battery voltage).

Since I have two alternators, I needed to create a circuit that allowed a line to go high any time either of the alternators were selected.  This is a very simple circuit with a pair of diodes.  I designed the circuit to plug in to the back of the alternator select switch (an Otto K1 model K1ABAPCABA) behind the panel so no wires need to be cut or spliced.  The back and front of the CAD models of the board are shown below (pictures of the actual board don't show the detail as well).  The side with female connectors plugs into the back of the panel switch.  The ALT OUT line goes to the backup battery circuit board to the corresponding location.

The EFIS and AHRS can be powered by the backup battery if necessary.  This allows me to power up the EFIS and AHRS before engine start and/or run the EFIS in flight if the main battery and two alternators become inoperable, understanding that the additional draw reduces the available flight time for the left SureFly.  It also allows me to monitor the backup battery voltage, giving me insight as to whether or not it's connected to the main bus (i.e., if the voltage is above its nominal 13.2 V, it's connected to the main bus) and if it isn't, what its loaded or unloaded voltage looks like.  

Adding the EFIS and AHRS to the backup battery places an additional 1.2 Ah load on the battery, giving the left SureFly, AHRS and EFIS a runtime of 1.5 hours under ideal conditions.  Again, switching off the EFIS and AHRS gives the SureFly more than 2x the runtime.  Again the backup Horis AI has its own independent backup battery, most probably making the EFIS and AHRS unnecessary in an emergency.

To consolidate things, I wanted to use the existing switch on my panel that selects if the backup Kanardia Horis AI is connected to its own, independent, backup battery.  That battery is enabled when its input line goes to ground.  So a requisite circuit was designed with a PFET and isolating diode.

That circuit board was designed to plug in to the back of the Otto K1 switch (model K1AABPCAD) that enables the backup battery for the Horis AI.  When the backup battery switch is enabled, the Horis AI is connected to its own backup battery as usual, however the left EFIS and AHRS are then also connected to the EarthX backup battery.  However, the left EFIS and AHRS can still be isolated from both the main battery and backup battery through the use of the locking DPDT toggle switch (NKK Switches M2021LL3G01) on the left side of the panel.  This allows the Horis AI to be enabled and the left EFIS and AHRS to be off if required. 

The CAD models of the front and back of that board are shown below.  The board includes provisions for fuses on the power output lines.  A high side PFET design is used for minimal loss when the circuit is enabled.

The left EFIS can then monitor the voltage of the backup battery and I also connected that power line to an analog input on the right EFIS so it too can monitor the voltage.  Thus, visual and aural anunciations are available if the backup battery voltage is below a programmed threshold and the backup battery voltage can be checked prior to engine start.

Screenshot from the right EFIS is shown below (immediately following a Vx climb, hence the OilT and CHTs).  The efficacy of the low R_DS PFETs in my design is apparent as the main bus and the backup battery are at the same voltage when the alternator is enabled and the main bus voltage is above 13.7 V.  Again, both EFISs are programmed to provide me a verbal and visible alert when the backup battery voltage indicates that it is disconnected from the main bus.

Finally, the aircraft's simplified high level electrical diagram now looks like the below.