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.
- If board loses its ground connection, the SureFly will still be powered by whichever battery has the highest voltage.
- Without ground, the backup battery cannot be recharged, however the EFIS would indicate the lower voltage of the backup battery, indicating disconnect from the main bus.
- 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.
- Fuses to protect circuit board and associated external connections.
- 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 right 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. And since the backup Horis AI has its own independent backup battery, that possibly make 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 RDS
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.
