During transition training to a Cirrus, many new owners get to experience their first aircraft with
multiple electrical buses. For most, it’s also likely their first airplane with multiple batteries. One of the
many preflight procedures presented in the POH, and by any Cirrus knowledgeable instructor, is to
switch on the BATT 2 first and see which bus powers up and which ones do not. It may seem like a
pointless routine, but skipping this critical test could be the difference between a successful CAPS
deployment and a failed one.

The Test

The standard electrical power-up procedure for all the SR series starts in the Pilot Operating Handbook
Section 4 with the Preflight in the cabin. Turning on battery master #2 (BATT 2) is the first step right after
checking that all the required documents are in the plane. There are some variations in the different
aircraft models for what happens with just BATT 2 turned on. Basically, you’ll see everything on the
Essential Bus is powered. For most of us, the easiest way to identify this is that the PFD powers up
and/or we hear the avionics cooling fan running, the Essential Bus voltage displays, and we note that the
flap position indicator is not on. This process gives us confidence that the Essential Bus is properly
powered but correctly isolated from the rest of the aircraft electrical system.

BATT2 turned on but the flap position indicator is NOT illuminated because it is not powered by the Essential Bus.

During my Embark training several years ago, the stated purpose of this test was to ensure that battery
#2 was connected to the Essential Bus and that all the isolation diodes between the Essential Bus and
the main electrical buses were working as they should. The idea is that power from the main bus can
passes to the Essential Bus but not the other way around. Skipping this test may hide a serious problem.

BATT2 turned on – PFD alive, backup AI spinning and warning annunciator showing a couple of system warnings

Some DC Charging System Basics

The #2 alternator (ALT 2) operates around 28.5 volts, which does a good job of keeping the 24 volt
battery #2 charged. When monitoring the Essential Bus voltage, if you see anything down near 24 volts,
less than 25 volts or so, you can be certain that the charging system (ALT 2) is not online. If the bus is at
or near 28 volts, then you know the charging system is doing its job. What the ammeter displays is
going to be secondary and mostly irrelevant in part due to common inaccuracies in the amps readings.

To help ensure the bus isolation diodes are doing their job, the Essential Bus charging system is
regulated about ½ volt higher than the Main Bus. This is an important bit of information when
monitoring the electrical system to know what is working and what isn’t. If the Essential Bus voltage is
the same or slightly lower than the Main Bus voltage, it is likely that ALT 2 is not operating.

Electrical current, like water, only flows downhill. Meaning, when an alternator is putting out 28 volts
and connects to a 24 volt battery, the current can only flow into the battery. I say this so all can
understand that when the bus voltage related to a particular battery is showing something above 25 or
26 volts, it is impossible for the battery to be discharging to that bus, regardless of what the ammeter is

Some Cirrus DC System Specifics

Battery #2 is mounted in the tail cone of the plane near the CAPS. It’s two small 12-volt batteries
connected in series to mimic a 24-volt battery which are mounted in a container with an electrical
connector. There are a few different layouts of how it is wired to the CAPS and the BATT 2 relay, but the
basics are all the same, assuming everyone has had the required CAPS ignition upgrade per SB2X-95-17
or SB2X-95-18. The positive output of battery #2 is connected to a 30 amp circuit breaker (CB) that
sends power to the BATT 2 relay and a 10 amp CB that sends power to the CAPS.

Once the BATT 2 relay is energized by turning on the BATT 2 switch on the bolster panel, the battery #2
power is then sent from the relay to a 30 amp CB mounted on the Essential Bus on the CB panel in the
cockpit. Unlike most circuit breakers that send power FROM a bus bar out to the systems in the plane as
labeled on the CB panel, this CB allows power to go TO the Essential Bus. So, when the BATT 2 is turned
on, everything on the Essential Bus should be powered up.

There are isolation diodes mounted between the various buses such that power on the Essential Bus can
not power any of the other buses. However, power from the Main bus(es) will send power to the
Essential Bus via those same isolation diodes. In theory, if there is any electrical power in the airplane,
the Essential Bus will always be powered.


In 2013 Cirrus published service bulletins SB2X-95-17(R) and SB2X-95-18(R) for the modification of all
aircraft to the new style rocket motor at the next CAPS system repack. These new rockets require
electrical power for ignition, so the bulletins include significant wiring changes to supply redundant
power to the CAPS igniter switch. This power is supplied from either battery #2 via the 10-amp circuit
breaker in the tail that I mentioned earlier or from the MCU output that also powers the ship’s clock.

Depending on which style battery #2 housing assembly is in the aircraft, the BATT2 relay may have two
internal diodes or one external diode to help reduce the impact of the reverse current caused by the
collapse of the electromagnetic field of the relay’s coil when it’s deenergized (turned off). These diodes
are a possible source of undetected trouble, as we recently learned at our shop.


The voltage reading displayed to the flight crew for the Essential Bus and BATT 2 is taken from the bus,
not the battery. If the BATT2 relay disengages for some reason, the #2 battery will be isolated from the
Essential Bus, but all the voltages presented to the pilot will appear near normal if the Main Bus is
powered or ALT2 is still online. Battery #2 will not be getting any charge from the alternator in this
situation. There is no warning mechanism in the system design for this failure after the Main Battery is
switched on.

This possibility came to our attention recently during an annual inspection. BATT 2 was turned on for the
preflight test, but nothing happened, nothing powered up. On further investigation we also found that
battery #2 was completely discharged. The owner made no mention of it in his pre-annual discrepancy
report but also noted that he did not routinely go through the normal preflight BATT 2 checks.
We eventually found that the small diode across the BATT 2 relay, intended to protect the system from
the normal collapse of the relay’s coil field, had shorted. When the BATT 2 switch was turned on, the
track on the printed circuit board on the switch panel. After that the BATT 2 relay could not be energized.
Due to the system design, that meant the battery #2 could not connect to the Essential Bus and could not recharge.

What used to be the diode in the wiring bundle at the BATT2 relay in the tail cone. It finally burned in half.

The printed circuit board located in the pilot’s bolster panel with all the switches. You can clearly see the fried foil that used to engage the BATT2 relay

The battery #2 is primarily responsible for energizing the CAPS, but during normal operation there is no
method by which the condition of that battery can be monitored. The only time we can have any idea
that battery #2 is actually connected and operating at proper voltage is when BATT 2 is the only switch
turned on.

As far as we know, this particular failure mode we discovered in this situation is very rare. Of course, we
don’t know how many pilots are skipping the proper start up test procedure. While we don’t have a way
to directly monitor the condition of battery #2 during flight operations, we can at least know that it
initially engages. It’s a very simple check that only takes a few seconds, but is our best method to know
battery #2 is online so it’ll be available when we need it most.