N104CD's Aux Battery Installation

I often get questions about why an Aux battery would be nice to have an how I did mine, so I'll take this space to document what I did with my installation.

I have a Main Bus, and a Critical Systems Bus (E-Bus),  and a very limited Aux Battery Bus.  The architecture can be seen by looking at my Power Diagram.
It can be a little confusing to read what I ended up with, as it doesn't directly mimic all of the popular Z-Diagrams, but instead pulls in many aspects of them, and adds things I wanted.  I have a bank of 4 small 6v 12AH batteries in a series-parallel arrangement to give 12V and 24AH in a thin but long pack.  I designed it this way to fit in some commonly "wasted" space in the RV-10....under the flap torque tube cover.  The batteries are Werker WKA6-12F (I'd recommend WKA6-12F2 instead because they're a .250" tab) from the local Batteries Plus store.  They're 5.94" x 1.97" x 3.74" and come in at 4.6lbs each.  You could do a bank of 2 or 2 banks of 4, depending on your load wishes.  Not only does this fill some wasted space, but it keeps the Aux Batteries mounted very close to CG, and doesn't alter the CG range as much as many other mounting options. If there's a drawback, it's that you have to wire the battery pack since it isn't just one single battery, but 4 of them.  This can also be a big benefit, because the 4 batteries can provide additional Ah capacity that might have required a large battery to be mounted somewhere with more space, like the tail...where it would affect CG greatly.

Here are some links to photos of the batteries.  Follow the links for more photos:


Main Battery Box
Main Battery Box Installed Photos
(see bottom of page)

Initial Aux Battery Locating
Aux Batteries Installed Photos
See closeup of batteries
Click image for closeup

Why would you want to use an Aux Battery?
Many people design systems with redundant alternators to provide power for systems should their main alternator fail.  Some people use single alternators with 2 batteries, some use dual alternators with dual batteries, and some use dual alternators with single batteries...oh yeah, and some people even use single alternators with single batteries.  These things are modular, in that you should be able to design the system to meet your requirements.  There may be a time when I'll add a standby alternator, but right now I'm holding out until Plane-Power comes out with one, which would be much cheaper than the common B&C offering.  For now, I find that I can get an hour's use or more out of my Aux Batteries, if my alternator fails....and even more run-time if you include the power left in the main battery.  If my main battery fries, I still have an Aux Battery to absorb the charge and keep the power smooth, but I can shut off my Master switch and hopefully let that Main battery be while I get on the ground. Either way, I feel it's very nice to have some power redundancy since I do have a very electrically dependent airplane.  I also utilize Electronic Ignition on one side, so having battery power is required to keep that system alive, although I could limp home on a mag-only if I needed to.
When sitting on the ground preparing for an IFR flight, you'll be doing things like contacting ATC for a clearance, entering multiple waypoints/legs/airways into your FMS/GPS, getting the local ATIS/AWOS, and in my case, getting your Datalink Nexrad Weather picture of the current conditions.  All of these things require battery power.  Sure, you can use the main battery, but as many will find, you just don't want to sit there for 15 or 20 minutes (or more!) burning your one and only battery for those uses.  Having an Aux battery system allows you to use the power off that battery as needed for these tasks, without even turning your master switch on. (You use the Aux Battery Master)
When you crank the engine to start the plane, your starter draws so much current that the overall system voltage can draw down to 8V or less.  At some point, your EFIS and somes systems with specs of 10-32Vdc and such, will choose to glitch or reboot from the power drop.  To protect them, some people choose to power those systems off during engine start. Additionally, if you use Electronic Ignition, the power drop can cause the system to misfire and cause engine kickback, which can damage your starter. In fact, some starters are DESIGNED to break from the kickback to protect the engine, and you'll be left sitting there with no way to start your plane. Some E.I. manufacturers recommend using an Aux battery to provide power to their system, even if just for emergency and starting use, to provide a stable power source during cranking. I have but one Battery Bus type item....my Lightspeed Ignition. It's set up so that I can hit one switch and draw E.I. power direct from the Aux Batteries, yet turn EVERYTHING else off.  This would be the absolute minimum power draw if I needed to keep the plane in the air without avionics.  I use this switch mode for starting the engine.  If I switch the switch OFF, it feeds the Lightspeed it's power from the Main bus.  You could just switch it to power OFF, but it gives you more options to be able to feed it from both places.

Regarding the EFIS specifically, some of them take a couple minutes to boot up (less time while in-flight to keep the system-out time down to a minumum if it knows you're flying), and having an Aux Battery can allow you to power your EFIS on pre-start, and then LEAVE it on during the engine start.  This can be a great convenience item.  The more complex the EFIS, the longer the boot up process. I usually fire up the EFIS and then start my walk-around, so when I get back to the cockpit it's up and ready to go.  I have a source select on my WSI Wx receiver so that I can choose to power it from either the Avionics Bus, or the E-Bus.  That way I'm not wasting battery if I have an emergency system, but if I want to have the convenience of Wx download pre-start, I can do that.  Another thing about the EFIS...there are some systems with critical periods during the bootup process where it's doing things that could leave you with corrupted software if the EFIS were suddenly shut off.  Having an Aux battery to keep things stable can be a great help in preventing these kinds of issues.

One other starting-related item....your Engine Monitor.  Many EFIS systems display the engine parameters too. If you aren't able to keep the system powered on during cranking, you'll never be able to see your engine data such as Oil Pressure and Fuel Pressure during engine start. Having an Aux battery can give you the ability to run that system during engine start.
I find that there is lots of time spent playing in the hangar, loading software updates, NavData, and even sitting in the cockpit making airplane noises. Again, during these times, you can be saving your main battery by utilizing your Aux battery for the power.

What goes on a "Critical Systems Bus"?
So now that you see the benefits of an Aux Battery system, what would you put on it? There are some very good schools of thought out there that you should only put the absolutely necessary, minimum items on an E-Bus, because by definition it's used for the best "ENDURANCE".  That's why I kind of avoid the term E-Bus. Yeah, I'm shooting for endurance, but I also don't want to be limited in function unless I HAVE to be. Fact is, I fly a 170kt aircraft, and I can absolutely find an airport within 30 minutes of time if I lose my alternator.  I can easily be on the ground inside of an hour, IF I have my quality instruments available. Without them, an IFR letdown could take a bit more work and concentration and planning.  So I choose to think of it as a "Critical Systems Bus", and load it up a little heavier with items that could be very handy in such a situation...especially since a combined Main and Aux battery in my plane will power me well over an hour and probably close to 2 hours or more, if the Alternator dies.

On my Critical Systems Bus, I have my PFD, my MFD, my AHRS, my Autopilot, My SL-30 (primary Nav/Com), my EIS, my ADI, my Audio Panel, and the ability to source select to get my WSI Wx if I'm not in a true emergency. No lighting is on that bus, because I have very handy red/white flashlights in highly accessible areas to the pilot, and I have independent alkaline battery powered click-lights in the rear that could be used in a pinch too.

Thinking about these items a bit:
The PFD could be your most important instrument if you're in the clouds. You get attitude reference, Navigation Info, the ability to switch it to an MFD and Engine monitor format, and just wayyyy too many things to throw it away for the 3A draw max that it has. And, it's on a pullable breaker so if you're really in that bad of a spot, just pull the breaker.
Considering you can access MFD functions on most (I have Cheltons) PFD's, this one is optional, but you'd have to use good judgement if you had an emergency. Depending on the situation, you may very much want the navigation and approach info a MFD can provide. I left it on the Critical Bus because I can always yank the breaker as I choose.
This is a no-brainer if you have the PFD/MFD there.  They're useless without it. It's also on a pullable breaker, so I can ditch it as necessary.
Again, if you're REALLY in a bad spot, you need all the help you can get. If I know I'll be landing soon, I may find that AP very helpful in getting down safely. It has it's own panel mounted power switch, AND a pullable breaker, if I don't think I need it, or know I can fly the approach without too much added stress.
Pullable breaker, but hey, you want to communicate, don't you? It switches off really easily with that little round knob too.
Would be nice to analyze fuel remaining, huh?  Well, leave the EIS on the bus. It's on a pullable breaker if you're really about to lose all power, and having it alive will help keep your EFIS from barking at you regarding all of your failed engine parameters if it were off.
Now here's a true minimum piece of equipment. If all else fails, I can shut everything else off and use the ADI. THAT is the worst case minimum power situation I want to be in if I'm IFR, assuming that I'm also powering my Electronic Ignition....which is not on the bus, but is switchable to run on the Aux battery bus.
Not the most necessary item if you're in need of endurance, but considering the help a good co-pilot can be, I'd rather get down quicker while communicating with a co-pilot than have to yell without a headset to continue a flight that will only take MORE time due to the complexity and lack of good communication. Pullable breaker, so it's there.
Convenience item only, so if I need to run on minimum power, this gets turned off immediately. It's just on the bus in case I want to get Wx before flight.

So, looking at the above, with the exception of the WSI, what's the common thread?  PULLALE BREAKER! Additionally, lots of that stuff isn't very high power drawing anyway. It gives you the ability to shed lots of other load quickly by flipping on the Critical Bus Alternate Feed switch, and turning off the master, but doesn't leave you handicapped at that instant...so you can keep your head on straight and have what you need to make the quickest landing you can. I like the E-Bus theory, but I'm not willing to toss out great safety tools prematurely.

So the Aux battery system in my plane is compact enough, well placed for CG and space concerns, gives plenty of power in an emergency, and can prove highly useful for many reasons. What was the hardest part of designing the system? Well, it went very well, but not perfectly. If you study the Power Diagram, (it's as accurate as I could keep it), you'll see the general layout. The odd feature you don't see often is the 2 Diodes. Many people use a single E-Bus Diode for feeding their bus from the main for charging, to prevent that bus from back-feeding the main if your main bus is off. I have such a diode in my system. I also added a 2nd diode. I did use an Avionics Master, which I really like for convenience of being able to shut everything off fully and easily, and prevent mistakes that would cause me to drain batteries. Not everyone likes an Avionics Master though, but with my 2 diodes, I find no big downside. The 2nd diode in the system basically allows the Critical Systems Bus items to stay live while the switch is turned from the active to inactive state. To say it differently, without that diode there to constantly power the Critical Systems Bus from the Avionics Bus, I'd end up rebooting my equipment during the switch change between running on the E-Bus during engine cranking, to switching back to being fed from the Avionics Bus for running. The nice thing about the arrangement though is that my critical systems can stay up all of the time, as long as the E-Bus Alternate Feed switch is on, and it doesn't matter what happens during engine cranking. Once the engine is running, I can shut the Alternate Feed switch off and let it be fed direct from the Avionics Bus.

Pay close attention to the diodes too.  Be aware that diodes will cause a drop in supply voltage between .3 volts and .7 volts depending on the diode used. I had one of the regular E-Bus diodes in the system already, but for the other I went with one from Perihelion Design that is a low-dropping schottky diode. In order for your Aux Battery to get a full charge, you want to keep that drop as low as possible. I'd use 2 schottky's if I did it all over again. They're harder to mount but be creative and they can work great. The system works absolutely fantastically the way it is laid out, which took a lot of thinking.

Here's my start procedure, for the interested, just to see the switch dance isn't too bad:
  1. Aux Battery Master On
  2. E-Bus Alternate Feed On - (CRITICAL AVIONICS and EIS now Alive)
  3. Master On
  4. Electronic Ignition Feed to AUX - (LIGHTSPEED IGNITION now fed from aux batt to prevent kickback)
  5. Fuel Boost On and Off as needed
  6. START ENGINE
  7. Avionics Master ON
  8. Electronic Ignition Feed to MAIN
  9. E-Bus Alternate Feed to OFF *you may leave this on if you wish
It looks like a long list, but think through it and sit in the cockpit and flip switches while you read the checklist and you'll see that you're just basically turning a couple things on before cranking, and off after the engine is running and Avionics Master is on. It takes a lot of looking at someone elses diagram before it sinks in well, but studying it well should reveal how it works with switches in certain positions. Hope this answers questions. I haven't found anything but good in the design for the most part, but that's my personal opinion and you may have vastly different needs. Just make sure to think about what you want out of your system well before you lay out the power diagram. You don't know ho many people come and ask what I did, only AFTER they find they are not satisfied with how their intial layout is working in practice.