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03-30-2019, 12:56 PM
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Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 358
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Twin Batteries- Not all Redundancy is Created Equal, part 2
Part one of this thread suggested five desirable traits for evaluating the redundancy of sub-system. Part 2 describes my design for a twin battery system and asks how well it incorporates those five traits.:
1- Symmetry.
2- Simplicity.
3- Familiarity.
4- Fool-resistance.
5- Parallel isolation.
Here is the wiring diagram for my system, which will feature twin EarthX EXT900-VNT batteries mounted in a center console located between the pilot and passenger’s legs in my RV-7A. It should be obvious right away that this design is absolutely bi-laterally symmetrical, so check YES on item one, Symmetry. This system is designed specifically for electron-dependant aircraft that rely upon a full-time operation of an electric fuel pump, and to insure that discharging both batteries after an undiscovered alternator failure is virtually impossible.
NOTE: it has been pointed out(Thanks Mich!) that the schematic below has an error that I will be correcting soon. The small relays powering the diodes should be flip-flopped to have the diode powered from the switched(output)side, and the coil should be powered directly from the main bus. Also, not shown for simplicity, each 25Amp Shottky diode is protected by a 20Amp pullable breaker, which I’ll also add to the next revision. I’m also deleting the shunts and ammeters in favor of simple voltmeters for simplicity and weight savings.

Features: As you can see , in addition to the main bus there is an always-hot, but circuit protected, 25Amp. essential loads bus(ELB), for each battery. The ELB’s are hard connected to each battery through a small shunt that feeds Voltage and Amperage info to the V/A meters. Those meters, the ELB’s, and the five mil-spec On-Off-On switches are arranged vertically in a small sub-panel on the console just above the fuel selector valve and below the throttle, prop and mixture controls and just a few inches away from the EarthX batteries. Note that each battery has its own standard contactor to the main bus, and also a mini-solenoid that will be explained below.
At the top, two panel-mounted switches are depicted. The lower one, a DPDT On-Off-On mil-spec MASTER, mounted horizontally as shown in the pic below, is the most important. Shown here in the off position, it is wired such that if moved to the left, it energizes the contactor for Battery1 via one pole, and ALSO energizes the mini-relay for Battery2, allowing it to be charged via a Schottky diode via the second pole. In this mode, Battery1 is serving as primary, and Battery2 is serving as backup that can only be discharged by active loads on its ELB. Flipping the Master instead to the right reverses the roles of the two batteries. It ABSOLUTELY DOES NOT MATTER which direction you flip the Master switch for a given flight, and SOP will be to alternate randomly from flight to flight for a reason described below. Note that this feature alone meets ALL FIVE of the criteria listed above!
The the other switch, shown below, is unique. It is a guarded, spring-loaded to Off, DPST momentary. If you flip the guard up and manually engage the switch against the spring-load, it energizes BOTH battery contactors momentarily. This serves two purposes. First, it is physically positioned in relation to the start button such that the left thumb can engage the momentary while another finger on the left hand simultaneously pushes the button, allowing the mighty force of both batteries together to spin the starter.

Second, the momentary can be engaged in-flight using the forefinger of the right hand, while the thumb of the right hand flips the master to the opposite “ON” position. This maintains power to all main bus loads during the switching of battery roles in flight. Why would you want to do that? Even at low-amperage charge levels, the Schottky diode charging the secondary battery robs about 1/4 to 1/3 Volt. LiFePo batteries are sensitive to charge voltage, and may not quite fully charge at this slightly reduced voltage, so switching the battery’s roles mid-way through a long flight would allow “topping off” of the battery that served as secondary during the first half of the flight. This is also the reason for routinely alternating the master switch selection from flight to flight.
Take another look at the lower section of the diagram. In addition to two micro-switches that energize the V/A meters, five mil-spec On-Off-On switches, installed horizontally in a vertical row access either ELB. Only the bottom four are true essential loads, the two ignitions and the twos fuel pumps. As with the Master Switch, it ABSOLUTELY DOES NOT MATTER which direction these ELB switches are engaged(though in practice the only load I’ll run full-time on the “backup” battery is one lightspeed ignition. in one direction they tap the battery that is currently serving as primary, and in the other direction they tap the battery serving as backup which, remember, is kept charged through the Schottky diode. The top switch, labeled “Aux. Avionics” is not, strictly speaking, an essential load. Instead, it enables power to be sent directly from either battery to the group of seven CB’s at the bottom of the CB block pictured below:

The Garmin LRU’s fed by these breakers have two power input pins each, P1 and P2. Each device draws power from P1, which is connected to the main bus via the top row of CB’s, but if for any reason P1 power is lost, it looks to P2. In effect, switching Aux. Avionics on does not actually result in any current flow unless the main bus gets shut down. This means that a selection of avionics remains available from battery power if the master has to be shut down, but also allows pre-start use of avionics on battery power without energizing the main bus, handy for flight planning. The three CB’s not included in the lower group are for the GTN750 and the GAD29. Conserving power after an alternator failure can be accomplished by selectively pulling some of the lower avionics CB’s.
This photo shows the other panel mounted switches that feed the CB block. The micro switches in the top row are no-load communicators to the GAD27.

So how does all of this stack up against the redundancy criteria? Symmetrical without a doubt! Simple? I say yes, this wiring diagram is dirt-simple compared to most backup power scenarios I’ve seen. Familiar? Definitely!, there are no emergency procedures, just slight variations on normal, everyday procedures. Fool-proof? Yes, you can literally position every individual switch arbitrarily or flip off the master and the airplane will chug along just fine. SOP will be to operate on one fuel pump, and have one ignition on each ELB, so the one thing the pilot should avoid is flying with both pumps and both ignitions on the battery serving as backup, but even that would be taken care of by current flow through the diode. (Note: not shown: the mini relays are in a relay/fuse box enclosure, and the relay output to the diode is fuse-protected) Parallel Isolation? Big time! - Otis
Edit note-3/31- after further thought and comments from readers, I decided to add circuit protection to the ELB feeds and annotated the schematic accordingly- Otis
Part 3 will cover the twin fuel pump system I’m building.
Here is the link to part 1: http://www.vansairforce.com/communit...d.php?t=170079
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2019 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"
Last edited by Hartstoc : 07-26-2019 at 06:32 PM.
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03-30-2019, 01:06 PM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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Where are the essential avionics; are there duplicates on each bus? If the avionics are on the 'aux avionics' switch....
Oh, edit: Fail the 'milspec' master switch. What happens?
edit2: From the drawing and accompanying text, it looks like start current will be on your main bus (?).
Last edited by rv7charlie : 03-30-2019 at 01:16 PM.
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03-30-2019, 01:37 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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I see each battery is a directly connect to the E buss, with no means of disconnection. Seems kinda dangerous to me, especially with earth x batt's. What happens if you have a dead short somewhere and smoke/fire starts from the current and melting insulation?
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 03-30-2019 at 02:01 PM.
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03-30-2019, 01:40 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by rv7charlie
Where are the essential avionics; are there duplicates on each bus? If the avionics are on the 'aux avionics' switch....
Oh, edit: Fail the 'milspec' master switch. What happens?
edit2: From the drawing and accompanying text, it looks like start current will be on your main bus (?).
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I also found it interesting that the entire redundancy system is driven by a single point of failure. Though maybe the backup is to hold down the momentary switch. You do realize that every bump in turbulence may result in an on off power cycle to the entire bus. Kinda rough on the avionics.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 03-30-2019 at 01:43 PM.
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03-30-2019, 03:07 PM
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Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 860
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Twin Batteries- Not all Redundancy is Created Equal, part 2
"twin EarthX EXT900-VNT batteries mounted in a center console located between the pilot and passenger’s legs in my RV-7A"
Balanced against the lightweight and high power of lithium based batteries is the fact that they contain...lithium! Even with Boeing's resources and certification requirements, they had 3 or 4 SERIOUS battery fires in 787s which if I remember correctly resulted in their grounding until the battery container was reinforced and properly vented overboard.
If I were to use lithium batteries, they would be mounted ahead of the firewall. However, I don't trust them to be 100% reliable and incapable of a thermal runaway (FIRE!). I'm using dual PC680s mounted ahead of the firewall.
If I may be permitted to add to the desireable (mandatory?) traits of an electrical system:
0. Safety
1. Symmetry
2. Simplicity
3. etc.
__________________
Terry Edwards
RV-9A (Fuselage)
2020/2021 VAF Contribution Sent
Last edited by terrye : 03-30-2019 at 03:10 PM.
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03-30-2019, 03:28 PM
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Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
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My backup battery is simple and no relays, just the 30A rated switch to tie into the three seprate fuse block busses, pump 1, ECM 1, Coil 1 on one fuse block, pump 2, ECM 2 Coil 2 on the other fuse block, and the avionics are on the third fuse block buss if I need to load sheed to keep the engine running longer, hand held for BU com. the Dynons monitor both battery volts and only switch on the BU battery during run up and taxi to top off, then switch off. To test the back up battery, BU battery on, alt field off, master off. fan still turning? then master on, alt field on, BU battery off.

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7A Slider, EFII Angle 360, CS, SJ.
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03-30-2019, 03:42 PM
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Join Date: Oct 2014
Location: Cedar Rapids, IA
Posts: 232
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Quote:
Originally Posted by terrye
"Even with Boeing's resources and certification requirements, they had 3 or 4 SERIOUS battery fires in 787s which if I remember correctly resulted in their grounding
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Boeings batteries were a different chemistry than what we put in RVs. Completely different animal.
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03-30-2019, 04:53 PM
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Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 358
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Quote:
Originally Posted by lr172
I see each battery is a directly connect to the E buss, with no means of disconnection. Seems kinda dangerous to me, especially with earth x batt's. What happens if you have a dead short somewhere and smoke/fire starts from the current and melting insulation?
Larry
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Note that each load on the essential buses is CB protected from each and switchable. It is alittle like having to turn off magneto switches apart from the master. The ELB’s are short and should be thought of as extensions of the battery posts. Anyone who has installed a lightspeed ignition knows that Klaus Savier rightfully insists that power supply to each unitbe connected as directly to the battery as possible, with no intervening switches or contactors. These is nothing intrinsically unsafe about short, well-installed hot ELB’s. (Edit note: after a bit more thought I did decide to add circuit protection to the ELB’s, and the schematic has been edited accordingly.-Otis)
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2019 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"
Last edited by Hartstoc : 03-31-2019 at 09:10 AM.
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03-30-2019, 04:57 PM
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Join Date: Mar 2006
Location: Newport, TN
Posts: 7,496
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What about the BMS system in each of the EarthX batteries?
It has electronics and some sort of micro controller built into it. Guess what, that requires firmware/software and you guessed it, both batteries will be the same.
There are eight fault light errors listed in the manual. Some of them require up to thirty minutes of monitoring to determine meaning.
There are at least two abnormal conditions where the BMS will disconnect the battery from the bus. Likely taking out both of them at the same time.
So far #1, #2, and #3 have not met your examples....
PS, this not a slam against EarthX. I will likely be using them in my RV-10 and would have one in my RV-7 if it were not for I need the weight where it is and I don?t want one mounted on the engine side of the firewall.
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03-30-2019, 05:04 PM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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Quote:
Originally Posted by Hartstoc
Note that each load on the essential buses is CB protected from each and switchable. It is alittle like having to turn off magneto switches apart from the master. The ELB?s are short and should be thought of as extensions of the battery posts. Anyone who has installed a lightspeed ignition knows that Klaus Savier rightfully insists that power supply to each unitbe connected as directly to the battery as possible, with no intervening switches or contactors. These is nothing intrinsically unsafe about short, well-installed hot ELB?s.
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Totally disagree, an unplanned short to one of these 'hot busses' can have serious consequences (I've seen it happen).
If you don't think one wire can create enough smoke/heat to cause a fire maybe you should try it.
Nothing worse than an electrical short that you can't shut off, all you can do is watch it burn.
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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