Hartstoc
Well Known Member
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/community/showthread.php?t=170079
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/community/showthread.php?t=170079
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