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Twin Batteries- Not all Redundancy is Created Equal, part 2

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.

2v2EtmKSCxBELK5.jpg

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.
2v2EYVUYnxBELK5.jpg

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:
2v2EtPoyVxBELK5.jpg

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.
2v2ELzPE8xBELK5.jpg

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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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 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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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 (?).

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
 
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"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.
 
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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.

uRbTSNC.jpg
 
"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

Boeings batteries were a different chemistry than what we put in RVs. Completely different animal.
 
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

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)
 
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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.
 
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.

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.
 
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 (?).

Remember, this is a schematic not an installation drawing. In the actual installation, the two main contactors will be co-located with their output sides joined, along with a heavy cable going from there to the nearby starter solenoid. So Yes, the starter, as per normal, will be on the main bus, which extends all the way down to the contactor outputlugs.

The mil-spec master is a 20Amp high qualiy switch that will never be asked to carry more than the one amp required to activate the contactor coils on one pole and the even tinier milliamp load of the mini-solenoids on the other pole. Not a likely point of failure, but completely unrelated to operation of the engine.

As for the avionics power pathways, they are pretty well explained. The Aux avionics switch provides a means by which a selection of avionics can be operated even if some problem forces the shutdown of the main bus, making it pretty much a non-event.
 
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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.

uRbTSNC.jpg

Bret- Beautiful! My first thoughts on the topic were along these lines and I find no fault whatever with your solution.

Where I’ve ended up going with this approach is intended to be a departure from the norm that takes full advantage of these magical new batteries we have available now. Their power to weight ratio is so high that it opens new possibilities. All of the battery related mods I’m installing for the system I describe here together weigh no more than one PC680! Why have a backup battery at all when it is so easy to have a full blown, interchangeable, full twin battery system. If I ever have any operational issues with one of the EarthX batteries I can completely isolate it from the system with the flip of a few switches, and be left with a completely normal operational system with no compromise of any kind, and I never have to worry that my backup battery may be getting a bit old, may not be fully charged, has reduced capacity, etc. also, my starter is going to be delighted to have 800CCA available on call.
 
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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.

Well, I appreciate your concern, but you would have to judge the installation rather than the schematic. If something direct-shorts anywhere along the cable connecting your battery post to your contactor input lug, you are in the same exact pickle you describe above. I’ll be working with much shorter hot runs than the one you probably have on your bird right now. The option to add another disconnect, of course, exists here, but you would also be adding a potential failure point. (EDIT 3/31–Walt- after thinking more about this, I did see an easy way to provide circuit protection on the ELB feeds, abd it also gives me more flexibility in battery location, so I’ve added it and will notate the schematic. Thanks for your input!- Otis)
 
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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.

Brantel- Thanks for your response. I distinguish between software/firmware and circuitry with hard-wired discrete logic, which I believe better describes the EarthX BMS, but of course you would have to ask EarthX about that directly. I do not believe that there is even a mechanism for ?firmware? upgrades(anybody who knows more please weigh in!) I see no reason why a simultaneous shutdown would occur since the two in my system are completely isolated from one another except during the very brief occasions when I activate the momentary switch, and I do not believe the BMS is a hair-trigger device.

I did talk to Reg and Kathy at EarthX about the momentary interconnect feature in my design. Reg told me there should be no concern if the two are within a few volts of one another. That is the reason I added the two V/A meters, and SOP will be to observe voltage on both prior to engaging the momentary. -Otis
 
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

Sorry Larry, I don?t get it. I?ve been flying for 39 years, mostly in airplanes with whimpy rocker-masters, and I?ve never once had one flip off due to turbulance. A mil-spec toggle holds position very stiffly, and given that this one is a DT mounted horizontally, less likey to be knocked out of position by turbulance than those mounted vertically. Also, don?t forget that Aux. Avionics switch on the ELB powering input #2 to nearly all of my avionics. They would not even notice it if you turned off the master on purpose.

Also, you hold the momentary up, not down.- Otis
 
Brantel- Thanks for your response. I distinguish between software/firmware and circuitry with hard-wired discrete logic, which I believe better describes the EarthX BMS, but of course you would have to ask EarthX about that directly. I do not believe that there is even a mechanism for ?firmware? upgrades(anybody who knows more please weigh in!) I see no reason why a simultaneous shutdown would occur since the two in my system are completely isolated from one another except during the very brief occasions when I activate the momentary switch, and I do not believe the BMS is a hair-trigger device.

I did talk to Reg and Kathy at EarthX about the momentary interconnect feature in my design. Reg told me there should be no concern if the two are within a few volts of one another. That is the reason I added the two V/A meters, and SOP will be to observe voltage on both prior to engaging the momentary. -Otis

Take a look at the pictures of the BMS, it is a far cry from hard wired discrete logic with passive electronics. Just because EarthX does not make firmware upgrades available to the end user does not mean it does not have any.....
 
Take a look at the pictures of the BMS, it is a far cry from hard wired discrete logic with passive electronics. Just because EarthX does not make firmware upgrades available to the end user does not mean it does not have any.....

I will take a look- really though my concern with software/firmware has more to do with ?upgrades? that can introduce unexpected behavior triggered by some unanticipated set of conditions. Far less concerned about established, proven code. I?m not at all concerned about simultaneous battery shutdown in this installation.- Otis
 
I will take a look- really though my concern with software/firmware has more to do with ?upgrades? that can introduce unexpected behavior triggered by some unanticipated set of conditions. Far less concerned about established, proven code. I?m not at all concerned about simultaneous battery shutdown in this installation.- Otis

History has shown it is so called proven code that takes out most systems in this failure mode.
 
History has shown it is so called proven code that takes out most systems in this failure mode.

In my experience as a home-automation technician, it has generally been firmware and software updates that lead to grief, with end-users left playing the role of guinea pigs left to discover the glitches.

I hope that the good folks at EarthX will weigh in here on the much-maligned BMS, which has to be pretty **** simple software-wise. The BMS is there to protect the battery and the user, and the EXT900-VNT’s I’m installing are on the verge of FAA certification for use in production aircraft. It is the ONLY battery available DESIGNED for safe installation in the passenger cabin and it features a closed containment vessle and overboarding vents for noxious gasses. I’ve been bench-testing the two going into my plane for some time, and I am blown away by thier performance.

Some people, perhaps you included, are going to remain sceptical about these new technologies for some time, but I have not heard anything here that will disuade me from making this installation. Don’t forget, I’ll have two of them and they will be colsely monitored. The indicator outputs will be wired into my G3X Touch and cofigured to get my atention at the slightest anomoly. If ANYTHING of concern pops up, I can instantly and completely isolate the offending battery from my working system, yet be left with a fully operational, no-compromise electrical system with huge stored power reserves. All this in a complete system that weighs about the same as one properly mounted PC680! What’s not to like about that?

That said, I truely appreciate every effort to blow holes in my design strategy. My fear was that someone would point out some fatal flaw in my design that would save my butt but make me feel like an idiot. Happily, nothing like that so far, but please keep it coming!- Otis
 
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In my experience as a home-automation technician, it has generally been firmware and software updates that lead to grief, with end-users left playing the role of guinea pigs left to discover the glitches.

I hope that the good folks at EarthX will weigh in here on the much-maligned BMS, which has to be pretty **** simple software-wise. The BMS is there to protect the battery and the user, and the EXT900-VNT’s I’m installing are on the verge of FAA certification for use in production aircraft. It is the ONLY battery available DESIGNED for safe installation in the passenger cabin and it features a closed containment vessle and overboarding vents for noxious gasses. I’ve been bench-testing the two going into my plane for some time, and I am blown away by thier performance.

Some people, perhaps you included, are going to remain sceptical about these new technologies for some time, but I have not heard anything here that will disuade me from making this installation. Don’t forget, I’ll have two of them and they will be colsely monitored. The indicator outputs will be wired into my G3X Touch and cofigured to get my atention at the slightest anomoly. If ANYTHING of concern pops up, I can instantly and completely isolate the offending battery from my working system, yet be left with a fully operational, no-compromise electrical system with huge stored power reserves. All this in a complete system that weighs about the same as one properly mounted PC680! What’s not to like about that?

That said, I truely appreciate every effort to blow holes in my design strategy. My fear was that someone would point out some fatal flaw in my design that would save my butt but make me feel like an idiot. Happily, nothing like that so far, but please keep it coming!- Otis

I wish you the best of luck in your endeavor. I find it strange that you are so eager to dismiss simple proven technology/concepts and so willing to bet the farm on a smoke filled black box (not literally, just an analogy) that you have no real knowledge of internally. Your mind appears to have been made up long ago and the true motive behind these post appears to be something different than stated.

As I said before, this has nothing to do with EarthX and their BMS, I would use them in a heartbeat. It is about how they break your criteria you specified earlier. Seems to be very double standardish in nature.

But hey, this is experimental aviation. Go for whatever you like and want to do!
 
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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 is absolutely correct here.

A cell short in one of the batteries, game over. I've had a battery in my Jeep die in this manner and could not jump start it.
 
I wish you the best of luck in your endeavor. I find it strange that you are so eager to dismiss simple proven technology/concepts and so willing to bet the farm on a smoke filled black box (not literally, just an analogy) that you have no real knowledge of internally. Your mind appears to have been made up long ago and the true motive behind these post appears to be something different than stated.

As I said before, this has nothing to do with EarthX and their BMS, I would use them in a heartbeat. It is about how they break your criteria you specified earlier. Seems to be very double standardish in nature.

But hey, this is experimental aviation. Go for whatever you like and want to do!

Brantel, Thanks for participating, I appreciate your input. It is a bit troubling to have my motives questioned, and I don?t understand at all what you mean in saying that I am dismissive of simple proven technology. What I?m trying to do is align the design of modifications to my airplane with current technology, and share this process with others facing similar challenges on this incredible forum.

High energy-density batteries and electron-dependent aircraft are both game-changers, and thinking outside the box of magneto-era solutions is very much called for. If you really spend some time with the schematic I presented, it should become clear that it does a very good job of meeting those goals where electron supply and management is concerned, and that it also scores very highly against any criteria for redundancy. -Otis
 
Yes

Yes, there are many here who choose the status quo because that?s how it?s always been done...

Had Van himself used that line of thinking, there would be no RVs to fly and this discussion would not be taking place...
 
Yes, there are many here who choose the status quo because that?s how it?s always been done...

Or maybe some of us prefer not to be test pilots in this hobby, give me a million hour proven method of doing things and I'm ok with that.

When I was younger (and invincible) I might have tried to put a V8 in the RV if someone suggested it, but I like to think I'm a little wiser now.
 
And

...and everyone is entitled to their own opinions in our wonderful hobby...

It is unfortunate that there are some that can?t accept that there is more than one way (their way) to do things, and still end up with a safe, reliable machine.

That being said, changes and modifications are not necessarily for everyone...and I am ok with that.
 
...and everyone is entitled to their own opinions in our wonderful hobby...

It is unfortunate that there are some that can’t accept that there is more than one way (their way) to do things, and still end up with a safe, reliable machine.

That being said, changes and modifications are not necessarily for everyone...and I am ok with that.

Very true, but …

I think the motivation for many posts, including my own, is to try to keep this hobby as safe as possible especially for those that may not have the insight/experience in this business to necessarily determine what's safe on their own.
 
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Otis - a very simple comment here...

Your "momentary" switch concept is interesting in concept, however I would advise that in its real-world application it may be a little more difficult than expected to actuate that momentary switch.

Solely from an ergonomics standpoint I would suggest replacing that momentary toggle switch with a "start" push button momentary switch as is available from both Stein and B&C. The push button is far easier to actuate. Its form factor nestles in very neatly beneath the flip-up plastic switch guard. The push button is just a lot easier for our fingers to manipulate.

Switch example:
https://www.steinair.com/product/starter-switch/
 
Trying to digest your main goal here, one of your statements about having 800 CCA for starting with two batteries, if you are using the Earth X 900, you will not need two of them, one of them will crank your prop fast enough to taxi with the starter alone :D Also these new batteries keep their charge for a long time with very little discharge, these can sit for months with no trickle charger and start just fine, even in cold weather. I tried the schottki diode with heat sink for charging the backup battery but it did not do well, so I just tied in a manual switch to top off charge it during runup and taxi, then off. Ready to go for backup at the flip of a switch. Also, keeping the secondary battery isolated protects it if a runaway alternator goes crazy, or the crowbar fails and takes the main battery off line. With my set up if this happens, I just shut off the master, turn on the back up battery and everything comes alive......for about 90 min.
 
Electrical systems reviews

Otis,
I wouldnt fault you for wanting to experiment and look for a better way. In a past life I spent a lot of time looking over the work of young engineers looking for a more inovative way to use new technology to solve avioncs and control systems problems. How to develop a culture of critical review without being disparaging or discouraging others to risk being dismissed as incompetent is critical to getting the best out of the process. Doing it at arms length over the internet in a hobby environment is even more difficult.
Many approaches to system design are flawed due to starting out with a concept and then trying to make it fit the situation - a solution looking for a problem if you will. The telltale signs are lack of balance in the design ( do we have a complex solution for a simple straightforward problem), patches to mitigate undesirable features or holes in the proposed design and finally the need for components with very tight tolerances or hand selected characteristics. Only you can make the determination of what is right for you for your electrical system - I would encourage you to do a critical review with those three concepts in mind before casting your design in stone. You may decide to proceed with what you have or you may decide to put it to one side and start over with a clean sheet of paper. I hope it is the latter.
KT
 
Earths offline

I had an Earthx that would go offline when starting, sometimes when flying. It was only 1 yr old. No way to get it back online till I put ground power to it. I have dual panel with some glass and some steam. Steam got me home every time. I sent battery back and they couldn?t not find anything wrong. I told them to keep it. If I was designing a system with a backup battery, it would be lead type just in case it needs to absorb some current when brought online. Lithium type don?t like to absorb current like a lead battery does.
CJ

I wish you the best of luck in your endeavor. I find it strange that you are so eager to dismiss simple proven technology/concepts and so willing to bet the farm on a smoke filled black box (not literally, just an analogy) that you have no real knowledge of internally. Your mind appears to have been made up long ago and the true motive behind these post appears to be something different than stated.

As I said before, this has nothing to do with EarthX and their BMS, I would use them in a heartbeat. It is about how they break your criteria you specified earlier. Seems to be very double standardish in nature.

But hey, this is experimental aviation. Go for whatever you like and want to do!
 
It is unfortunate that there are some that can’t accept that there is more than one way (their way) to do things, and still end up with a safe, reliable machine.

Walt was pretty clear with his objection. How would you do things? What approach is best in your opinion?
 
Below is a picture of a symmetrical dual-battery electrical system that really is simple.
It is up to the builder-installer to make absolutely sure that there is no
chance of a short circuit of the main power bus or its feeders.
In case of electrical fire or smoke, the pilot should shut off all switches except the master and those for the engine.
The failure of any one connection will not result in loss of electrical power.
A warning placard must be posted warning against shutting off both master switches during flight.
A fuse should be in the alternator B lead located near the battery contactor.
No separate E-Bus or avionics bus or battery bus is needed. Adding one will only add an unnecessary failure point.
enhance
 
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I had an Earthx that would go offline when starting, sometimes when flying. It was only 1 yr old. No way to get it back online till I put ground power to it. I have dual panel with some glass and some steam. Steam got me home every time. I sent battery back and they couldn’t not find anything wrong. I told them to keep it. If I was designing a system with a backup battery, it would be lead type just in case it needs to absorb some current when brought online. Lithium type don’t like to absorb current like a lead battery does.
CJ

This is not really surprising. Remember that these batteries have an electronic, digital processor that controls it. This processor and circuitry has the ability to disconnect the actual battery cells from the cases terminals. For those relying upon these batteries (referring to earthx, not necessarily all lithium) as your backup for IFR or engine operation, please keep that in mind. I think we all know that electronic, microprocessor circuits can and do fail; Sometimes completely and sometime intermittently.

Larry
 
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This is not really surprising. Remember that these batteries have an electronic, digital processor that controls it. This processor and circuitry has the ability to disconnect the actual battery cells from the cases terminals. For those relying upon these batteries (referring to earthx, not necessarily all lithium) as your backup for IFR or engine operation, please keep that in mind. I think we all know that electronic, microprocessor circuits can and do fail; Sometimes completely and sometime intermittently.Larry

Larry, in fairness, we've never gotten a straight answer about how the battery management module actually works, just result. The official word is it tapers charge current beginning at at 15.5V, and "blocks" current at voltages 16V and above. That's not necessarily a physical disconnect, although it is equivalent in many respects, notably the loss of buffer effect in an overvoltage event. There is no reason to believe the battery won't readily supply electrons to the bus as soon as bus voltage drops back below nominal battery voltage.
 
No pic. Any chance you can check your link?

Below is a picture of a symmetrical dual-battery electrical system that really is simple.
It is up to the builder-installer to make absolutely sure that there is no
chance of a short circuit of the main power bus or its feeders.
In case of electrical fire or smoke, the pilot should shut off all switches except the master and those for the engine.
The failure of any one connection will not result in loss of electrical power.
A warning placard must be posted warning against shutting off both master switches during flight.
A fuse should be in the alternator B lead located near the battery contactor.
No separate E-Bus or avionics bus or battery bus is needed. Adding one will only add an unnecessary failure point.
uvU4aN67G8eq76RW3bUQyKo1kqShIw_5mf1rsS9_sJXrT57mPOllEcvlClRMe1ODFqJ83NGAUVKX23XlZsss6OrSpQ3IfRyicMrN3cMdyqmh4m4zSZ8-mEwJt8xP8e9iXNfQeFllXqaUPUqVPdDlzfbXs3MS46ifK502czCDyRH3gQqaS4WE0oCyBFbSsX7U2ICJyCARsTLsynwUT11fqJ6XK0PhMaRGZ9c857CsQDI2bMcRZpEFxmLhH1IJn9fbWKIHxrQtj4VKF9ysNGhC25SU0nFYSyBaGoq1cam8uYhMYJ5cnCiFX1QYDBZguBG5ew2emC--fVzeqF1zQDicsaNc_oMrVtZQjxH0qv_pXE5LvhyRvmT3j5mozb3fhWZls8Yor6pPolCi9k1tUlRBT3302nLgzD1hwoUrbzpRy1cw8W1sq0Bd-A80haLjbFBG0lb2fzxtAMxrPLpWOyGTue0CcxJWhKtIh8DBLwB-V0Ma7taT6BuDcyU8ejCKIz0gxBdfcmhfuDgWfH4Mts1IwsMJ8JUC25Yd5NDNDBYblQoWB062mL7odmqIoFhpiBn9bK8pAj_fcff47Bp0JPU5c8RxITIs7RC02CifMG4QLlrKflI8VM6R57xUQszaHoiPeMZ27x-5b4kna91hh0L2D20ou8jwGB4=w589-h762-no
 
Google Photos will not work long term as an image hosting source. Pics will randomly disappear......
 
You are correct that one cable is redundant. However, the O.P. wanted a symmetrical layout.
One of the batteries and one of the contactors are also redundant.
Some builders want a fail-safe architecture without a single point of failure.
I am not necessarily recommending this design. Just offering it as an option. My plane, which
is not electrically dependent, only has one main bus feeder and one battery.
Remove one cable from master sol. to buss. Or am I missing something?
 
Larry, in fairness, we've never gotten a straight answer about how the battery management module actually works, just result. The official word is it tapers charge current beginning at at 15.5V, and "blocks" current at voltages 16V and above. That's not necessarily a physical disconnect, although it is equivalent in many respects, notably the loss of buffer effect in an overvoltage event. There is no reason to believe the battery won't readily supply electrons to the bus as soon as bus voltage drops back below nominal battery voltage.

You may be right and I possibly made unfair assumptions based upon limited data. I thought that I read a post from earth x a while back indicating that their BMS has the ability to disconnect the battery in case of low voltage conditions. It may have been a misundertanding on my behalf or I may have misread the post, but my recollection was that the battery cells will disconnect from the terminals when it reaches a defined low voltage state, in order to prevent battery cell damage. While I realize that feature isn't an inherent risk, the BMS's "ability" to disconnect the cells would create the potential for an unforeseen failure in the BMS to disconnect the battery.

My point was what the BMS has the physical ability to control over, more than it's defined features. Whatever it has control over could fail in an undesireable way based upon a circuit failure, microprocessor failure or firmware bug. Not really bashing the product, only exposing potential risks. It seemed relevant in this thread discussing potential failure scenarios. Also not suggesting this battery is bad to have in an airplane. Only that all risks need to be evaluated when something is used as an emergency backup.

Larry
 
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....my recollection was that the battery cells will disconnect from the terminals when it reaches a defined low voltage state, in order to prevent battery cell damage.

Yes, that is understanding also. And I think we would both like to hear how it's done.
 
Below is a picture of a symmetrical dual-battery electrical system that really is simple.
It is up to the builder-installer to make absolutely sure that there is no
chance of a short circuit of the main power bus or its feeders.
In case of electrical fire or smoke, the pilot should shut off all switches except the master and those for the engine.
The failure of any one connection will not result in loss of electrical power.
A warning placard must be posted warning against shutting off both master switches during flight.
A fuse should be in the alternator B lead located near the battery contactor.
No separate E-Bus or avionics bus or battery bus is needed. Adding one will only add an unnecessary failure point.
enhance

If the engine is running shutting off both masters will not power down the bus. The alt will continue to supply power until the field switch is also opened.
 
Doug Mattson, you are right. However, the voltage could be unstable.

Most regulators can keep the voltage quite stable, under normal circumstances, without a battery in the circuit. It is the VR, not the battery, that is maintaining a stable voltage. The battery only helps with absorption of spikes and other transients, that are typically not present.

Larry
 
I guess I was just trying to point out that if you wanted to kill the power to your bus and motor in case things were going south you would have to remember in addition to shutting off the master switches you would also have to remember to shut off the field to the alternator. This isnt just for two batteries. I think my single battery setup works the same way.
 
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There is a flaw in the schematic. The diodes prevent the small relays from energizing.

Mich- Thanks for this- it is amazing how easy it can be to overlook the obvious so kudos to your astuteness! You are correct, but the fix is simple. The diode and ELB should be on the switched(output side, and the coil on the hot(in this case main bus) side. The purpose of the solenoid is to make sure that reverse leakage of the diodes cannot occur when the aircraft is shut down, and that the diode charging circuit is hot only when the master is on and, presumably, the alternator functioning normally . I’ll substititute a revised schematic soon. - Otis

P.S.- I’ve been away from the forum for a week or so and see quite a few posts deserving of replies. My apologies for the delay and I’ll get to them soon. One general comment, though. I saw one post suggesting that I somehow appear to think that this twin battery system is the cat’s meow for all aircraft, or the “only” solution.

Nothing could be further from the truth, but It does offer some features that I think are new for electron-dependant aircraft, especially those relying upon electric fuel pumps. 1- It has the ability to isolate either battery completely and take it off-line very quickly without any sacrifice of functionallity of the aircraft. This should assuage some of the aparrently widespread concern about using new battery technology. 2- It allows one ignition to operate full time on the battery serving as secondary, but also insures that the vast bulk of that battery’s capacity remains available even if an alternator failure goes unnoticed for an extended period of time. That capacity is sufficient to operate the aircraft through nearly an entire tank of gas, even while driving a (properly sized) fuel pump, providing a far greater set of options than a small backup battery. Also, if the “main” battery is used alone for starting, the ignition running on the second battery will see no voltage drop during engine cranking. 3- It makes the full force of both batterys available for starting without any heavy added switching hardware. I may not use that feature much, as one person pointed out a single EXT900 already has twice the CCA of a PC680, but the happiest starter is pne that sees the lowest possible voltage drop when cranking, and it could offer an advantage for hot starts at times. The main purpose for the momentary is to allow seamless switching of the two battery’s roles in flight without concern for voltage spikes or loss of current to sensitive avionics. 4- the whole setup weighs the same as one PC680. 5-It is operationally dirt-simple, especially as regards responding to an alternator failure. More soon- Otis
 
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Otis - a very simple comment here...

Your "momentary" switch concept is interesting in concept, however I would advise that in its real-world application it may be a little more difficult than expected to actuate that momentary switch.

Solely from an ergonomics standpoint I would suggest replacing that momentary toggle switch with a "start" push button momentary switch as is available from both Stein and B&C. The push button is far easier to actuate. Its form factor nestles in very neatly beneath the flip-up plastic switch guard. The push button is just a lot easier for our fingers to manipulate.

Switch example:
https://www.steinair.com/product/starter-switch/

I’ve been away from VAF for a bit but wanted to respond to a few of your comments, ALL of which I’m very grateful for- Otis

Canadian Joy- Thanks for your comment. Making the momentery al little hard to get to was intentional. It’s use is optional for starting and that does require a dedicated effort, but you are sitting on the ground with no distractions. Airborn, it would likely be used no more than once per flight to reverse the battery’s roles to “top-off” the one that has been charging through the diode. Its proximity to the master makes this a very easy, one hand operation, but inadvertant engagement of the momentary is hard to imagine.- Otis
 
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