Dual G5 Questions

[jlunseth]

I am improving my panel in November. I currently have the KFC200 with the classic related King vacuum instruments (525, 256). I was going to add a 275 as a backup AI, and as a primary Turn Coordinator (the 275 is not my question). Someone suggested the "Dual G5" installation (if memory serves it was @artvandelay but I don't remember for sure) so I have been looking in to that. I have a couple of questions about the G5 if anyone knows the answers.

The announcements about the G5 all appear to indicate that it is compatible with my KFC200 (with a magnetometer and GAD29B), but I am being told by my avionics shop that the G5 does not supply pitch information. It would not support the KFC200 in flying a Glideslope coupled approach, although it would supply the horizontal course information. Does anyone know if the G5 can support the KFC200 to fly a coupled glideslope? I currently have an ICARUS SAM gpss which I really like and it will cause the KFC200 to fly all the horizontal routes involved in any approach (or other procedure) loaded into the GPS, but to fly a coupled approach with glideslope the KFC200 must be put into Approach mode and then gets pitch from the GPS as I understand it (for an LPV, for example).

Second, I am interested in the G5 because of the 4 hour battery backup. For my flying, none of the half hour or one hour backups (i.e. the 275) are enough. Doing a failure analysis, I assume that the alternator goes out and there is no electrical other than the backup battery in the G5s. This means, I believe, that the G5 would not get information from either the magnetometer or any GPS source outside of the G5 (such as a GTN 750). To get that information, the G5 would need to have a GPS antenna of its own, I believe, and probably some way of switching to that antenna if all other sources of directional information fail.  Is that possible, and does anyone know how that can be done.

I did search for prior threads on these issues. I found a thread on the external-antenna-for-the-G5 issue, but it did not seem to resolve how the G5 would get information from that antenna once the G5 has been hooked up to external GPS signal sources (and even if those sources fail).

Any thoughts? Help on this?

[Oscar Avalle]

Good question.

My calculations are:

I lose my alternator. I lower the gear... (I don't want to find myself having to lower the gear manually later...)

Then I start disconnecting stuff. Leaving only my GTN750. 

I should have at least 1.5 hours of juice left on the plane battery. That should give me enough time to return or to land somewhere.

If that is not enough and my battery dies too, I would still have 4 hours of power on the G5s. In total that gives me around 5 hours of flying time... Which should be enough... considering that I seldom plan a flight that long.

[jlunseth]

Same calculation for me except I have actually done it. You get about a half hour on the single 12V battery. The Dual G5's would provide attitude only and not directional information in that failure scenario, unless there is a source of directional information such as an external antenna, and unless the G5 has the ability to automatically switch to that antenna and use its internal source in the event of failure of an external directional source.

The problem with the single battery in the back of our aircraft, and the problem with all the half hour and one hour backup systems that are out there right now, is that actual backup time is about half what the rated time is, especially if the event happens five or ten years after the backup was installed.  How much storage do you believe you have in your 5, 6, or 7 year old 12V? Its not much. Also, there appear to be drain sources acting on your 12V even if you turn the Master off.

It was not a big deal when it happened to me in VFR conditions, but if you are IFR, let's say crossing from Great Falls to Kalispell, and it is another 45 minutes or hour before you can get to anything but granite, and then you might have to do a descent from a fairly high altitude (about 14K to make that crossing as I recall) and then you have to fly an approach, it is not enough. 4 hours I will take, which is what the Dual G5s provide, but if the G5s can't provide anything except attitude information they are not very helpful. And, if I am not in failure mode, if the G5 installed in place of my current King instruments prevents my AP from flying a couple GS approach as I can do now, they are not worth the investment.  All I need is a backup AI at that point. 

[Bob - S50]

The G5 can only work as an HSI for the KFC200, not an AI. The GI275 can be use as an AI and/ or HSI. You could install a dual GI275 and be compatible with the KFC200  and a future GFC500. No backup required but you could add a G5 if you wished.

[JimB]

2 hours ago, jlunseth said:

I am interested in the G5 because of the 4 hour battery backup. For my flying, none of the half hour or one hour backups (i.e. the 275) are enough. Doing a failure analysis, I assume that the alternator goes out and there is no electrical other than the backup battery in the G5s. This means, I believe, that the G5 would not get information from either the magnetometer or any GPS source outside of the G5 (such as a GTN 750). To get that information, the G5 would need to have a GPS antenna of its own, I believe, and probably some way of switching to that antenna if all other sources of directional information fail.  Is that possible, and does anyone know how that can be done.

The G5s have an internal GPS antenna and it works ok in my installation but you can have an external antenna hooked to it if you want. I also have a GNX375 for position information for it as well. You can turn off the G5 HSI and just use the Attitude one. It will display heading and course information independently. The GMU 11 only draws .5 amps. You won't lose all electrical power if you lose your alternator. You still have a main battery and it will last a long time if you shed all the non-essential stuff. Just don't turn on your landing light unless you have an LED. 

[jlunseth]

@ BobS - Thanks. That is what I was afraid of. The 275 has only an hour of backup. Reality is, five years or seven years out when you have a failure event it is no longer an hour, and Garmin does not say it is an hour either, they say "up to" an hour so if it turns out you really have fifteen minutes, well, there was no guarantee. So that is out. Adding three new glass instruments when I can just add one backup 275 to the King instruments I have in the system now, is alot of $$ for no advantage gained.

@ JimB - I have had occasion to turn the Master off with no working alternator, I was in VMC although the conditions were deteriorating, and by the time I got to Willmar about a half hour away I was out of juice, so there goes that. If your 12V is brand new maybe you have time. In five years, you don't have the time, the house battery is not a reliable backup.

Edited September 23, 2020 by jlunseth

[JimB]

 

[Steve W]

Your Avionics tech is half right. The G5 does not provide pitch information in that it can't replace the AI on the KFC. But your current HSI also doesn't provide pitch information to the autopilot. The glideslope information comes directly to the Autopilot from the analog outputs on your GPS or NAV source and it doesn't matter if you have a King HSI, a G5 or no HSI at all.

 

[jlunseth]

@SteveW - I am sure my Avionics tech is all correct, it is probably lost in translation through me. However, he was concerned that the G5 does not allow a coupled GS approach, and he has been through some installations. That information is from a couple of years ago though. I will talk to him about it further. Just so I understand, in my definitely non-Avionics brain, when I put the KFC200 in APPR mode it is using analog inputs into the Flight Computer of the KFC200 (the KC295 as I understand it). If it is an ILS, the analog inputs would be the horizontal and vertical signals from the ILS. If it is an RNAV (GPS) type approach, then the GPS (I have a 430AW and will be installing a new GTN 750 Xi) will supply analog outputs for the course and glideslope to the Flight Computer, which the FC then uses to control the aircraft down the glideslope? Something like that? It sounds, though, as though the G5 cannot act as a Flight Director and properly display the glideslope in that event, so I would not know if I am on slope or not on slope, I would have to just trust that the computer is working?

It is confusing to me, here is what the Garmin site says about the G5: The unit displays both vertical and lateral GPS/VOR/LOC course deviation when available. Perhaps it is not fully compatible with the KFC200?

 

[Steve W]

21 minutes ago, jlunseth said:

@SteveW - I am sure my Avionics tech is all correct, it is probably lost in translation through me. However, he was concerned that the G5 does not allow a coupled GS approach, and he has been through some installations. That information is from a couple of years ago though. I will talk to him about it further. Just so I understand, in my definitely non-Avionics brain, when I put the KFC200 in APPR mode it is using analog inputs into the Flight Computer of the KFC200 (the KC295 as I understand it). If it is an ILS, the analog inputs would be the horizontal and vertical signals from the ILS. If it is an RNAV (GPS) type approach, then the GPS (I have a 430AW and will be installing a new GTN 750 Xi) will supply analog outputs for the course and glideslope to the Flight Computer, which the FC then uses to control the aircraft down the glideslope? Something like that? It sounds, though, as though the G5 cannot act as a Flight Director and properly display the glideslope in that event, so I would not know if I am on slope or not on slope, I would have to just trust that the computer is working?

It is confusing to me, here is what the Garmin site says about the G5: The unit displays both vertical and lateral GPS/VOR/LOC course deviation when available. Perhaps it is not fully compatible with the KFC200?

 

The analog glideslope output from the 430 or 750 go straight to the KFC computer just like today. The G5 is not involved(neither is your current HSI, it's wired in parallel to just display the GS). The G5 displays the same glideslope data from a digital output from the 430/750 which is why it can't be used with legacy analog only radios. The G5 interface(GAD 29B) to the KFC is to emulate the old HSI lateral modes which is the  course pointer selection and heading bug and provide GPSS. The wiring diagram for the G5 and KFC 200 doesn't include glideslope wires as they're unchanged from your current installation and the functionality is exactly the same from the user's point of view. Glide slope coupling is not mentioned in the flight supplement since there's no change in operation.

The only people who run into problems are those who hoped the G5 would replace the Attitude Indicator and Flight Director, which requires the 275(or Aspen or G500)

[Ragsf15e]

This doesn’t help you much, but I’d heard a while back that the -275 batteries were designed to last as long as the G5 batteries but the certification process caused them to have to write it as 1 hour (or 30 minutes depending on where you look).  The G5 didn’t go through the full certification process.  Did they test the G5 batteries at -35C which is what I normally see at FL300?  Maybe the G5s were room temp?

The GI-275s are certified and can be used in way more aircraft, possibly at much more extreme temps, altitudes, ect.  I suspect the typical general aviation use would give a much longer battery life before failure, but we’ll never know.  As I said, that doesn’t really help you, sorry.  
 

Maybe someone has the battery specs for both?

The G5s are real nice though.  If you have the internal antenna hooked up, you could switch it from external to internal gps inflight during a total electrical failure but why?  It’s just track at that point.  Navigate with a phone or ipad or (gasp) the compass. .  You cannot use that internal gps to steer to waypoints or shoot an approach or anything.  You’re on your ipads or map/compass at that point.   The gps input normally helps it maintain precise attitude but it’s not required during power failure.

[jlunseth]

Well, I think I understand. The G5 won’t replace the King AI and DGI. The 275 will, but only provides a certified backup of “up to” one hour. 

Thank you for the help.

Edited September 24, 2020 by jlunseth

[oldguyscanfly]

On my first test flight after my dual G5 installation, I lost the generator on my 65E. I  I was doing pattern work at an airport about 20 miles from the avionics shop. I think I had possibly 2 gear cycles after the generator had died. I left the gear down and flew back to the other field, and on approach the 5's went to back up power, after I assume the voltage on the ship battery got to low. I left my GNX375 and com radio on, all other accessories off. It did continue to have a GPS guidance after it went to battery power. the 375 and the radio continued to work until shutdown. Upon replacing the generator with a plane power alternator, the shop discovered my battery was in pretty poor shape also.

So I think with a good ship battery, one could count on at least 2 hrs. of G5 operation plus a gear cycle.

Cant help on the autopilot questions.......someday if Garmin or BK gets it together.

[jlunseth]

I live in MN where we have lots of battery issues because of the cold winters.  The problem with relying on the ship’s battery, in fact any 12V wet cell, is that they appear to work fine for a long time and then suddenly start to fail. There is no way of knowing when that point is. Our aircraft here in MN sit and fly in very cold conditions which helps to cause battery failures. I have been in temps as cold as -54 at altitude. So sure, if you have a brand new 12V as ship’s battery you are likely good for a while. But when you are no longer good and you have just a little storage left you don’t know, even if you get the thing tested at annual an internal failure can start the next day. Having had some car batteries fail over the years, well, they work right up until the day they don’t, and that is usually right when the weather starts to get cold. I am not going to put my life and that of my passengers on “might work.”

[Bob - S50]

18 hours ago, jlunseth said:

@ BobS - Thanks. That is what I was afraid of. The 275 has only an hour of backup. Reality is, five years or seven years out when you have a failure event it is no longer an hour, and Garmin does not say it is an hour either, they say "up to" an hour so if it turns out you really have fifteen minutes, well, there was no guarantee. So that is out. Adding three new glass instruments when I can just add one backup 275 to the King instruments I have in the system now, is alot of $$ for no advantage gained.

@ JimB - I have had occasion to turn the Master off with no working alternator, I was in VMC although the conditions were deteriorating, and by the time I got to Willmar about a half hour away I was out of juice, so there goes that. If your 12V is brand new maybe you have time. In five years, you don't have the time, the house battery is not a reliable backup.

Look at it this way.

Before you need the GI275 or G5 battery you have to lose your alternator and drain the ship's battery. When that happens you no longer have Nav, Comm, or transponder either. If you are VFR who cares? But if you are IMC, ATC can't see you, you can't squawk 7600 or 7700, you can't talk to anybody, and your only possible navigation source is your tablet.

You have a lot bigger problems than being limited to an hour of battery life. And don't you heat the cabin which keeps the batteries warm?

[jlunseth]

Bob, thanks. As I have said before, I have been in these actual situations so I know what happens, I am not guessing and it is not theory. There is no Nav, Comm, transponder, and in my aircraft no engine gauges whatsoever (no MP, CHT, EGT, TIT, etc.) because I have a JPI 930. When it is -54 the cabin heater does not keep up, the cabin does not stay warm, and heating the cabin does not help the battery, which is in the tailcone,distant from the heater in the center console and walled off from the cabin.  

For purposes of deciding what to put on the panel there are two basic analyses to conduct. One is the "No Failure Analysis," assuming no failure, will the new instrumentation provide at least the normal functionality of the existing system. If, for example, the G5 cannot do a coupled approach with glideslope, and the existing King instruments can, then there is no reason to spend money on two new Garmin instruments because I will wind up with less than I now have. This rules out the dual G5 installation.

The second analysis is the failure analysis, what happens in the worst case if the aircraft loses all electrical? From experience, I can tell you that keeping a valid AI for the remaining duration of the flight is critical. I have a Stratus 2, I would not stake my life on it even as a backup. Among other things, if the Stratus, which is not fixedly mounted, moves even slightly, then the AI readings are no longer valid. Partial panel is mediocre at best and does not even get to mediocre in turbulent conditions. I cannot rely on the ship's battery because, five years out, there is no way of knowing how much storage is left and it is probably not nearly what you might think. As I said, the one time complete electrical failure happened to me (in VMC), the ship's battery, fully charged, had about a half hour of capacity. If I replace my good King vacuum instruments with glass instruments, such as the 275 (in other words, the vacuum instruments are taken out), then half hour and one hour backup times are not enough. For one thing, again from experience, if the failure happens five or seven years out from the installation, the backup time is generously, no better than half the original backup time.

To replace vacuum instruments that are working fine I need glass instrumentation with a minimum of a certified, 2 hr. backup time. The 4 hours of the dual G5 installation would be fine with me, but see the "Non-failure analysis," above. I would apparently lose the ability to fly a coupled glideslope approach. 

I think the best I can do currently is to replace the existing electric Turn Coordinator with a 275 that is certified as primary for the TC function (that requires an added OAT gauge) and keep the King vacuum instruments (256, 525). The 275 becomes my backup AI, to the existing vacuum AI, and the 275 has a certified one hour backup time, which is an hour more than I have with the present electric TC that is in the aircraft (that TC becomes the backup AI under partial panel if the vacuum system of the vacuum AI quit).

[Bob - S50]

7 hours ago, jlunseth said:

....and heating the cabin does not help the battery, which is in the tailcone,distant from the heater in the center console and walled off from the cabin.

I was talking about the G5 or GI275 battery which is located in the cabin.

For purposes of deciding what to put on the panel there are two basic analyses to conduct. One is the "No Failure Analysis," assuming no failure, will the new instrumentation provide at least the normal functionality of the existing system. If, for example, the G5 cannot do a coupled approach with glideslope, and the existing King instruments can, then there is no reason to spend money on two new Garmin instruments because I will wind up with less than I now have. This rules out the dual G5 installation.

As stated earlier, the G5 will not drive the AI portion of the KFC200 but the GI275 will.

The second analysis is the failure analysis, what happens in the worst case if the aircraft loses all electrical? From experience, I can tell you that keeping a valid AI for the remaining duration of the flight is critical. I have a Stratus 2, I would not stake my life on it even as a backup. Among other things, if the Stratus, which is not fixedly mounted, moves even slightly, then the AI readings are no longer valid. Partial panel is mediocre at best and does not even get to mediocre in turbulent conditions. I cannot rely on the ship's battery because, five years out, there is no way of knowing how much storage is left and it is probably not nearly what you might think. As I said, the one time complete electrical failure happened to me (in VMC), the ship's battery, fully charged, had about a half hour of capacity. If I replace my good King vacuum instruments with glass instruments, such as the 275 (in other words, the vacuum instruments are taken out), then half hour and one hour backup times are not enough. For one thing, again from experience, if the failure happens five or seven years out from the installation, the backup time is generously, no better than half the original backup time.

I'm probably wrong, but I think battery capacity is supposed to be tested each year as part of the annual.  We had a pretty good idea that our battery was not long for this world based on cranking power when we started the airplane.

If I'm IMC, I'm sure as heck not going to take 90 minutes to find a place to land.  I'm deciding where I want to go, then I'm declaring an emergency and then I'm getting there as quickly as I can.

To replace vacuum instruments that are working fine I need glass instrumentation with a minimum of a certified, 2 hr. backup time. The 4 hours of the dual G5 installation would be fine with me, but see the "Non-failure analysis," above. I would apparently lose the ability to fly a coupled glideslope approach.

You are correct.  A vacuum AI will keep working with total electrical failure, but again, you'll be using your tablet for navigation IF you have one that has a built in GPS receiver.

I think the best I can do currently is to replace the existing electric Turn Coordinator with a 275 that is certified as primary for the TC function (that requires an added OAT gauge) and keep the King vacuum instruments (256, 525). The 275 becomes my backup AI, to the existing vacuum AI, and the 275 has a certified one hour backup time, which is an hour more than I have with the present electric TC that is in the aircraft (that TC becomes the backup AI under partial panel if the vacuum system of the vacuum AI quit).

We aren't trying to talk you into a G5 or GI275.  You were the one that brought it up.  If you feel better keeping the KI256 and KI525, that's up to you.  The FAA apparently finds the GI275 to be acceptable since they allow you to eliminate your entire 6 pack of instruments with a dual GI275 installation.  While I'm not positive, if you are really worried about battery life, I would think you could install a dual GI275 system and also put a non-required G5 in the panel.

The G5 can also replace the TC.  It can replace the AI ..or.. the TC, but not both.

See my comments in blue above.

[jlunseth]

Bob, I appreciate your comments and the information. My flight situation is a little different from what you might experience in a J. First, my aircraft is turbocharged so I am often in the high teens or flight levels when the winds aloft are favorable. To give you an idea of how that impacts my decision making on my panel, a normal descent from, say, FL210 to 1,000 AGL for landing takes over 40 minutes at 500 fpm. It is certainly possible to accelerate that to 1500 fpm safely, but that is still 15 minutes just to get down from altitude. I have had occasion to dive at about 3,000 fpm with a failing engine, through IMC, and I will not put myself in a position to have to do that, at least not without a functioning AI (partial panel does not cut it in that circumstance).

I also fly out west quite a bit, and to the Bahamas. I have been to Billings twice this summer and all over the Dakotas. There are areas in MT where the airports are over 100 nm apart. There are also circumstances, such as crossing the Rockies or over water, where it is not an option to set it down for quite awhile.  KGPI to KGTF is at least 45 minutes. 

Typically, I am not in IMC, I am way over the top of it, in smooth clear air. But if there were an emergency there are many times when an approach would be necessary to get in to an airport. If you have a towered airport and an ILS that can be cut to about 10 minutes by vectoring on to final. Nontowered airport in mountain country there may only be a long RNAV, maybe 20 minutes or better to fly it, and you can’t cut corners to final because of the rocks. There are also circumstances where descent through the clouds is not wise, better to fly somewhere where it is clear or semi-clear however long that takes, because it is winter and those clouds are full of ice.

In sum, it is not possible to say that in the event of an instrument failure, I would find an airport and get down right away. On the one occasion when I lost the vacuum that is exactly what I did because I was in the clear and Bismarck was just 20 miles away, spent the night on a cot and flew the next day with a new pump. But there are circumstances where that is just not possible and I am trying to build my panel to safely accomodate that worst case. It might take 52 minutes, but I want a system where I am not worrying while I do all this emergency flying that the time on the backup battery is 57, or whatever it happens to be. Certainly 30 minute times are no good.

The ship’s battery can’t be counted on. Yes, you started the engine, but that was in the warmth at sea level. Now you have been in the flight levels for three hours, the temp has been -35, and that battery is cold. I make it a point to get my ship’s battery checked at annual precisely because of these issues, I don’t know if it is required or not. But from more than 50 years of driving cars in Minnesota, 12V wet cells are subject to sudden failure in the cold, especially when they have been in service for more than about three years.  As I said, the one time I had to rely on mine I had 30 minutes if that on a battery that I know was fully charged.

Currently I have a great, working system. I don’t have an interest in buying glass that has less functionality just because it is glass. what I don’t have is a good, backup AI. If I could put in a dual G5 and continue to get the functionality I currently have with my KFC200, 256, 525, I would put in the glass, but apparently I can’t. The backup time for the 275 is not long enough for me. “Up to” one hour does not do it given the above circumstances. So I think what I am going to do is put in a 275 to replace the TC. The 275, with an OAT sensor, can be primary for TC, there are little markers at the top of the screen that show turn rate. The 275 itself will be my backup AI. I will keep the King system. 

I don’t have a problem navigating with the iPad. It would be nice to have a backup DG because it gives instantaneous heading information where the iPad lags, but the iPad and a compass are better than nothing. It is primarily the AI I am concerned about.

Edited September 25, 2020 by jlunseth

[Ragsf15e]

Sheesh, just put a G5 in backup in place of the TC.  That’s legal too.  Have it wired with it’s own antenna and it will give you gps track and groundspeed info during your total electrical failure scenario for approximately 4 hours.

[squeaky.stow]

On 9/25/2020 at 12:48 PM, jlunseth said:

The ship’s battery can’t be counted on.

I agree. You need a backup that will last long enough to get you to the worst case enroute alternate. That could be a lot longer than an hour. 
I have one of these to power my standby AI. 

https://www.aircraftspruce.ca/catalog/pdf/ESP_Information.pdf

Will power an RCA electronic standby instrument for up to six hours. Whether this could be used as backup power for a G5 or other electronic A/I is something that could perhaps be answered by some of the avionics folks on this forum. 

[carusoam]

@elimansour had mentioned using the ESP battery in his installation...

So I extend an invite to see if he has insight...

Best regards,

-a-

[jetdriven]

On 9/23/2020 at 4:05 PM, jlunseth said:

@ BobS - Thanks. That is what I was afraid of. The 275 has only an hour of backup. Reality is, five years or seven years out when you have a failure event it is no longer an hour, and Garmin does not say it is an hour either, they say "up to" an hour so if it turns out you really have fifteen minutes, well, there was no guarantee. So that is out. Adding three new glass instruments when I can just add one backup 275 to the King instruments I have in the system now, is alot of $$ for no advantage gained.

@ JimB - I have had occasion to turn the Master off with no working alternator, I was in VMC although the conditions were deteriorating, and by the time I got to Willmar about a half hour away I was out of juice, so there goes that. If your 12V is brand new maybe you have time. In five years, you don't have the time, the house battery is not a reliable backup.

There is a backup battery test the GI275 runs. We got almost two hours from it. Also the GMU11 is powered by the GI275, so you get that for the full duration of backup battery.

you should be doing regular capacity tests of your ships battery   If it doesn’t thane 80% capacity it’s not airworthy       .  My last prebuy it lasted 7 minutes...Like 10% capacity...The seller bought him a new 600$ 28v battery   

also, gear down at alternator failure may not be the best idea. You may need to fly 200-300NM for good VFR weather and it would be most efficient at 125kts and 7 gph. Vs the 110kt and 11 gph.  Which will take longer than most backup batteries for AI’s. And certainly longer than than the ships battery.  And you can’t retract it with the emergency system. So keep it clean if you need range.  It cranks down fine. 

[jlunseth]

@ Rags - As a backup AI and a replacement for the TC, I am fine with the 275. It is a little less expensive and seems to me to be a better instrument (a little more up to date) than the G5. There would need to be two system failures to get to the battery backup in the 275 as a backup instrument, a failure of the vacuum/gyro system and a failure of the electrical. Possible, but the chances are very slim. One hour is fine with me under that circumstance. What is not acceptable is setting up the ship’s systems so there is a single point of failure, say the electrical, and then have just an hour or half hour rated backup battery before total blackness.

@Squeaky - that is a very good idea. I briefly looked at supplemental backup systems, had not seen that one, I will check it out. The one concern I have is that the battery is lithium and they don’t have the best reputation for in air systems (risk of fire). Also, don’t know if it will wire up, but I can check that out.

@ Byron - I agree with all your comments on the ship’s battery. Mine is checked every year at annual. My concern is that from experience, in the cold climate I live in, 12V wet cells are subject to sudden failure, plenty of capacity one day and then not so much the next, and in my ship there is only one. As I mentioned, the one time I experienced total electrical failure in the air that battery did not have the duration I expected, even with the Master off. As far as I am concerned, in the event of a total electrical system failure that battery should be disconnected (Master off) and saved for end of flight for the gear and to hopefully have radio and maybe GPS nav if needed, but it needs to be off until that point. I have a handheld that can be plugged into an antenna, my main concern about that is the task loading that happens in a critical emergency does not allow time or distraction to go fish for things in the back seat, especially if you are single pilot and the AP is out. If it is not on the panel and working, chances are pretty good you will not be able to put it into use.

On the backup duration of the 275, it may be that the thing, when new and just hooked up, will give you two hours. What I am concerned about is what happens 5 or 7 years later, when that backup has been in the plane for awhile and then the incident happens. As I said, my experience on that is you have half, or less than half, of the “as new” rated capacity. That is not enough for me, as a backup to a single point of failure system.

[elimansour]

9 hours ago, carusoam said:

@elimansour had mentioned using the ESP battery in his installation...

So I extend an invite to see if he has insight...

Best regards,

-a-

I ended up scrapping that idea and going with a GI275 as a backup.  Installer felt that the ESP would be difficult to fit with all the other changes in the panel.

[jlunseth]

That is exactly the direction I am headed.