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  #1  
Old 12-14-2020, 06:14 PM
gnorm gnorm is offline
 
Join Date: Jan 2011
Location: Southfield, MI
Posts: 5
Default GRT EFIS backup battery

I'd like to power up my dual GRT Horizon EFIS before engine start. GRT says to use a TCW backup battery although they didn't go into detail how. A friend told me to connect it to a 2nd power input of my EFIS with a switch. He said connect it to my main power batter with a shottkey diode so I can keep it charged with the alternator when not in use.
Anyone have a better explanation or a schematic? What size diode? Connect parallel? Fuse it? All clear as mud to me. Any advice is greatly appreciated.
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  #2  
Old 12-14-2020, 08:54 PM
Lars Lars is offline
 
Join Date: Nov 2005
Location: Davis, CA
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Quote:
Originally Posted by gnorm View Post
I'd like to power up my dual GRT Horizon EFIS before engine start. GRT says to use a TCW backup battery although they didn't go into detail how. A friend told me to connect it to a 2nd power input of my EFIS with a switch. He said connect it to my main power batter with a shottkey diode so I can keep it charged with the alternator when not in use.
Anyone have a better explanation or a schematic? What size diode? Connect parallel? Fuse it? All clear as mud to me. Any advice is greatly appreciated.
TCW manufactures several flavors of backup battery, but unless your needs are unusual, the IBBS-12V-3AH is plenty. No diode needed. Connected to the main buss, it will charge itself when ever the engine is running. I advise going to TCW's website and downloading the manual.

Most GRT equipment except for the engine monitors have at least two diode isolated power inputs. For example, any EFIS, the AHRS, SafeFly GPS. The above-mentioned backup battery has enough capacity to (for example) light up all three of the items I just mentioned for about an hour. When you first look at the wiring diagrams in TCW's documentation it may look a little intimidating, but don't be intimidated. Ask here. I've installed a bunch of them, so have many others on this board.
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  #3  
Old 12-14-2020, 09:08 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
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TCW is nice but a bit expensive, since it has its own regulated power supply for charging. A less expensive option is to get a small (maybe 3-5 amp hrs) battery of the same type as your main battery (lithium, wet cell lead-acid, glass mat, etc), and as your friend told you, connect it via a switch to the efis, and keep it charged with a Shottkey (low forward drop) diode (rated at 10 amps or so) between the main buss and the aux battery. In flight keep both main and backup power switches on.
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  #4  
Old 02-26-2021, 06:35 AM
gnorm gnorm is offline
 
Join Date: Jan 2011
Location: Southfield, MI
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Quote:
Originally Posted by BobTurner View Post
TCW is nice but a bit expensive, since it has its own regulated power supply for charging. A less expensive option is to get a small (maybe 3-5 amp hrs) battery of the same type as your main battery (lithium, wet cell lead-acid, glass mat, etc), and as your friend told you, connect it via a switch to the efis, and keep it charged with a Shottkey (low forward drop) diode (rated at 10 amps or so) between the main buss and the aux battery. In flight keep both main and backup power switches on.
Thanks Bob,
So why have an extra switch for the backup battery if the battery is connected to the main buss and will be used at startup (so the EFIS display will not drop during cranking).
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  #5  
Old 02-26-2021, 11:36 AM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by gnorm View Post
Thanks Bob,
So why have an extra switch for the backup battery if the battery is connected to the main buss and will be used at startup (so the EFIS display will not drop during cranking).
1. Do not connect the backup battery to the main buss. If you do and there is a main buss fault both batteries will be lost. Connect the backup power directly to the second power input on the GRT. The GRT will pull power from which ever battery is at a higher voltage - normally the main battery, since the backup is charged thru the Shotkey diode it will be a few tenths of a volt lower. The grt power inputs are diode-insulated, so power will not flow back out to a shorted buss.
2. You need a switch so you can turn the backup power off at the end of the day.
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  #6  
Old 02-26-2021, 12:57 PM
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rv8ch rv8ch is online now
 
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Quote:
Originally Posted by BobTurner View Post
TCW is nice but a bit expensive, since it has its own regulated power supply for charging. ...
It is pretty nice. It does some things that a diode/battery won't do, like pass-thru power from the main bus when it's there, and automatically switch over when it's not. I could have rolled my own, but this little guy was just so well-designed I couldn't pass it up. What I do is power up my efis and friends with the IBBS as soon as I plop into the cockpit, giving the ahrs time to wake up and get synchronized. Starting the engine doesn't cause a reboot of course. When I shut down the engine and the master, I leave the IBBS on for a minute or so to give the efis time to write the flight log files to the USB key.
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Last edited by rv8ch : 02-26-2021 at 01:00 PM.
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  #7  
Old 02-26-2021, 06:39 PM
BobTurner BobTurner is offline
 
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Originally Posted by rv8ch View Post
It is pretty nice. It does some things that a diode/battery won't do, like pass-thru power from the main bus when it's there, and automatically switch over when it's not. .
Well, actually, wired as described, the simple diode/backup battery does that too. I thought the TCW was nice for other reasons.
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  #8  
Old 02-27-2021, 12:20 AM
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rv8ch rv8ch is online now
 
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Quote:
Originally Posted by BobTurner View Post
Well, actually, wired as described, the simple diode/backup battery does that too. I thought the TCW was nice for other reasons.
True! And perhaps they are using the same method you described, which is probably the simplest. I was visualizing a PCB in there doing something magic but the voltage differential can't get much simpler or more reliable. Thanks for the correction and clarification.

Just to help me further justify my purchase what other things did you see with this unit that you liked?
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  #9  
Old 02-27-2021, 06:23 AM
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azrv6 azrv6 is offline
 
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Quote:
Originally Posted by gnorm View Post
Anyone have a better explanation or a schematic? What size diode? Connect parallel? Fuse it?
Would someone please be able to answer this question from the original post? Thanks.
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  #10  
Old 02-27-2021, 07:47 AM
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DanH DanH is offline
 
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No problem, can do.

You'll want to drive the EIS also, so you need a way to tie the backup battery power to it and the second EFIS power input. Two diodes get 'er done. I like a junction block, rather than building them into a wiring harness, because it allows easy diagnosis of any future power issue, and is easily modified for upgrades. Tapped power for the MAP sensor there too; the EIS is kinda useless without it.

See the small toggle to the right of the master? EFIS/EIS backup power, per the diagram.

I don't know if GRT sells this particular battery any more, but almost any will do. The GRT-supplied pack had its own little isolation board.
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Last edited by DanH : 02-27-2021 at 07:56 AM.
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