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GRT EFIS backup battery

gnorm

I'm New Here
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.
 
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.
 
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.
 
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).
 
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.
 
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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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.
 
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 :D what other things did you see with this unit that you liked?
 
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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1N5820 Schottky Barrier Rectifier Diodes 3A 20V DO-201AD (DO-27) Axial 5820 IN5820 3 Amp 20 Volt should be dandy.


Dan covered the rest. If you have multiple items you might use a Schottky on a heat sink.
 
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I know its an old thread, but I'll ask thoughts anyway...

I came across this Bioenno Power Lithium Iron Phosphate (LiFePO4) Battery Model BLF-1203AB which specs as a flat pack 4 cell Lithium 12v 3A/hr battery. Specs show it to have an integrated charger etc etc all for about $50. Seems like this is ideal for just getting over the blink out issue during start. I have a 12" GRT - I'm not worried about the other ancillaries as they come up quicker than the screen does after start, and I have Z14 architecture anyway.

Seems like it would charge just the same with the described Schottky setup.

Thoughts?
 
I think it would charge as you describe. I don’t know anything about that battery or it’s reliability, but it doesn’t sound like it’s that critical anyway. On my RV6 with a 10” GRT Horizon, I have a 5 ah sealed lead acid battery from Battery Mart (Duracell) as my backup battery controlled through an Aux battery toggle switch to power an Aux bus. The only thing on my Aux bus is the GRT Horizon and the uAvionix Echo ADSB as secondary power sources - less than 2.5 amps draw. Instead of using a Shotkey diode, I have a simple toggle switch, since that bus only draws 2.5 amps.

Your Shotkey diode is automatic, although it draws down the voltage slightly for charging your backup battery. My power switching is manual through a bus tie toggle switch that I have to turn off in the event of a primary power failure to conserve backup battery power - which really isn’t that critical in my airplane either, but allows the full 14.4 volts for charging. It’s easy to manage. My Low Volts light comes on, switch off the bus tie and I’m good - no diode needed. The Aux bus would continue to power the Horizon and ADSB, through whichever power source has higher voltage. Neither battery is being charged at this point because you just lost your alternator. If I want to isolate the backup battery to conserve its power, I can just turn off the backup battery switch. My electronic ignition (SureFly on the left) is wired hot through a fuse to the backup battery. If I turn off the the Aux battery toggle switch, the backup battery is supplying power to the SureFly only. This procedure isn’t critical, because I have a magneto on the right. So I have independent redundancy for engine ignition, and independent power for my primary flight display and ADSB information. Pretty robust for a VFR airplane.

Sounds like you have the same thing with your setup, although you may not need to wire in a diode unless you don’t have a clear indication of primary power failure (alternator). I would make sure you understand the battery chemistry and charging circuitry protection for the Bioenno battery before installing it.
 
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.

Bob,

I'm studying the ideas on backup batteries...

Would someone need to put some kind of resistor on the charging wire to the battery in the event of a discharged backup battery? Would a discharged battery soak electrons fast enough to easily open a circuit breaker?

Also...could a small 3aH LiFePO battery charge acceptably by just having 14.5 volts being pushed at it, or does it need some kind of charging circuitry?

I'm not an EE, but I'm trying. :)

David
 
Bob,

I'm studying the ideas on backup batteries...

Would someone need to put some kind of resistor on the charging wire to the battery in the event of a discharged backup battery? Would a discharged battery soak electrons fast enough to easily open a circuit breaker?

Also...could a small 3aH LiFePO battery charge acceptably by just having 14.5 volts being pushed at it, or does it need some kind of charging circuitry?

I'm not an EE, but I'm trying. :)

David

Second question is easy: I think pretty much all Li batteries come with some sort of charging electronics, so yes, they re-charge just fine with 14.5 volts applied.
As to the second question, I would look up the max charging current (which can be large, but for a small battery hopefully not too large), use appropriate sized wire and a fuse or CB to protect the wire in case the battery fails shorted.
Not asked, but for keeping an EFIS or two up during start, or maybe an hour or two of use following a main battery failure, you don't need a lot of amp-hrs. Since I'm visualizing this battery in the cabin, I don't want a Li battery there. I'm thinking sealed Pb-acid.
 
Thanks Bob for your reply.

Curious...if my small (2Ah) or so 12 volt AGM battery (that, let's say, happens to be somewhat discharged after use) is then fed unrestricted 14.5 volts from the aircraft's power system for recharge, will the size of the 2Ah battery effectively limit it's amperage intake on it's own? Or do I need to add a resistor to keep the amperage flow within the wire's capability?
 
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