GRT Backup Battery Details

Ya'll gimme a little peer review here...

Subject is providing backup power to a GRT Sport EFIS and a GRT EIS using the GRT backup battery.

The battery install sheet suggests a "backup battery bus master switch" on the battery output. Such a scheme would require (a) switching after main power failure and waiting for the EFIS to reboot, or (b) engaging the backup bus switch as part of the post-start proceedure. I'm thinking a DPDT avionics master switch would provide uninterupted power via the two isolated EFIS power inputs and eliminate the extra switch operation. Thoughts?

Uninterupted power to the EIS is not so simple as it only has one power input. Can't just hook two power sources to the single input, as a ground short on one would kill both. I'm thinking a pair of diodes as shown would do the job. Any downside? Any special diode required? Other methods? Did you even bother with backup power for the EIS (don't need it to fly)?

Dan,
Your setup makes sense to me. The key characteristics of the diode you want is the forward voltage drop. You want that to be as low as possible. I used this from DigiKey. Their part number is 90SQ030-ND. It is way overkill in terms of being able to handle 9A's. However, according to the data sheet (figure 1), the max voltage drop you should see is around .35V. This is much better than the typical .6 - .7 drop you get from a silicon diode.

I hope this helps.

Dan,

I am currently installing the Sport EFIS as a retrofit in a flying plane. As far as I understand, both the main battery bus and the backup battery bus can be connected at the same time. The Sport will select the higher voltage to power itself. The purpose of the switch between the Sport and backup battery bus is to allow the Sport to be turned off after flying. Without it, the backup battery would continue to power the Sport until it is discharged. Normally, the backup battery switch would be turned on before engine start, powering up the Sport. This has two functions. One, it shows that the backup battery is charged and working. Second, it allows the Sport to be powered during engine start. I have heard that starting the engine may draw the voltage down below the required level of the Sport. Without the backup battery, the Sport would restart. My EIS does not seem effected by engine starting and is available during the engine start proceedure. As I said before, this is what I understand from talking with GRT tech support.

Good Luck!
Tom RV-7A N175TJ Flying

Hi Dan,

Your diode feed for the EIS should work fine - you're essentially building the same thing as what the EFIS Display Units have internally (but with only two feeds). I have only a single power feed for my EIS, and don't see it drop off during start - also consider it non-essential if I have a failure big enough to knock out the main bus.

I actually don't have switches for my EFIS screens at all - they power up with the main, essential, or backup busses (whichever comes up first). If I want them off for maintenance or checkout, I just pull the breakers.

Paul

Is there a scenario where you would want to be able to turn off the Avionics Master but retain backup battery power to the EFIS?

Probably just fine ... and more feedback

Feedback on a similar installation ...

I replaced the circuitry (similar to what Paul Dye uses) I had in a friend's RV with the module from GRT attached to a larger backup battery than they specify. THe reason was the output showing the "status" of the battery on the EFIS. Had to modify one of the "values" in the EFIS to scale correctly.

The output is through a switch and that is used to drive a little "buss" that is fused. From there two EFIS'es are driven (along with something else I forget).

Before power up, the "AUX power" switch is turned on and the EFIS'es boot up. During start there is NO lowering of the voltage from that battery as it is isolated so they don't reboot. So now I know there is some "juice" in the AUX battery. The switch is left ON during flight.

During flight the AUX battery gets charged but it never reaches the voltage of the MAIN battery so the EFIS'es are run off the MAIN battery/alternator combo.

If charging is lost and ignored (it happened once), sooner or later a radio fails and something else quits. Even then the AUX (assuming big enough) will drain at a lower rate and you will have some power left over to get home (or to nearest airport. If you were to immediately turn OFF the AUX until voltage got to say 9 volts and then turn it on, you would have a LOT of power left for the EFIS. THe main point here is that you do not have to wait until things die to be powering the EFIS and thus no reboot.

I did NOT bother to power the EIS as it was considered non-essential but the way you are doing it seems like a fine idea.

James

Several good points, thanks.

Should have mentioned I was planning to use the EIS for engine monitoring during start (apparently it doesn't mind being on the main bus while cranking), and connecting battery pin 4 to an EIS aux input for battery condition.

I'm ditching the diode idea and considering the EIS non-essential. With the diode pair I can't turn the EIS off.....duh on me.

Well, actually the diode-fed EIS can be turned off....just needs a slight change:



Looks like we have three basic schemes.

Paul's example is the bus architecture approach, certainly a fine way to go for an IFR airplane.

The standard GRT method allows a graphical engine display on the EFIS during start, but adds an additional switch to the start and shutdown proceedures.

What I've proposed doesn't power the EFIS while cranking (EIS display only), but provides backup while appearing conventional to the average GA pilot.....master on, one mag on, crank, second mag on, avionics master on.

Backup power for the EIS is a compromise choice. On one hand, maintaining engine information after a main bus shutdown is nice. On the other hand, it cuts backup power time roughly in half. Heck, maybe I will go with it. The practical view says no engine information would drive the pilot decision process towards landing soon anyway.

Here's how I wired up my aux battery to allow me to power the EFIS and other avionics completely independent of the main bus so I don't risk rebooting during engine start. I switch the Main DC Power to ON and move the Avionics Master Switch to the Aux position. Once the screens are powered up I can start the engine. Then I move the Avionics Master up to ON and all the buses and batteries are tied.



This layout is based on the Z-13 design with an aux battery added, but could be adapted to other electrical systems.

DanH said:

Backup power for the EIS is a compromise choice. On one hand, maintaining engine information after a main bus shutdown is nice. On the other hand, it cuts backup power time roughly in half. Heck, maybe I will go with it. The practical view says no engine information would drive the pilot decision process towards landing soon anyway.

Click to expand...

If you do the standard reverse flow (avionics off, check magneto grounding, pull mixture for shutdown, turn off magnetos, turn master off), you would lose the EIS when you turned the master off.

What you show should give you:

• EIS power during startup (Main is ON)
• Backup Battery power while Avionics Master is ON

What it won't give you is:

• EIS power after shutdown, unless you leave Main ON
• Ability to shed loads by turning Avionics Master OFF

Though I suppose you could turn Main OFF and leave Avionics Master ON, which should power just the EFIS/EIS via the backup battery ...

Edit: It does feel a bit funny to me that the backup power is tied to the avionics master which is tied to the main bus. If you look at how a G1000 is installed and it's startup flow, it actually goes like this:

backup battery turned ON
check voltage of backup battery
turn master ON
start engine
turn avionics master ON

Then reverse for shutdown:

avionics master OFF
shutdown engine
master OFF
backup battery OFF

ccrawford said:

If you look at how a G1000 is installed and it's startup flow, it actually goes like this:

backup battery turned ON
check voltage of backup battery
turn master ON
start engine
turn avionics master ON

Click to expand...

My goals were simplicity and conformance to common operating proceedure. If that's a how a G1000 airplane is rigged, then it is fairly called the new common operating standard. Sold.

Thanks for the feedback guys.