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GRT starting voltage sag

jjconstant

jjconstant

Well Known Member
Hi All

I have my plane wired up with 2 PC680 batteries. I start the engine on both batteries. Problem is, my GRT Horizon 1 dual screens come on with the master but the starting voltage sag causes them to reboot. Is this bad for them...do I need to install a switch or switches and keep them off until after starting? I didn't think with 2 batteries the starting current draw would still reboot the systems, but...

Jeremy
 
Does this happen with other EFISs?

Does the reboot-on-engine-start also happen on

Garmin G3X?
Aspen PFD?
Dynon?

I am building a similar electrical system (two batteries that can be -- but need not be -- paralleled for starting) and am wondering if I need to plan on a little battery to keep the EFIS alive.

I fly a Diamond DA42 at work that has a G1000 and it never reboots during start...I wonder what the differences are?

Thanks,
Martin
 
Jeremy,

My plan (not flying yet) is two batteries (one PC680 plus a 9Ah aux SLA battery) but starting the engine only off the PC680. My AFS EFIS/EMS will be isolated from the PC680 and connected only to the aux battery until after engine start.

Lots of people say that an avionics buss is nolonger necessary but I just feel happier protecting all the high value electronic items from the electrical vaguaries of the start process.

Happy New Year!
 
Most avionics are designed to operate down to 10 volts. So when you start a 12v system, most of the time the voltage drops down below 10v. The DA42 is a 24v system, so there is more "headroom" during start. There may a small aux battery tied into the G1000 as well - don't know for sure.

The GRT/AFS/Dynon/MGL all have small aux batteries that can be purchased separately, and we generally recommend that both for starting and for backup power in flight.
 
Hi JJ,

One neat thing about the GRT's (and I believe some of th others), is that they have three power feeds, all dioded inside the boxes, so you can feed them from three different sources. Yes, the voltage sag of engine start will reboot them every time -unless one of the power feeds is connected to a little Aux battery. The boxes draw from whatever source has a higher voltage, and they never mis a beat this way. I always boot my systems prior to start (so that I am ready to taxi when oil pressure stabilizes), and never have a problem - unless I forget to flip on the Aux battery before hitting the start switch...

Paul
 
Since someone asked about Dynon, the Dynon stuff has a few options, such as an internal battery that keeps it on when the voltage sags. If you have an internal battery, it just works, no need to wire something funny up to keep it on. AFS offers this as well. GRT does not. MGL can do stuff with an external battery.

At Dynon, we also design our stuff to run way below 10V. In fact SkyView can run as low as 3V. Our current EMS systems run down to 5V, since EMS is what people really need right after start. Even with a reboot, the EFIS-D100 only takes about 6 seconds to boot so it's not much of an issue.

Most 12V planes sag to about 8V when starting, and permanent magnet starters can be much worse. If your stuff only stays on to 10V, you do need something to keep it on during start.

We 100% suggest you have our stuff on during crank. I know it's contrary to what you may expect from older electronics, but we design for that. It's actually much less sensitive than older stuff. We've sold over 8,000 units and I don't believe I've ever heard someone say "It was on, but it died when I cranked the engine."
 
With GRT, if you also have the EIS installed where you can see the display, it will not re-boot on start, so you'll have RPM and Oil pressure during start if it's on. I don't run an aux battery, but start with EIS on, and EFIS off until after start and it seems to work just fine.
 
I would recommend to move the HX off your master and have it start with the avionics switch. I asked GRT specifically about this and the Horizon is no protected like the EIS. I have only my EIS on the master and reason for that is that I want to know about my oil pressure right away.
As for the HX rebooting, it might be because it draws the most power right at the beginning for its screen. I can't remember the exact amount of draw but I know for sure as mine was once rebooting when I had run the battery some what down. But if it booted, then it would run OK even with low voltage on the battery.
 
yup, EIS stays on...

but it looks like I need to install either a switch or an aux battery. I'm leaning away from yet another battery...Coin flip or is there something a bit more meaningful to base the decision on:rolleyes:?

Just for discussion purposes, I have 2 identical pc680 batteries and 2 masters. Procedure is to turn both on for start, taxi and runup, then turn one off for the duration of the flight. Next flight, fly and charge the other battery. The thinking is that I will always have a known good battery in my back pocket if needed.

Jeremy
 
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jjconstant said:
Just for discussion purposes, I have 2 identical pc680 batteries and 2 masters. Procedure is to turn both on for start, taxi and runup, then turn one off for the duration of the flight. Next flight, fly and charge the other battery. The thinking is that I will always have a known good battery in my back pocket if needed.

Jeremy
Click to expand...

You could do a fairly simple re-wire that allows you to power the EFIS from one battery when the other is isolated for the start. That is where the mupliple power feeds are useful. But then again, if you don't mind waiting for EFIS boot after start, the EIS is plenty enough to monitor oil pressure - as is a simple oil pressure switch.

Paul
 
Multiple solutions ...

jjconstant said:
Hi All

I have my plane wired up with 2 PC680 batteries. I start the engine on both batteries. Problem is, my GRT Horizon 1 dual screens come on with the master but the starting voltage sag causes them to reboot. Is this bad for them...do I need to install a switch or switches and keep them off until after starting? I didn't think with 2 batteries the starting current draw would still reboot the systems, but...

Jeremy
Click to expand...

Having been down this path I would suggest getting a THIRD battery of at least 7.5AH capacity and feed that to the third voltage input. Fuse as appropriate and route through a switch. Feed the third one for charging via appropriate diode setup OR just purchase backup battery module from GRT.

At engine start, the voltage drops a LOT more than most people probably realize. It does not take much to keep the the GRT alive but if the voltage goes below 8V they are going to check out. Even a small 9v battery would keep them alive but you will want something you can charge AND something that keeps them going for a few minutes even if all the big stuff fails.

Another reason for doing it this way is that you can just switch on the GRT's and NOT have to turn on the master and drain your main batteries when you want to check things out on the ground.


James
 
Intelligant Power Stabilizer

TCW Technologies makes a product called an Intelligent Power Stabilizer that will keep the power at 11.25 VDC or greater with input voltages as low as 5 VDC. The power drop and EFIS re-boots is exactly what this unit was designed for and can provide up to 8A output for that short time. That would even run a Garmin G900X. The largest system weights 14 oz, much less than a battery.

Christer

SteinAir Inc.
A&P, Avionics Tech
Cessna 120, RV-8 under construction
 
Back up battery and charging

A couple of mentions about the back up battery on the GRT EFIS but no details on the charging circuit. Is it simply wired to the main bus with a diode or is it more complex than this? I've been meaning to add this to my GRT Sport and need to get after it.

Always appreciate the input.
 
carlrai said:
A couple of mentions about the back up battery on the GRT EFIS but no details on the charging circuit. Is it simply wired to the main bus with a diode or is it more complex than this? I've been meaning to add this to my GRT Sport and need to get after it.

Always appreciate the input.
Click to expand...

Mine comes off the battery bus with a diode and resistor (to limit current in case of a problem). Also have a circuit breaker/fuse in there for safety. It charges anytime the smaller battery is at a lowr potential than the main battery bus. I put in a new 5 Ah Aux battery about every two years.

Paul
 
Ditto Paul's comment plus ...

Ironflight said:
Mine comes off the battery bus with a diode and resistor (to limit current in case of a problem). Also have a circuit breaker/fuse in there for safety. It charges anytime the smaller battery is at a lowr potential than the main battery bus. I put in a new 5 Ah Aux battery about every two years.

Paul
Click to expand...

You can do it this way **OR** you can now get a setup from GRT. Last time I chatted with them, they were using a battery smaller than I wanted to I put the circuit onto a larger battery. There is a "sense" wire that you can connect to the Horizon and display a **sort of** battery charging status. It works as advertised but we agreed it could be improved on the "user interface" side.

James
 
Questions

Here is the wiring chart for the GRT Back-up battery, which I find a little obtuse. Do you use just one for all your EFIS's (EFII?), or one each? Should the aux battery pull power off the main or avionics bus? When you say battery bus Paul, is that the same as the main bus? NASA nomenclature? Do you then just run with the aux battery on, or do yo shut it off after startup?

I have questions about using the Intelligent Power Stabilizer too. The wiring diagram for it shows it feeding off the main bus. So this means that each unit that is wired to it will come on as soon as you turn on the main bus, right?

Paul, do you have anything other than your EFIS Displays on your aux battery?

I am really wanting to set up an aux system if nothing else to avoid the reboot of the EFIS's

Hans
 
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A few quick answers Hans -

When I say battery bus, I mean the "always hot" side of the battery (through a fuse). Essentially, the batteries are paralleled, but isolated in one direction. It is not the same as the main bus - different side of the main battery contactor.

I power the Display Heads and AHRS from the Aux Battery feed, and keep the Aux battery on from before start to after shutdown - no reason to turn it off, and i you have a main power problem in flight, you don't have to take a step to turn the Aux on. Simple is your friend!

I have no experience with the Intelligent Power Stabilizer, so can't help you there.

Paul
 
nucleus said:
Here is the wiring chart for the GRT Back-up battery, which I find a little obtuse. Do you use just one for all your EFIS's (EFII?), or one each? Should the aux battery pull power off the main or avionics bus? When you say battery bus Paul, is that the same as the main bus? NASA nomenclature? Do you then just run with the aux battery on, or do yo shut it off after startup?

I have questions about using the Intelligent Power Stabilizer too. The wiring diagram for it shows it feeding off the main bus. So this means that each unit that is wired to it will come on as soon as you turn on the main bus, right?

Paul, do you have anything other than your EFIS Displays on your aux battery?

I am really wanting to set up an aux system if nothing else to avoid the reboot of the EFIS's

Hans
Click to expand...

I used their circuitry but with a 7.5 AH battery to power two displays plus AHRS. You want to "power" it (the circuit) with over 14 volts from a logical point in your system so it will charge the battery.

I then took the output and sent it to a small fuse block via a switch. A tap each from there to Display 1, Display 2 and AHRS. Each with appropriate fuse (around 1 or 2 amps ... I forget at the moment).

Operating mode:

1. Get in plane and turn on the "backup battery" (or whatever you want to call it) switch. This starts power up of EFIS (es).

2. Master, fuel pump etc, START. The main batteries can sag all they want but the EFIS(es) keep right on running as this circuit is isolated.

3. Leave switch ON during flight (as Paul does). Battery gets recharged and if everything else failed in the dark of night in the clouds in the middle of a storm over the mountains, blah, blah ... you still at least have the EFIS (with AHRS) telling you which way is up.

Again, this is basically the same as what Paul does with his "homebuilt" version :) . I even had built basically the same earlier. It just has the added benefit of having been done by GRT and having the sense wire with settings to tell something about the health of the battery.

Feel free to contact me offline if there are other questions.

James
p.s. AGAIN, at around 8 volts or so you WILL lose the GRT EFIS. Just keep that in mind.
 
IPS product solves this problem

Christer got it right,

Our IPS series of products was designed to solve this exact problem. The IPS-4a unit provides protection for up to 4 amps of load and is perfect for systems such as the GRT EFIS. The IPS-4a weighs in at only 8 oz and unlike a battery requires no additional maintenance.

Our IPS-8a version provides up to 8 amps of load current and weighs 14 oz.


All the details are available at www.tcwtech.com


Thanks,
Bob Newman
TCW Technologies, LLC.
RV-10 40176
 
I have a similar setup (two 680s)

and find that I can start up my panel (thru the endurance bus switch) on the left battery and then start the engine on the right battery only. Never had a problem starting on just one 680 (180 hp 8.5 compression ratio). I figure if I ever have a starting problem, I'll just switch off the panel and start the engine on both batteries.

BTW, I always fly with both batteries in the system and charging. I figure if I ever have an alternator problem, I'll have two fully charged batteries to manage my power options (L, R, or both) as the flight requires.
 
great responses

Thanks guys! I think in the end I'm going with simple, cheap, light and easy: an on/off switch.:)

One of the deciding factors was that I don't have an endurance bus, just 2 batteries and 2 master solenoids, both or either powering the bus.

Jeremy
 
Ironflight said:
I power the Display Heads and AHRS from the Aux Battery feed, and keep the Aux battery on from before start to after shutdown - no reason to turn it off, and i you have a main power problem in flight, you don't have to take a step to turn the Aux on. Simple is your friend!
Click to expand...
Agreed. Using the "OR"ed power inputs and a standby battery allows for engine start with the EFIS/EIS on, automatic and uninterrupted feed from the aux battery if the main power feed goes out (for any reason) and requires no pilot input. No having to flip a switch and reboot the units.

Simple is good.

TODR
 
A little more detail?

Ironflight said:
Mine comes off the battery bus with a diode and resistor (to limit current in case of a problem). Also have a circuit breaker/fuse in there for safety. It charges anytime the smaller battery is at a lowr potential than the main battery bus. I put in a new 5 Ah Aux battery about every two years.

Paul
Click to expand...

Understand the need for a diode to protect the aux battery from discharging into the main battery when capacity has diminished. A bit rusty on current limiting though. Assuming a 5 - 7 AH gel cell, what value and how is the current limiting resistor wired in?

I also understand a question about our airplane wiring pales when you're getting ready for a launch. Whenever you have a moment - the input is more than appreciated.

Thanks
 
Thanks for the Input Guys

I think I am going to emulate Paul's set up, it will be inexpensive and do what I want -freedom to keep my EFII and AHARS "spun up" as well as a back-up system in case something goes haywire.

Hans
 
carlrai said:
Understand the need for a diode to protect the aux battery from discharging into the main battery when capacity has diminished. A bit rusty on current limiting though. Assuming a 5 - 7 AH gel cell, what value and how is the current limiting resistor wired in?
Click to expand...

The current limiting resistor is in series with, and upstream of, the diode. It is simply to limit the current if, for instance, the Aux battery is drained very low. In this case, you have a large voltage difference between the main battery and the Aux, and the current can get pretty high, turning your diode into a NED (Noise Emitting Diode) or a SED (Smoke Emitting Diode) - both are "one-time-use" by the way...;)

My chicken scratchings say that I used a 50 Ohm resistor, 2 watt rating, and a 1/2 watt, 0.7 volt drop diode. I will caveat this with the usual notes - I am not a EE, I have friends who are, and they gave me this design. In addition, the Aux battery is there primarily to provide a stable voltage during start, not to give me a long-life backup to get me to the ground (I have a standby alternator for that). Because of the voltage drop across the diode, you are never going to get a full charge on the Aux battery unless you are running for a long time at alternator voltage, so don't depend on it to run your EFIS for 30 minutes. It might....but it might not.

Paul
 
Digging through the miscellaneous parts boxes....

Ironflight said:
The current limiting resistor is in series with, and upstream of, the diode. It is simply to limit the current if, for instance, the Aux battery is drained very low. In this case, you have a large voltage difference between the main battery and the Aux, and the current can get pretty high, turning your diode into a NED (Noise Emitting Diode) or a SED (Smoke Emitting Diode) - both are "one-time-use" by the way...;)

My chicken scratchings say that I used a 50 Ohm resistor, 2 watt rating, and a 1/2 watt, 0.7 volt drop diode. I will caveat this with the usual notes - I am not a EE, I have friends who are, and they gave me this design. In addition, the Aux battery is there primarily to provide a stable voltage during start, not to give me a long-life backup to get me to the ground (I have a standby alternator for that). Because of the voltage drop across the diode, you are never going to get a full charge on the Aux battery unless you are running for a long time at alternator voltage, so don't depend on it to run your EFIS for 30 minutes. It might....but it might not.

Paul
Click to expand...

Paul,

Many thanks for the response. I'll test for the SED effect prior to installation!
 
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