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Battery backup versus standby alternator

JHartline

Well Known Member
Folks,

Considering redundancy options for a G3X panel and I would prefer to use battery backups to give me roughly an hour of functionality before the screens go dark. What that functionality would be exactly is negotiable depending on complexity, cost, and design issues. I have sent a query to Garmin to see what they have to say but wanted to poll the community and see if anyone has gone a similar route.

I want to be able to have autopilot, comm/nav/GPS/ILS and at least some external lighting. Pitot heat is on the table.

If any builders have gone this route and would like to share thoughts I would love to hear about it. Stein will be doing my panel - I'm not an avionics guy or an electrician unfortunately.
 
Ask Stein

Stein and his crew are about as experienced as you will find. One of the things I really appreciate about Stein is his candor. He is not shy about giving his opinions and explaining the pros and cons of a particular component or configuration.

Working with Stein and his folks has been a great experience and both my son and I have learned a lot so far.

For the RV-7A that we are building, we are planning a single B&C externally regulated alternator, TCW backup battery and a G5 with its own backup battery.

For our RV-10, we will very likely add a second alternator and battery - as we are planning a fully electronic SDS EI/EFI system.
 
Well my philosophy is in the event of an alternator failure I don't want to load shed while in IMC -- IOW I want all of my tools at my disposal and don’t want to make a bad day worse by tying one hand behind my back when I need it the most. As a result, my design goal was to attempt to make fuel the only endurance variable to the maximum extent possible. The idea was that I wanted to be able to provide my system's normal operating load for as long as I had fuel. There's a number of ways to accomplish this but I chose to go the 2 alternator (belt driven primary, gear driven back-up) route with a single 28AH battery. Does my setup drive the risk to zero--nope, but I'm satisfied that it's been lowered sufficiently. YMMV....
 
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I, too, go the single battery and dual alternator belt. I think there is a better chance of an alternator failing/belt breaking, than a battery failure on one that has been properly cared for. The second alternator will even get you home, as the standby alternators today can pretty much carry the whole panel.

Vic
 
Backup alternator provides a known amount of current for remainder of your flight. A backup battery will have an unknown quantity. The main battery is your backup reservoir (of unknown quantity). No need for another. My 2 cents worth and this is what I've done.

Bevan
 
Recent panel rebuild for me included G3X Touch (2), G5 and a new B&C up front belt driven and a new B&C out back on the accessory pad.
 
Backup, vacuum-pad mounted B&C alternator, single battery.

Very comfortable with that decision...get outta the goo on the backup, then either find a fix for the bad primary or wait and fly home when in VFR on the backup.

Edited to add:

I completely agree with what Todd wrote above about having the max amount of tools available if I'm battling a major systems malfunction while single pilot IFR. I'm going to stack as many chips in my favor as possible in that situation.

Among other gear...Pitot heat, VOR/ILS, Primary GPS, landing lights all need a lot of electrons flowing to function and will flatten a backup battery perhaps much quicker than I'm comfortable with/capable of getting the airplane on the ground...

Great peace of mind with the extra alternator available.
 
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Dual alternators and single battery here also, I don't want to give up capability in IMC just because my primary alternator takes a vacation. I have the B&C 20 on the vacuum pad and it will handle all normal flight loads except lights - so in the event I lose my primary alternator I simply go dark until making the approach to the airport and let the battery supply the minor additional amperage for the lights for the duration of the approach.

I also test my backup alternator every flight, or nearly every flight, for several minutes to make sure it's all happy.
 
A second battery is always a high maintenance, pay many times, decision. A second alternator is a low maintenance, pay once, route.

Before you make any decisions write down the equipment that you must have after primary alternator failure, and the stuff you would like. Next write their current draw at 12.5v and 14.5v (12.5v for the battery and 14.5 for standby alternator). Add up the numbers to find out what how much current you need to keep you from sweating. Expect to have to do a little research to find accurate numbers. Question why you must have the more current hungry devices.

Now figure how much battery is required to keep everything running (use 70% of nameplate capacity as a first guess), and what kind of stand-by alternator would be required. When you have the data it will be straight forward to make a decision.

Pete
 
Lots of stories on VAF about failed alternators. Batteries are a maintanence item that can have a low probability of failure if properly maintained. I plan a dual alternator system. I may have a small secondary battery too but my first line of defense is the backup alternator.
 
Batteries degrade (fail) slowly over time in that they store less useable energy, and yet we still fly with them never really knowing their capacity at that particular time.
 
two alternators vs two batteries

It also depends also on the type of ignition one has. I have dual lightspeed ignitions, so I prefer two batteries. I have a setup to test the backup battery (whose only function is to feed the second ignition) using a 12v lamp. This draws a known current, so I can calculate the effective amp-hrs available until it drops to some specific voltage. I test it annually.
 
There is no one answer.

I entirely agree with the dual alternator strategy for an IFR bird, and lean toward dual alt/dual battery for EFI/EI.

That said, the KIS approach to VFR with a low-draw (<1 amp) electronic ignition is a small secondary battery. KIS is good.
 
It also depends also on the type of ignition one has. I have dual lightspeed ignitions, so I prefer two batteries. I have a setup to test the backup battery (whose only function is to feed the second ignition) using a 12v lamp. This draws a known current, so I can calculate the effective amp-hrs available until it drops to some specific voltage. I test it annually.

For those with dual electronic ignitions I would encourage you to check the backup battery as often as possible, certainly more than once per year. I have seen more than one airplane come through my shop with the backup battery reading 5 volts! Sometimes it was due to a charging system failure and sometimes it was a corroded connector on the battery. Klaus has a schematic that allows you to test the backup battery prior to takeoff.

Vic
 
For those with dual electronic ignitions I would encourage you to check the backup battery as often as possible, certainly more than once per year. I have seen more than one airplane come through my shop with the backup battery reading 5 volts! Sometimes it was due to a charging system failure and sometimes it was a corroded connector on the battery. Klaus has a schematic that allows you to test the backup battery prior to takeoff.

Vic

Good point - I do check the voltage of this battery prior to startup before every flight. I also full time monitor its voltage during flight to be sure the proper charging voltage is being applied. The test I do annually is a capacity check, where I run it down to perhaps 10 volts, to determine amp-hr capacity.
 
Thank you all for the replies

I feel like the available backup-battery solutions are getting better but maybe not the ideal situation for me just yet. And the points about how to avoid getting stranded somewhere are well made. While my mission won't take me into austere locations we have all seen how deserted ramps are late on a weekend evening when we're trying to get home.

I will be talking with Stein very soon and I'm sure he will provide some great options to consider. Thanks for the input - it is always valuable and always appreciated.
 
Well done

Good point - I do check the voltage of this battery prior to startup before every flight. I also full time monitor its voltage during flight to be sure the proper charging voltage is being applied. The test I do annually is a capacity check, where I run it down to perhaps 10 volts, to determine amp-hr capacity.
Well done!

Vic
 
Klaus has a schematic that allows you to test the backup battery prior to takeoff.

Vic, got a link? Found these two (both appear to be voltage monitoring)...

http://lightspeed-aero.com/Manuals/PS_Diagram.htm

http://lightspeed-aero.com/Images/dualindicator-LSEr4.pdf

...but nothing to test battery capacity.

Re voltage monitoring, these are less than $5 delivered, and can be configured to alarm:

http://www.vansairforce.com/community/showpost.php?p=1159312&postcount=166
 
Vic, got a link? Found these two (both appear to be voltage monitoring)...

http://lightspeed-aero.com/Manuals/PS_Diagram.htm

http://lightspeed-aero.com/Images/dualindicator-LSEr4.pdf

...but nothing to test battery capacity.

Re voltage monitoring, these are less than $5 delivered, and can be configured to alarm:

http://www.vansairforce.com/community/showpost.php?p=1159312&postcount=166
Look at the dual power supply wiring diagram. By pulling the 10 amp breaker and putting the B ignition to the aux source, and watching the voltmeter, you should be able to test the aux battery. You could even run it in flight that way every once in a while.

Vic
 
Good point - I do check the voltage of this battery prior to startup before every flight. I also full time monitor its voltage during flight to be sure the proper charging voltage is being applied. The test I do annually is a capacity check, where I run it down to perhaps 10 volts, to determine amp-hr capacity.


Here's a method of very accurately determining battery capacity I used in when I was heavy into glider flying:

http://aviation.derosaweb.net/batterytest/BatteryTestingFeb2012.pdf

I flew with 2 identical SLA batteries, and this load test was a great way of discovering one that was starting to decline.

Very important in a glider (with all the electronic gizmos nowadays) that doesn't have a non-electric variometer...
 
Look at the dual power supply wiring diagram. By pulling the 10 amp breaker and putting the B ignition to the aux source, and watching the voltmeter, you should be able to test the aux battery. You could even run it in flight that way every once in a while.

Ah, voltage monitoring over an extended time. That works. I was hoping he had some sort of interesting capacity test circuit....sorta like a standard automotive resistance load tester, or the test rig in the article Rob linked.
 
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