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AUX AV Battery wiring - advice requested.

ssonixx

Active Member
Hello VAF braintrust...

I have been through Aeroelectic. I have searched on this forum. But, I could use some help from you all.

I'd like to wire in an AUX BATTERY through a 20A relay and fuse to an AVIONICS BUS. Note: I am not interested in a dual alternator set up or a completely redundant battery. My interest is really to try and keep things simple.

Questions:

1) Is it okay to run the AUX Battery circuit under normal operations? This would also allow me to charge the AUX battery at the same time, correct?

2) Any considerations for using a solenoid vs a 20A relay in this case? I am anticipating a 12-13A steady state draw on the avionics bus. The AUX Battery relay connects to the AVIONICS BUS through of a 30A bladed ATC fuse and fuse block.

3) Should I consider adding a 20A current limiter? Any reason to consider doing so?

AUX_AV_BATTERY.png


All opinions welcome! Thank you very much.

Regards.
 
Why not just remove the relay all together? That way, with no pilot action, the aux battery will feed the avionics buss when the feed from the main buss goes away. The diode between the main buss and the avionics buss will prevent the aux battery from feeding anything other than the avionics buss.

I have suggestions on what diodes and diode heat sink to use. PM if interested.

Carl
 
1. Yes, it is OK to normally have the aux battery connected.
2. A battery contactor is not necessary. Automotive relays are available in the 30 to 40 amp range.
3. Depending on the length of wires and where components are located, you might want to add a 30 amp fuse at the aux battery. There are two sources of power, the main battery and the aux battery. Both sources need to be protected against short circuits.

Keep in mind that the diode will drop about a 1/2 volt. If the charging system puts out 14.2 volts or greater, that should not be a problem under normal conditions. But if the aux battery is ever discharged, it will need 13.8 volts or more to recharge.
 
Yes, you could eliminate the aux battery relay and just have a switch, depending on the battery location. There should always be a way to shut off power at the source in case of smoke in the cockpit or an imminent forced landing. If the aux battery is located any distance from the switch, then a relay is recommended for remote control, to shut off power right at the source.
 
I had almost the same setup----minus the relay. Fuse was at the battery, and switch fed directly to the buss.

I did have a second alternator dedicated to the aux battery.

My system worked just fine for the 6+ years I flew the plane.

How do you intend to keep the aux battery charged???
 
Thank you.

Re 3: I will add an inline fuse on the battery. This is going behind the bulk head.



1. Yes, it is OK to normally have the aux battery connected.
2. A battery contactor is not necessary. Automotive relays are available in the 30 to 40 amp range.
3. Depending on the length of wires and where components are located, you might want to add a 30 amp fuse at the aux battery. There are two sources of power, the main battery and the aux battery. Both sources need to be protected against short circuits.

Keep in mind that the diode will drop about a 1/2 volt. If the charging system puts out 14.2 volts or greater, that should not be a problem under normal conditions. But if the aux battery is ever discharged, it will need 13.8 volts or more to recharge.
 
Switched auxiliary for Avionics

I was originally going to use a really tiny battery just for a few pieces of Garmin equipment for back up. But as I considered this, the price of a small battery for a subset of avionics vs a larger back up AUX AV battery for the whole avionics bus was not that big of a gap. So.... now I am down this path.

I didn?t want the avionics bus hot all the time, hence the need for a switched power.

- Allows me to flip on avionics (including emon and radios) through AUX AV battery prior to startup ? good for IFR set up.
- Keeps the power circuit to this equipment independent and stable during startup.
- AUX battery charges from the bus as long as the circuit is closed

I saw this AV BUS like an endurance bus but more appropriately, for me, called an Avionics bus since it is really just full of Garmin G3x stuff. So the AUX battery provides some time predictable go juice in the event of a ALT or main battery issue, depending on how I size an AUX LifePO battery to meet my endurance intent as a backup.
 
Since you intend to use an aux lithium battery, you might not want the voltage drop from the diode. Below is a circuit to get full charging voltage.
The E-Bus is isolated during engine starting.
enhance
 
Since you intend to use an aux lithium battery, you might not want the voltage drop from the diode. Below is a circuit to get full charging voltage.

With the "Start Enable" switch set to disable the start button, the e-bus will not disconnect from the main battery at shutdown. Eventually the relay coil will discharge the main battery. It will disconnect then, all by itself, leaving no diagnosis trace. The perfect crime ;)

Better to tell you now than have you scratching your head over a "bad battery".
 
With the "Start Enable" switch set to disable the start button, the e-bus will not disconnect from the main battery at shutdown. Eventually the relay coil will discharge the main battery. It will disconnect then, all by itself, leaving no diagnosis trace. The perfect crime ;)

Better to tell you now than have you scratching your head over a "bad battery".

I guess I saw the schematic a bit differently.

The situation you describe would only happen it looks like if the master remains on. With the "start enable" switch set to disable, it won't open the e-bus relay because the e-bus itself has no power unless the master switch is also on. So, once you kill the master switch, the e-bus relay will open.

If the master is on, then the e-bus will be provided power across the diode, and the e-bus relay will then close and allow the main battery to bypass the diode and flow to the e-bus directly and charge the AUX battery without the voltage drop across the diode.

Do I have this right?

...I like this.
 
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Small backup batteries cannot accept a high current charge, if you use a battery that can accept the full alternator output then you need to size wires, relays, diodes, etc. appropriately.
 
Do I have this right?

Dan is right. Ignore all of your schematic except for the main battery, the E-bus relay, and the start enable switch. Ignore the master solenoid completely.
Then draw the E-bus relay with the relay closed instead of opened, which is how it will be when you turn off the master switch. You've pretty much designed a latching relay.
 
Actually Joe designed it, but yep, it's latched, unless the shutdown procedure includes cycling the start enable.

15p2xkw.jpg
 
Understood. Thank you.

Small backup batteries cannot accept a high current charge, if you use a battery that can accept the full alternator output then you need to size wires, relays, diodes, etc. appropriately.
 
I modified my drawing so that the E-Bus relay can be turned on regardless of the battery contactor state.
I like to think of the switch as an E-Bus switch, not as a start enable switch. The E-Bus switch must be off
in order to start the engine. Yes, leaving the E-Bus switch on will run the battery down same as if the
master switch is left on. Bob N. has his E-Bus connected the same way. Mount the Master switch and E-Bus
switch and aux battery switch next to each other and shut all off simultaneously.
enhance
 
I modified my drawing so that the E-Bus relay can be turned on regardless of the battery contactor state.
I like to think of the switch as an E-Bus switch, not as a start enable switch. The E-Bus switch must be off
in order to start the engine. Yes, leaving the E-Bus switch on will run the battery down same as if the
master switch is left on. Bob N. has his E-Bus connected the same way. Mount the Master switch and E-Bus
switch and aux battery switch next to each other and shut all off simultaneously.
enhance

what software are you using to create these diagrams?
 
Sometimes I use AutoCad and sometimes Express.sch
AutoCad is not intuitive. Even after getting proficient at using it, it is easily forgotten.
After not using AutoCad for a year, it was frustrating trying to re-learn it.
Express.sch is free and a WHOLE lot easier to learn and use.
 
Alternator

Small backup batteries cannot accept a high current charge, if you use a battery that can accept the full alternator output then you need to size wires, relays, diodes, etc. appropriately.

To Walt?s point, what in Joe?s diagram protects the small backup battery from high alternator charge current?
 
Don't you just use a resistor on the feed from the main bus to the small battery after a diode so the small battery cannot power the main bus? Allows about 1/3 of the amp-hour rating of the small battery to charge the small battery? 1 have 1 AH SLA that gets 300 milliamps via a 50 ohm resistor. Even with a schotky diode with low forward loss, it never exceeds 13.8 volts in flight. V=IR.

My small battery is only for brownout protection at engine start of a GRT EFIS and Trutrak Gemini PFD. The Gemini has two input paths, both with diodes allowing power only in from the small battery or the essential bus, whichever has higher voltage. The GRT has 2 power inputs and internal diodes, 1 the essential bus, the other the small battery.
 
Read THIS THREAD about the need for putting a resistor in series with an aux battery. Search the thread for "aux battery".
A resistor is not desired in the aux battery circuit. Even one ohm is too much. Supposing that the load on the aux battery is 10 amps.
10 amps x 1 ohm = 10 volts. If 10 volts are dropped across the resistor, how many volts are left to power the E-Bus?
 
So does the aux battery need to also be capable of handling the alternator output as Walt mentioned?

I?m asking but I am using an Etx16c as an aux battery which is not capable of handling the alternator current at max output so I have it diode isolated and and run through a breaker switch for protection. I have no way to charge it other than connecting a charger on the ground periodically. I would rather a circuit that would charge it as well if possible.
 
SLA batteries charge well at a current of 1/3 their AH rating. The resistor cuts the current to that value without robbing excessive charging voltage, in my simple application. There are more efficient ways to do so, but I think the primary idea is to not charge a battery without a current limiter, especially if it has been significantly discharged.

The cited Matronics thread is actually one I read in deciding on my setup. It is not the most efficient in delivering charging voltage but it has been effective in stopping brownouts during engine start. It just needs to push an amp at 12 volts for under 30 seconds.

So far I swap it out every other annual. Takes 30 seconds and costs under $20. No OEM branded backup, TCW, VPX or fancy stuff for me.
 
The size of the alternator does not determine the current through a load.
The load resistance determines current. When batteries are charging, they
are loads and the battery determines the charging current. The charging
current into a small aux battery is limited just like current into the the main
battery is limited. And that is by the battery's voltage and internal
resistance. If the battery is discharged down to 11 volts, then the charging
voltage is 3.5 volts (14.5 -11). A 60 amp alternator does not force 60 amps
of charging current into the main battery because the battery limits the
current. Same with the aux battery. Yes, if the aux battery is completely
dead, the charging current will be high. I agree with Walt that circuit
components need to be sized appropriately. If it is desired to limit the
charging current, then put a diode and resistor in parallel. The diode will let
full current out of the battery and the resistor will limit charging current into
the battery. But like Bob Nuckolls said, 99.9 percent of the time it is not
necessary to limit charging current. Have you ever completely run down a
battery, then used jumper cables to start the engine? How did you limit the
charging current? I would not worry about it. If you are really concerned,
read the battery manufacturer's datasheet.
 
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I like to think of the switch as an E-Bus switch, not as a start enable switch. The E-Bus switch must be off in order to start the engine. Yes, leaving the E-Bus switch on will run the battery down same as if the master switch is left on. Mount the Master switch and E-Bus switch and aux battery switch next to each other and shut all off simultaneously.

Ahhh. It makes sense when labeled as an E-bus switch operated as described.

Many folks wire a "start enable" switch as a safety and anti-theft device, often hidden in some out-of-sight cockpit location. It's mandatory with a start engage button on the stick grip. The prop can't be swung with the starter unless the start enable is closed, and to do that, the uninvited pilot has to find the switch. Said switch is opened after cranking.

This relay system makes the starter button live if the master alone is closed, but has no other downside I can see. It would be fine with a stick grip switch installation; three grouped switches ON disables the start function. I like it.
 
Thank you for the review, Walt and Joe.

If your main battery is SLA/AGM and your backup is another chemistry like LiFePO4, you don't connect them to the same bus at the same time because they have different internal resistance, right?
 
The size of the alternator does not determine the current through a load.
The load resistance determines current. When batteries are charging, they
are loads and the battery determines the charging current. The charging
current into a small aux battery is limited just like current into the the main
battery is limited. And that is by the battery's voltage and internal
resistance. If the battery is discharged down to 11 volts, then the charging
voltage is 3.5 volts (14.5 -11). A 60 amp alternator does not force 60 amps
of charging current into the main battery because the battery limits the
current. Same with the aux battery. Yes, if the aux battery is completely
dead, the charging current will be high. I agree with Walt that circuit
components need to be sized appropriately. If it is desired to limit the
charging current, then put a diode and resistor in parallel. The diode will let
full current out of the battery and the resistor will limit charging current into
the battery. But like Bob Nuckolls said, 99.9 percent of the time it is not
necessary to limit charging current. Have you ever completely run down a
battery, then used jumper cables to start the engine? How did you limit the
charging current? I would not worry about it. If you are really concerned,
read the battery manufacturer's datasheet.

Joe, I would suggest you do some additional research on this topic before posting, your advice above borders on dangerous.

Ever notice your ammeter after cranking for a while, the current into an even moderately discharged battery is very high and is only limited by the alternator capacity. Modern AGM and Lithium batteries have very low internal resistance and if discharged will attempt to take everything the alt can give. Try charging a small AH battery at 60 amps and the battery will self destruct. Every battery has a maximum charge current limit it can handle safely without overheating, and a recommended charge current for maximum life.

The "normal" batteries we use in our cars and airplanes (PC680, Concord etc) can all handle the output from a "normal" 60amp alternator so you can jump your car airplane without fear.
Have you ever felt how hot a battery gets when being charged at this maximum rate?

Thinking because its just a back up battery so it should never get fully discharged would be a big mistake.
 
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Wow

I?ve been following this discussion and wonder how we got to this point of complexity, and frankly very different camps on care and feeding of batteries.

For the builders out there now pulling their hair out on what to do, I suggest a calming review of battery specifications before adopting anything design action.

Carl
Two PC-625 batteries, two avionics busses and one 60 amp alternator - flying for 16 years in 4 RVs and soon the new RV-8 project.
 
From my original post, I'm still trying to figure out if one can simply charge a smaller AUX battery that has a "max charge amps" design limitation at say 20A from a power bus. And, whether a current limiter can accommodate the deficiency.

It seems using a matching pair of batteries was the recommended approach from Bob's book. His justification for it was to enable an easier annual management cycle of the main battery.

My goal is candidly a bit simplistic. I'm just trying to just get an small(er) AUX battery with known endurance to be available on my avionics bus.


I?ve been following this discussion and wonder how we got to this point of complexity, and frankly very different camps on care and feeding of batteries.

For the builders out there now pulling their hair out on what to do, I suggest a calming review of battery specifications before adopting anything design action.

Carl
Two PC-625 batteries, two avionics busses and one 60 amp alternator - flying for 16 years in 4 RVs and soon the new RV-8 project.
 
. SNIP
My goal is candidly a bit simplistic. I'm just trying to just get an small(er) AUX battery with known endurance to be available on my avionics bus.

Sure - but perhaps go back to your objective for why you want to do this instead of just figuring out how to implement the one solution you selected. What are you using for a main battery? Many of the aux/main battery setups I?ve seen have no weight savings over two PC625 batteries, and forgo the capabilities offered by two such batteries.

Carl
 
If you're not willing to the work to develop your own back up battery system, then I suggest you spend the money for a TCW backup battery and be done with it.
 
Last I checked, the tcw battery does not have enough capacity to suit my needs. I’m not sure how you got 5he impression anyone was not willing to do the work. On the contrary, this is part of the process.
 
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Originally Posted by ssonixx

My goal is candidly a bit simplistic. I'm just trying to just get an small(er) AUX battery with known endurance to be available on my avionics bus.

You and me both....

Last I checked, the tcw battery does not have enough capacity to suit my needs. I’m not sure how you got 5he impression anyone was not willing to do the work. On the contrary, this is part of the process.

Well you said you wanted a simple solution, apologize if I got that wrong, the TCW option is a "simple" solution.

You can design your own constant current charger or use the rather primitive diode/resistor network. After using the diode/resistor myself for a while I was never really happy with that charge method so I opted for a "Sterling" battery maintainer. Did a bunch of bench testing on it and was satisfied with its operation so installed one in the RV, been working great for the last 2 years.
It keeps the backup battery (EarthX12AH) topped up and won't overcharge it.
http://www.sterling-power-usa.com/sterling-power-usa12vto12vbatterymaintainer.aspx

There are other chargers out there similar to the Sterling but I have no experience wth them.
 
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I appreciate your guidance.

But I hope my questions here are not implying that I am NOT willing to work. On the contrary, I enjoy working. In fact, I have been trying to understand the fundamentals rather than just adopting SGOTI (some guy on the internet) designs. I have looked at TCW and if that is the only solution then I guess I have no choice but I?d prefer to better understand the trade offs before I have to make that decision.


If you're not willing to the work to develop your own back up battery system, then I suggest you spend the money for a TCW backup battery and be done with it.
 
Well you said you wanted a simple solution, apologize if I got that wrong, the TCW option is a "simple" solution.

You can design your own constant current charger or use the rather primitive diode/resistor network. After using the diode/resistor myself for a while I was never really happy with that charge method so I opted for a "Sterling" battery maintainer. Did a bunch of bench testing on it and was satisfied with its operation so installed one in the RV, been working great for the last 2 years.
It keeps the backup battery (EarthX12AH) topped up and won't overcharge it.
http://www.sterling-power-usa.com/sterling-power-usa12vto12vbatterymaintainer.aspx

There are other chargers out there similar to the Sterling but I have no experience wth them.

That?s an interesting idea. I was looking at the tecmate option for that as well, which advertises it as lithium capable. They switch to standby mode at 12.8v so I was assuming they would not top off the battery. Thanks for your confirmation that a dc to dc Charger is capable of topping off the EarthX battery.
 
Walt, I see nothing dangerous about my aux battery circuit posted above.
There is a fuse at each end of the wire that connects
the E-Bus and Aux battery. If the current gets too high, a fuse
will blow. That is not dangerous. The E-Bus still has two supply
current paths. I based my previous statements on Bob Nuckolls' postings
and I provided a link to those. Here is quote from Odyssey Publication No: US-ODY-TM-AA
September 2016. Notice that it says "ALL" Odyssey batteries.
Size does not matter. Also note that 14.7 volts is higher than most aircraft
electrical systems. If the voltage is lower, the current will be lower.
All ODYSSEY batteries can be quickly charged. Figure 7
on the next page shows their exceptional fast charge
characteristics at a constant 14.7V for three levels of inrush
current.
My previous posting were based on using an AGM battery.
So I took your advice and researched Lithium batteries. You are right and
I was wrong about Lithium batteries.
According to https://earthxbatteries.com/faqs, the maximum current charge
rate is 5 x true amp hour. So for a true 7.8 Ah EarthX ETX24C Lithium battery (Weight 2.6 lb.),
the maximum charge current should not exceed 40 amps. If a ETX24C Lithium Aux
battery is fused at 30 amps, it will be safe. The pilot should be informed that
allowing the Lithium Aux battery to completely discharge could result in a
fuse blowing once the charging system is operating.
 
Found this on EarthX website for dual buses with their (presumably) smaller aux batteries.

https://earthxbatteries.com/dual-bus-lithium-battery-design

Dual bus lithium battery design. Main and Essential Buses with dual batteries

Advocating the use of a forward drop diode between buses.

That?s a good find. Thanks for sharing. It is Interesting to see the relationship between voltage and amperage. I guess it doesn?t really matter if the diode is between the busses or between the bus you would charge off of and the battery to be charged.
 
I modified my schematic for using a Lithium Aux battery. A Schottky diode in
series with the aux battery will reduce charging voltage by 0.4 to 0.75 volts
depending on charging current (0 to 40 amps). The diode will drop more
voltage as the current increases. The diode is recommended by this EarthX Document
to limit charging current for smaller batteries. The diode is
not needed if a lead acid battery is used. Criticism and comments are welcome
to point out any design flaws. I am not necessarily recommending this
architecture, just offering it for those who want it. I believe in keeping the
electrical system simple. Dual diode part number: 78-VS-QA250FA20
Singe diode part number: 747-DSA300I45NA
enhance
 
Joe,

If the only reason you have the 5AH battery is for engine start brown out, then disregard this note. The simple option for brown out is just start the engine with the panel off.

If you want it to keep the panel alive after an electrical fault, I recommend you translate what a small 5AH battery does for you if that is what you fly on.

For most panels, 5AH may be 30 minutes, at best.

In other words, if you decide on going with two batteries you might want to consider other design approaches that gain you more capability.

Carl
 
Carl,
Thanks for your comments. I agree with you. Yes, the purpose of the small battery is for preventing brownout during engine cranking. The circuit is not for me but for those who want a brownout battery. My plane just has one battery. I routinely leave all avionics turned on during starting. The brownout doesn't hurt anything. It is just annoying. My EFIS has its own internal backup backup battery. So it does not reboot.
 
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