Avionics Standby Power with Battery Charger

[jackking123]

OK don't get mad. Ha ha. I know this is beat to death, but some schemes, complicated, many alternators, batteries, panel switches, relays. Others on battery and a diode. OK. None of them have a battery charger? What? I will explain but look at my 4 options: (I want opinions and personal experience, but be nice )

If you think this is dumb idea or beat to death, OK. What did you do. Look it over and think about it before poo-poo'ing it.

1 thru 4:
1) no back up except ships battery. Why not? If the alternator fails that will get you on ground? (G5 has on board battery, handheld Com & iPad's internal battery)
2) parallels ship/backup battery with diodes. Backup battery has BMS (Earth X).
3) parallels ship/backup battery with diode and charger.
4) Serial avionics powered only by standby/avionics battery. Main bus charges backup battery. (Option, emergency bus tie, switch or CB pulled with collar that can be pushed in.)

Notes: I know diodes have 0.7v drop but can be much less if you "oversize" them. Most devices run at 11v or less. Batteries will still charge if input voltage is above it's nominal 100% voltage. The more headroom the more current the battery takes. If too much voltage or prolonged periods of higher voltage, it can damage battery.

My avionics or emergency avionics: 2 EFIS: G5, GRT EFIS (and all their extra items, ARINC429 modules, magnetometers). engine monitor system, Com Radio. Transponder and ADS-B in not on standby bus. Not done a formal analysis, but estimate well under 5 amps, no transponder, VHF Com RX only.​

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Rational and HOW to get a Battery Charger On-Board?

Why battery charger? Airliners use battery chargers to charge the batteries. It is a MEL item, no go without charger working. In our GA world we let the alternator blast the main battery with full rated voltage. May be why batteries don't last, heat, vibration and over charging. I suggest BATTERY CHARGER to isolate the standby battery from overvoltage, keep it charged, under use, while extending battery life.

Finding a DC-DC battery charger? RV's industry makes them. Problem is input voltage has to be higher and they drop-out with low voltage. These could be OK off the shelf solution. A 20 amp unit is $100.

Another cool device, not expensive, works well, DC-DC boost/buck converters that has steady voltage output, regardless on input voltage (within limits). You would adjust output to be at moderate or "trickle charge" rate. This may or may not keep up with all avionics load, if all avionics is powered only by standby battery. So you need to parallel with main bus but that demands (diodes. May be diodes are great, fine, dandy? Opinion or experiences?

Best solution may be using an inverter (12v DC to 110v AC). Inverter doesn't have to be high wattage, small, light, inexpensive. Now that you have 100W or 200W of AC 110V, you can plug in a AC battery chargers, many to chose from with a lot of smarts tailored to AGM, Li-Ion. You can go with bigger inverter and get 110V AC plug in your plane for laptop charging? The 110V AC may be handy for a O2 concentrator for flying at altitudes. These are made for home, not too much and saves filling tanks? .
#2 With a LiFePo4 using internal BMS to regulate the charge seems best. The cost about $180. The AGM may do fine charging via a diode only without a charger direct from main bus. It may under charge or overcharge with out danger, except short battery life. Cost is small so just replace battery every year.

 

[Mich48041]

I pick circuit number 2 with the avionics switch deleted and replaced with wire. Simpler is better.
If the avionics switch is not installed, then it can not fail.
The backup battery will prevent avionics brownout while cranking the engine.
Alternators put out 14.5 volts or less. That is not over voltage.
The diode will drop between 0.4 and 1.4 volts depending on the type of diode and the current through it.
The current through the diode will keep the aux battery charged. No need for a separate charger.
Diodes rarely fail if mounted to a heat sink with heat conductive paste.

 

[Carl Froehlich]

Hard to respond without knowing the mission this setup will support. Day VFR only or night IFR? Electrically dependent engine?

Depending on the battery, it may not do well running on the lower voltage output from a diode.

For non electrically dependent engine aircraft my rule of thumb is 12 amp of load for IFR flight (no external lights, pitot heat or heated seats/vests). For IFR I design for 2-3 hours of full panel IFR flight following the loss of the alternator(s).

Keep in mind the battery (if properly maintained and not abused) is the most reliable element in our aircraft power systems. The loss of an alternator is an inconvenience, but not an emergency assuming the battery(s) has the capacity to meet the mission. Some failures are much less probable but the outcomes are far more severe. For example the failure prevents power from the good battery(s) getting to where you need it. I consider this the Achille’s heel of many designs.

Simple examples:
- Failed master switch
- Failed avionics master switch
- Failed ground path

Backup batteries that are now common with many EFIS system are fine, if your mission is supported by how long they work.

Carl

 

[1bigdog]

Hard to respond without knowing the mission this setup will support. Day VFR only or night IFR? Electrically dependent engine?

Depending on the battery, it may not do well running on the lower voltage output from a diode.

For non electrically dependent engine aircraft my rule of thumb is 12 amp of load for IFR flight (no external lights, pitot heat or heated seats/vests). For IFR I design for 2-3 hours of full panel IFR flight following the loss of the alternator(s).

Keep in mind the battery (if properly maintained and not abused) is the most reliable element in our aircraft power systems. The loss of an alternator is an inconvenience, but not an emergency assuming the battery(s) has the capacity to meet the mission. Some failures are much less probable but the outcomes are far more severe. For example the failure prevents power from the good battery(s) getting to where you need it. I consider this the Achille’s heel of many designs.

Simple examples:
- Failed master switch
- Failed avionics master switch
- Failed ground path

Backup batteries that are now common with many EFIS system are fine, if your mission is supported by how long they work.

Carl

I very much agree with Carl. You are not clear on the mission. His case and my case are IFR so the panel and the power buses were structured to support an extended period where a backup as you propose might get exhausted.
In my case I used a separate gear driven alternator so loosing the first alternator is not an issue and there are no limits on time. Hope that help.

 

[jackking123]

I very much agree with Carl. You are not clear on the mission. His case and my case are IFR so the panel and the power buses were structured to support an extended period where a backup as you propose might get exhausted.
In my case I used a separate gear driven alternator so loosing the first alternator is not an issue and there are no limits on time. Hope that help.

I realize this is CRAZY... Ha ha. I have thoughts and VAF Forum members are great for kindly correcting me and dispelling these thoughts. Charger is out.

So question.

What back up battery 5 Ahr i+/- 1Ahr. LiPoFe4 with BMS. AGM. I like AGM primarily for the lower chance of it turning into a mini Chernobyl. Likely behind panel near bus bar and avionics. I know chance is rare but no LiFePo4 in cockpit (except phone, iPad, ELT, Garmin g5 backup battery, Doha! Ha ha.).

What Diode? I was told look at digikey or mouser and get a 100amp job for less Fwd Volt loss. The diagram below, will this drain the backup battery? I do love the idea of NOT having another switch. I have seen some horrendously complexed power systems in EAB's with switches, relays, dual everything. That is great.




Mission? To keep the sky's free from tyranny for Peace, liberty and pursuit of happiness. Ha ha. Could not resist. Acro, XC and IFR and may be if I dare give transition training (LOD). I'm and ATP and a CFI/II/ME and active. I teach for fun part time at a flying club. Have one IFR student now, Flt Reviews, IPC's. LPV, terminal enroute GPS, only. We already had an epic discussions in another thread. flying with a GPS175 only for IFR. No need to repeat. My hail Mary backup is Garmin G5 with it's own backup power. I want to also have (with back up battery) GRT EIS (Eng Monitor), GRT 10.1 EFIS (and it's ARINC 429), G5 (and its' GMU11, GAD29), GPS175 , Com Radio. Autopilot and Transponder will be on main bus. Ignition x2 P-Mags, also on main bus (or may be backup)

My backup to backup is iPad and handheld transceiver. The later is a poor substitute for COM. I have tested it for Com. It also does VOR/LOC ok. Two words "Personal Limits". Not flying in IFR night to mins in convective weather. I will pick my conditions to avoid needing an ILS to Cat 1 mins and squeaking in to 100' HAT with ALS.

 

[bbaggerman]

The diode will drop between 0.4 and 1.4 volts depending on the type of diode and the current through it.

Use a Schottky diode. It has a lot less voltage drop.

 

[Desert Rat]

I went basically with #2. I have a EarthX 12 amp hour main ships battery, an IBBS 3 Amp hour Backup and a G5 with an internal battery.

I have a full Garmin suite with two 10" screens and the other stuff you'd expect from a solid IFR platform. My total electrical load with everything on including pitot heat is 27 amps. I can load shed down to about 10, which gives me more than 90 minutes until the main ships electrons run dry plus whatever I then have on the G5. If I cant find somewhere to land within 2 hours of an alternator failure then I probably shouldn't be flying.

 

[Mich48041]

Your revision to circuit number 2 will work, but all diodes have reverse leakage current that could possibly run the aux battery down over time when the aircraft is not flown for awhile. Check the diode specifications. You want low forward voltage drop and low reverse leakage. Or you could put the switch back where it was in circuit 2, version #1, and not worry about reverse leakage current. Avionics don't draw a lot of current. A 100 amp diode is larger than necessary.

 

[Walt]

I use and recommend the TCW backup battery, available in 3 or 6 AH sizes. Well designed and proven service history.
Sure, you can roll your own for less, but unless you use some kind of current limiter for charging then you may be abusing the smaller BU battery.

 

[Mich48041]

In case of an imminent forced landing or smoke in the cockpit, The pilot should have the ability to shut off all sources of electrical power as close to the source as possible, that is unless the reason for a forced landing is running out of fuel. Relays can be used for remote control.

 

[DanBeadle]

I realize this is CRAZY... Ha ha. I have thoughts and VAF Forum members are great for kindly correcting me and dispelling these thoughts. Charger is out.

So question.

What back up battery 5 Ahr i+/- 1Ahr. LiPoFe4 with BMS. AGM. I like AGM primarily for the lower chance of it turning into a mini Chernobyl. Likely behind panel near bus bar and avionics. I know chance is rare but no LiFePo4 in cockpit (except phone, iPad, ELT, Garmin g5 backup battery, Doha! Ha ha.).

What Diode? I was told look at digikey or mouser and get a 100amp job for less Fwd Volt loss. The diagram below, will this drain the backup battery? I do love the idea of NOT having another switch. I have seen some horrendously complexed power systems in EAB's with switches, relays, dual everything. That is great.


View attachment 74937

Mission? To keep the sky's free from tyranny for Peace, liberty and pursuit of happiness. Ha ha. Could not resist. Acro, XC and IFR and may be if I dare give transition training (LOD). I'm and ATP and a CFI/II/ME and active. I teach for fun part time at a flying club. Have one IFR student now, Flt Reviews, IPC's. LPV, terminal enroute GPS, only. We already had an epic discussions in another thread. flying with a GPS175 only for IFR. No need to repeat. My hail Mary backup is Garmin G5 with it's own backup power. I want to also have (with back up battery) GRT EIS (Eng Monitor), GRT 10.1 EFIS (and it's ARINC 429), G5 (and its' GMU11, GAD29), GPS175 , Com Radio. Autopilot and Transponder will be on main bus. Ignition x2 P-Mags, also on main bus (or may be backup)

My backup to backup is iPad and handheld transceiver. The later is a poor substitute for COM. I have tested it for Com. It also does VOR/LOC ok. Two words "Personal Limits". Not flying in IFR night to mins in convective weather. I will pick my conditions to avoid needing an ILS to Cat 1 mins and squeaking in to 100' HAT with ALS.

I rolled my own, but I am also an electrical engineer.

The heart of the system is a solid state load switch. It has a .1 drop when turned on. Hardware logic controls the switches - no software required. If the main Avionics Bus source drops below 12volts, the LiFePO4 battery is switched in.

All of my avionics work from 10V to 32V. I run the avionics bus at 16V using a DC-DC step up. Step up the main avionics bus voltage with a 9-32 Volt step up to charge the backup battery. The battery capacity is enough to supply a light IFR system - 2 PFDs, ADSB I/O, Com, etc. For 2-3 hours.

There are a couple of rare kinds of failure that are tough to get around.
1. A short on the wiring between the box and the avionics. I mitigate this in my design with 6 output “electronic Circuit Breakers”. If one of the feeds short out, the remainder of the feeds continue to supply power

2. The box itself can fail. The key mitigation is to use “dumb”electronics - All power steering is done in hardware and is partitioned.

3. The charging logic is controlled by a single dumb chip. It is designed to charge the LiFePO4 battery and is current limited to 5A of charging.

Of course, as an engineer, I had to throw in a micro computer. It has no only one control output, but a lot of inputs. It collects data of real-time current use on each of the channels and forwards that to my GRT EFIS. It is fully integrated with the GRT to display the status (Current, voltage and CB status) to the GRtT. It also displays on the JPI EDM. The only output is to control the circuit switch.

 

[Walt]

The above Sounds like a TCW backup battery and VPX all rolled into one.
Personally I don’t see how all your complexity made a simple system more reliable?
What advantage does running DC-DC voltage step up to 16v have over standard system running at 14.5 volts?

 

[DanBeadle]

The above Sounds like a TCW backup battery and VPX all rolled into one.
Personally I don’t see how all your complexity made a simple system more reliable?
What advantage does running DC-DC voltage step up to 16v have over standard system running at 14.5 volts?

The key thing is to fully top off the backup battery, it needs about 15V for the charger to bring the battery to 14.5 v

But, engineers do tend to over-engineer.

A requirement leading to complexity is a smooth fail over from the main avionics feed to the backup.

I have not analyzed TWC, but it would not surprise me to have a design with a lower voltage battery and a boost regulator to the desired switchover voltage. That kind of system can work well, but I wanted a 2 hour backup with an off-the-shelf EarthEx battery and the appropriate charging for it.

 

[kswanson]

I am basically running #1 for simplicity. G3X VFR day/night VFR only, 2axis AP. LED everything, EarthX battery. I am a KISS guy, if anything ever stops working the way it is supposed. I put it on the ground and deal with it on hard surface. I can shed everything non essential off the main buss. Main leaves me with a single g3x g5 Aera 660 and transponder (less than 4 amp draw, less than 3 if g5 and 660 are running standalone). Absolute worst case I am NORDO 7700 to the nearest airport. As I see it, on a fresh 12 amp hour battery that should be plenty. If I added a backup battery it would be another etx680c. They are a little less than twice the weight of the 6a tcw units (still less than 4 pounds) and twice the amp hour for less doll hairs.

 

[Carl Froehlich]

I am basically running #1 for simplicity. G3X VFR day/night VFR only, 2axis AP. LED everything, EarthX battery. I am a KISS guy, if anything ever stops working the way it is supposed. I put it on the ground and deal with it on hard surface. I can shed everything non essential off the main buss. Main leaves me with a single g3x g5 Aera 660 and transponder (less than 4 amp draw, less than 3 if g5 and 660 are running standalone). Absolute worst case I am NORDO 7700 to the nearest airport. As I see it, on a fresh 12 amp hour battery that should be plenty. If I added a backup battery it would be another etx680c. They are a little less than twice the weight of the 6a tcw units (still less than 4 pounds) and twice the amp hour for less doll hairs.

I note you are assuming the only failure you will ever have is a loss of alternator. For a day VFR plane I agree this is reasonable. I’ve seen day VFR planes land with a dark panel because the power to it was gone - and not because they lost the alternator.

This discussion is not applicable to electrically dependent engines.

Carl

 

[kswanson]

I’ve seen day VFR planes land with a dark panel - and not because they lost the alternator.

Well I have never seen a glass failure. In fact, the last airplane that tried to kill me had no glass in it at all. It was a vacuum system failure in night IMC in our m20k which caused the kap140 to roll us over when the attitude tumbled. For me, glass wins every time, and now that mooney is all glass as well.

Simple bus layouts with the ability to shed amp draw quickly for nonessential flight items is the way to go. I wouldn't concern myself with inline battery chargers or anything of that nature. I don't understand fully how VPX works but from the little I understand about it, that would probably get my vote if I had to redo my entire system tomorrow. In the example above #2 would get my vote if a second battery was a personal requirement.

 

[Avanza]

Try to keep the electric system simple. Don't invent the wheel again.
If you have a single failure of some sort, it should not require turning on/ off switches.
A low voltage alarm is required.
If flying IFR or with electric dependent ignition/ fuel you should consider a back up alternator that picks up the load automatically.

Good luck

 

[Carl Froehlich]

If you have a single failure of some sort, it should not require turning on/ off switches.
If flying IFR or with electric dependent ignition/ fuel you should consider a back up alternator that picks up the load automatically.

Agree on backup modes, the first being continued IFR flight with no pilot action after a single worst “probable” failure takes out half the panel (e.g. EFIS #1, Comm #1). On my planes most such faults can be restored with minor pilot action (a single switch to move to another designed backup mode).

I strongly disagree that just a back up alternator is satisfactory for electrically dependent engines. This makes the big assumption that the only possible failure is the loss of a primary alternator. I would not fly that plane.

Carl