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Submitted By:   terryml5c2p6@sympatico.ca
Email List:   AeroElectric-list,RV-List,RV7-List,RV8-List,RV10-List,RV9-List
Name:   

Terry McMillan

Date:   

Oct 6 2008

Subject:   Dual Battery System
Description:   

This is a comment on the design/operation of Bob Nuckoll's Z19 electrical system, and a proposal for an alternative system. It offers a fairly in-depth comment on a popular electrical system design by Bob Nuckolls, plus a possible alternative approach.

There have been a few posts in the last couple of weeks regarding variations on Bobs Z19 single alternator dual battery system. Although I am going dual battery/dual alternator, I had a look at Z19 to see if there are any significant features not found in the Z14 dual/dual design. There are a couple of points I noted on Z19 that I think need revision or clarification.

With the fuses shown, a fault on the Nav/Com feed off the Endurance bus would likely take the whole bus with it. The fuse supplying the bus needs to be a higher rating than any circuit on the bus so that the first fuse to blow will be the feed out of the bus. The main bus feed and the Nav/Com circuit are both 7A. Nav/Com needs to be <7A or the bus feed >7A.

The ENG BAT switch wiring looks odd to me, a single pole 3 position switch would do the job and be more reliable than 2 poles in series as shown, though I expect this is because the switch specified doesnt bring out the centre position to an external terminal. A final minor point is that there is no way shown to control fuel and ignition (ECU) independently, and for most aircraft engines at least you usually have an electric boost pump only as a backup to the mechanical pump, for starting, takeoff and landing. A separate fuel pump switch would take care of this, and simplify servicing and troubleshooting.

On a different note, one thing I have found to be a bit problematic for me in understanding the Z diagrams is that there is usually no explanation of what would be the operational sequence for the more complex switchgear, especially in failure/emergency modes. As recently as a May 24 post this came up. Also, it takes a while for me to sort out the physical aspects of the designs, specifically which side of the firewall various components are. Im an electrical engineer so I can usually reverse-engineer the intended operation sequences after I figure out how the whole thing works, but from the level of questions I see on the forum it is obvious electrical design is a bit of a black art to a lot of people. I suspect a few sentences on typical operation sequences for at least the more complex designs would help people more quickly understand whats going on. Maybe this kind of info already exists somewhere and I just havent seen it yet.

Re the physical configuration issues, I know that one of the challenges is that there is no hard standard on where most of the components are located, especially the battery. This is important because its good design practice to try to minimize firewall penetrations, as this is a potential failure point. Also, it can add significant time to the drafting process when you try to arrange stuff so the physical relationship to the firewall- or things like rear-mounted engines- are more obvious. Its not realistic to expect universal designs to accommodate this, but my attached drawing of a variation of Z19 shows how a customized layout can clarify this.

The last thing Ive noted is the number of switches in some of the designs. Theres no question that the Z drawings work as intended, but there is inherent in some of them the need to perform switching in specific sequences to obtain the desired outcome. This is fine for commercial aircraft in which the pilots use the stuff on a daily basis plus they get regular training on emergency procedures, backed up by detailed emergency checklists. For part-time pilots like most homebuilders, not to mention that a large percentage of us are older with less-than-pristine memory circuits, I feel that if any custom mods are made to these designs, it would be time well spent to see if the controls could be simplified. A post on May 18 specifically asked for suggestions on this point. I fully agree with Greg Richters comments on this subject- heres a brief excerpt from his excellent Aircraft Wiring for Smart People writeup found at his Blue Mountain Avionics website (http://bluemountainavionics.com/download.php#appnotes) My thought is simple: No manual overrides, cross-feeds or other Apollo-13-wanna-be switches in the electrical system.The first task in an emergency is to Fly The Airplane, which is probably a better idea than trying to remember how to bring another alternator on-line and cross feed your essential bus from a backup system while not blowing your remaining breakers. This sort of thing can be made automatic by simply designing for it, so theres no need for the manual overrides. Again, my attached drawing shows one way to do this.

I suspect that the idea of a separate essential bus and intricate switch networks reflects design practice common to large complex commercial aircraft with heavily loaded electrical systems. However, few homebuilts have radar, stormscopes, electric prop deicers, large on-board entertainment systems, etc. My glass-cockpit RV power analysis shows a demand of less than 10 amps (including LED nav lights but not the FADEC) if the strobes are turned off. This will last a long time on one battery, let alone two. Im not convinced that the complexity and pilot workload of a multi-bus system is warranted in a majority of homebuilts.

There are several concepts in the attached drawing that I developed for a more detailed design that I will be using on my FADEC-equipped RV7. Specifically, the starter solenoid is connected directly to the battery rather than via the master relay, the master relays are smaller devices located in the cockpit at the other end of fuse-protected feeds from the battery, and there is only one main bus, fed via isolation diodes from the two battery sources. I have previously posted that design looking for critiques, but perhaps because of the level of specific detail, down to things like annunciator LEDs in the control rocker switches, it was a bit intimidating to most people. To make this one more approachable I have stripped out a lot of detail and am using terminology and symbols from the Z19 design.

Also, per my own suggestion above, I am including a basic listing of the pilot inputs required for a number of routine and emergency situations, for both systems. Here they are:

Z-19 Dual Battery Single Alternator System (see http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z19m_1.pdf and http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z19m_2.pdf

Preflight
Engine Primary Power ON, check Main Battery voltage
Engine Primary Power OFF, Engine Secondary Power ON, check Secondary Battery voltage

Starting
Engine Primary Power ON
DC Power Master to Bat + Alt position
Fuel Pump switch to ON
Engine Secondary Power OFF (if 2nd battery used for avionics keep-alive circuits) or:
Engine Secondary Power ON ( if required for extra starting power)

Taxi, Flight
As Starting, except ENG BAT to AUTO
If electric fuel pump is only for boost to mechanical pump during start/takeoff/landing, turn it OFF at other times

Alternator Failure
LV Alarm light comes ON, Engine Battery Contactor drops out.
Pilot sets Essential Bus Alternate Supply switch to ON, DC Power Master to OFF
Essential Bus circuits are now powered from Main battery. All circuits on Main Power bus go dead, engine electronic ignition and fuel pump run off Engine battery
Pilot makes regular checks of individual battery voltages. If engine battery voltage gets too low relative to main battery, or engine begins to run rough because Engine battery is failing, turn Engine Secondary Power to On and Engine Primary Power to OFF. Essential Bus and engine will now be powered from Main battery.
To bring back any circuits on Main Bus (like landing lights) pilot again checks main and engine battery voltage levels: if Main battery has best voltage, sets DC Power Master to BAT position, if Engine battery has best voltage, sets Eng Bat switch to ON

Short Circuit on feed to Main Bus
LV light, smoke and sparks warn pilot of problem, Engine Battery Contactor drops out
Pilot turns DC Power Master to OFF and Eng Bat switch to OFF as quickly as possible, to prevent fire and/or damage to other circuits like fuel/ignition power in same firewall penetration.
Main Bus and Essential bus circuits go dead, engine ignition and fuel pump circuits remain on line
Pilot turns E-Bus Alt Feed to ON, essential bus circuits come back on line, some avionics may need to go thru reinitialize sequence.
Pilot makes regular checks of individual battery voltages. If engine battery voltage gets too low relative to main battery, or engine begins to run rough because Engine battery is failing, turn Engine Secondary Power to On and Engine Primary Power to OFF. Essential Bus and engine will now be powered from Main battery.
Main Bus circuits will not be available until repairs are made

Open circuit on either battery
Problem will not be evident until next preflight check

Short circuit on either battery
Battery short circuit usually occurs when deposits at bottom of a cell short that cell, but this will normally affect only one cell. Result is a 1/6th drop in battery voltage output, which will show up on preflight check. If it occurs in flight the alternator will overcharge the remaining cells causing electrolyte loss and excess heating but the problem will not be evident until the next preflight check.

Revised Z19 System
See the link to the PDF file at the end of this post

Preflight
DC PWR MASTER to BAT, check Main Battery voltage
DC PWR MASTER OFF, push ENG BAT TEST, check Secondary Battery voltage (press V Check pushbutton)

Starting
DC PWR MASTER to BAT + ALT, Electronic Ignition ON, Fuel Pump ON
ENG BAT OFF (if battery used for avionics keep-alive circuits) or:
ENG BAT ON ( if required for extra starting power)

Taxi, Flight
As Starting, except ENG BAT to AUTO
If electric fuel pump is only for boost to mechanical pump during start/takeoff/landing, turn it OFF at other times

Alternator Failure
LV Alarm light comes ON, Engine Battery Contactor drops out.
Pilot can manually turn off any circuits (ex strobes) not needed to continue flight. Batteries will automatically share load until exhaustion
When items like landing lights are required at termination of flight they are manually turned on again.

Short Circuit on either feed to main bus
Fuse blows, associated master relay drops open. Alternator continues to supply main bus via healthy feed from other battery.
All circuits remain available, blown fuse & short repaired following next landing

Battery Open, Short Circuits
As per standard Z19 circuit, no immediate action required

Advantages of this revised system include the following:
* Fewer control switches:
   1) No E-BUS ALT FEED
   2) Ignition Power and Fuel Pump Power unambiguously describe functions controlled, and replace ENG PRIMARY POWER, SECONDARY POWER, and Fuel Pump Power
* One less power bus (or 3 less buses if Main Battery and Engine Battery buses are eliminated)
* One less double diode
* 5-8 fewer firewall penetrations depending on configuration
* No complex switching sequence required to deal with loss of alternator and no frequent voltage checks required after event; just turn off unnecessary circuits and turn them back on when needed. Batteries will automatically share load.
* Shorting of main feed cable is a non-event, no action required, limits damage done to wiring

The additional components to achieve these advantages are 1 extra bus power feed cable, two ANL fuses, the battery tie contactor, and the engine battery test pushbutton. I feel the added security and reduced pilot workload in emergency situations alone more than outweigh the marginal cost if any of this alternate approach. BUT..

Its possible I have misinterpreted the design intent or certain details of Z19. Its possible there are design issues in my alternate proposal which still need work, its hard to anticipate every possible situation that could arise in the real world. So please let me have your comments, particularly on any drawbacks to this proposal. Its not my intention to knock a well-proven design that has worked for decades, but to see if there are alternative approaches that can advance the state of the art. The homebuilt aircraft industry is the source of a great deal of innovation because it hasnt always been content to stick with proven traditional solutions.

If there is sufficient interest in this alternative I can revise details as appropriate and add further info like reducing lighting switch numbers, plus make the original CAD files available.

Terry McMillan

 
    Z19RevPDF.pdf

   Schematic Diagram






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