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Dual Battery Dual Alternator Power Distribution System

 
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lwesterlund



Joined: 02 Nov 2018
Posts: 4
Location: Beaverton, OR

PostPosted: Fri Nov 02, 2018 2:48 pm    Post subject: Dual Battery Dual Alternator Power Distribution System Reply with quote

I’m looking for some help in reviewing a draft Dual Battery/ Dual Alternator (DBDA) power distribution architecture for an RV-14A. I started with Bob Nuckolls’ Z-14 system and I’ve attempted to modify it to capitalize on the dual power feed capabilities of the newest generation of avionics.

My mission is IFR in the Pacific Northwest where there’s lot of clouds and lots of rocks in those clouds. Accordingly, the proposed system utilizes two alternators and two EarthX batteries. The 60W primary alternator will handle all normal loads at 60% capacity. The 20-30W accessory-pad mounted auxiliary alternator will handle its normal loads at 50% capacity, and when called upon, all IFR critical loads indefinitely at 66%+ loading. The two batteries provide a third level of redundancy.

The batteries are not wired for cross connection for starting. I removed that from Bob's Z-14 - if I can’t start the engine with just the primary battery, it’s time to get a new battery.

Power is distributed through two buses:
1. The Primary Bus feeds all dual feed components as well as the non-critical single feed components. I’ve elected to use the Vertical Power X Pro to control these loads. I realize the VPX does somewhat increase the risk of a single point of failure (not withstanding its two independent controllers and two buses) but even if the VPX did go dark, I could still get home on the completely independent Aux Bus with at least as much capability and safety as I get flying spam cans now.

2. The Aux Bus provides redundant power to all critical IFR dual feed components (PFD, ADAHRS) as well as to several always-nice-to-have dual feed components (MFD, AP, Transponder). It also provides sole power to the WAAS GPS/NAV/COM (which only has a single power feed) and several other redundant loads.

The Aux Bus will be set to run at a slightly lower voltage than the Primary Bus, so the Primary Alternator will normally carry the full load (excepting the GTN650, stall warning, and optical fuel level detector). Should Primary Bus voltage fall during starting, or due to a Pri Alternator failure, the dual feed components will automatically be picked up by the Aux Bus with no pilot intervention required.

Regarding the ubiquitous always-hot battery bus, my hypothesis is that it’s no longer necessary given advances in technology. In Z-14 (and earlier in Chapter 17 or The AeroElectric Connection), Bob lists the following loads as worthy of a direct connection to the battery:
• Ignition
• Fuel pump(s)
• Dome light
• Clock
• Radio mem

My initial attempts at developing an architecture included a battery bus for those loads. However, as I started to work through failure modes, the list of battery bus-worthy loads got smaller and smaller. For instance, with dual P-mags (which the manufacturer recommends not connecting to an always hot battery bus) you only need power to one P-mag at start. Above 700 RPM both with self-excite so even in the unlikely event you lose both bus contactors, the plugs will fire and the engine will spin.

The only recommended battery bus load that gives me a little pause is the fuel pump. But, as shown, a simple single pole double throw switch should let me connect to either bus. I would need to lose both buses and the mechanical fuel pump to lose fuel to the engine. Absent battle damage, it seems like the chance of fuel starvation due to an electrical problem is pretty remote.

Regarding bus management, normal operation would be to turn on the G5 (to check G5 back-up battery operation) then the Aux Bus during pre-start to check voltage and set up the avionics. The Primary Bus, electric fuel pump and P-mags would be turned on just prior to engine start-up. If the twirley thingy in front spins, you’re good to go.

When done flying, the procedure would be to turn off the Primary Bus (to confirm load pick-up by the Aux Alt), then secure the Aux Bus, then the G5 which has its own battery, and finally the P-mags.

In the event of an electrical fire in IMC, opening the Primary Battery relay would dump about 70% of loads and feeds. I would still have a full complement of IFR critical equipment running on the Aux Bus. If opening the Primary Battery Relay doesn’t isolate power to the fire, opening the Aux Battery relay would dump the balance of loads and feeds. At that point, I’d still have self-excite power to run the P-mags and the back-up battery to power the G5 EFIS to keep the plane upright. In VFR conditions, I would dump both Pri and Aux buses immediately.

So, what I have I missed? Or misunderstood? Or miscalculated? Does eliminating the battery bus increase risk in some unintended way? I’m 100% sure I’ve missed something small, complex, and nuanced. I’m also pretty sure I’ve missed something really, really big. Any feedback would be much appreciated. Thanks.

Lance
Beaverton, OR, RV-14A - Emp & Fuse Done, Finish Kit in Progress, QB Wings Ordered


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bobmeyers



Joined: 05 May 2017
Posts: 2

PostPosted: Mon Nov 05, 2018 11:04 am    Post subject: Re: Dual Battery Dual Alternator Power Distribution System Reply with quote

I used Z-14 for my 14. I have over 220 hours on it now and am very pleased with it. It is pretty much a straight up implementation except for not using the automatic cross feed at engine start.

My only comment about your proposed set up is your plan for the aux bus. Almost all of my Garmin equipment comes with dual power inputs. I have each go to Bus 1 and Bus 2. There are a few avionics items that don't have dual power inputs. For those I just used a couple of bridge rectifiers and feed them from Bus 1 and Bus 2 just like the ones that have the built in dual power inputs. This technique is shown in the Aeroelectric book.

All heavy load items, lights, pitot heat etc are only hooked up to Bus 1 so I would have to manually switch on a cross feed switch to power those with a loss of the main alternator. My only load shedding item is pitot heat, The secondary alternator will handle everything else with no problem.

I flip on both buses when entering the airplane and everything is ready to go by the time I have strapped in and am ready to begin the engine startup checklist.

I do understand Bob N's preference for a one battery two alternator setup but in my case I elected the heavier Z-14. With my wife and I, full fuel and baggage, we are still 50 pounds under max gross. If the weight was going to be 20 pounds over max gross, I would have used Z-13.


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