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0. 09:11 AM - Please Make A Contribution To Support Your Lists (Matt Dralle)
1. 10:12 AM - Re: Dual Battery Dual Alternator Power Distribution System (Bill Watson)
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| Subject: | Please Make A Contribution To Support Your Lists |
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| Subject: | Re: Dual Battery Dual Alternator Power Distribution |
System
Lance, I've been flying an RV-10 with a Z-14 for over 7 years and 1,000
hours. I think it's the 'Bee's Knees'. I've lost a LR3 controller.
I've slow-killed a few batteries due to some bad management and some
clocks on a battery bus. I have a panel full of yesterday's avionics
with dual feed capabilities. Some people think the Z-14 is overkill but
it has never given me a moment's worry about not having the juice to
complete a flight as planned. In fact, it has enabled me to confidently
plan flights with a dead battery or a dead controller until those
components could be replaced in my home shop.
The main suggestion I would offer is not to start by modifying the
Z-14. Rather implement it as designed. Do it straight up and operate
it as designed. It's a 'belts and suspenders' design in the first
place. You might even be diminishing it's capabilities by modifying it
the ways you have described.
Main power distribution diagram hopefully attached. The thing that
initially caught my eye about the Z-14 is it's simple symmetry - dual
everything with a cross feed.
Some comments made below: (No expert here so take my comments FWIW)
On 11/2/2018 6:48 PM, lwesterlund wrote:
>
> Im 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 Ive 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 theres 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 cant start the engine with just the primary battery,
its time to get a new battery.
I would include and use the cross connection for starting. I started
with the same thought since I mainly wanted to be able to operate my
panel independent of the engine start so I would light up the bus with
most of the avionics on it for flight planning, filing, clearances,
waiting out local wx, then start on other battery. I'm using Odyssey
680s which, when combined with the light weight starter that was a
standard part of the Vans engine package, gave me marginal starts. But
it always started even in the winter. With that said, that's the wrong
way to do it. First, don't use the 'light weight' starter if there is
such an option on the RV14 engine - they turn fast but without much
punch. Second, always start with both batteries. I'm told that's for
good battery health. Then fly with the cross feed disconnected so that
any failures on either side become quickly evident.
> Power is distributed through two buses:
> 1. The Primary Bus feeds all dual feed components as well as the non-critical
single feed components. Ive 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.
Two potential problems here as I see it. First, by not using a cross
feed contactor as laid out in the Z-14, you are bypassing the ability to
run EVERYTHING in your aircraft if there is an alternator or controller
failure. Second, (it may only apply to my particular configuration with
(3) GRT HX MFDs with no on/off switches so I'll save it). But I
initially had my dual input gear (the 3 GRTs and the G430) connected to
both buses but over time came to understand that it was unnecessary
redundancy with the cross feed capability and sometimes it was a
liability. Maybe we can discuss the experiences that led me there later.
> 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.
This sounds good. I added a TCW power stabilizer to keep the avionics
on during deep discharge starts. The reality as I've experienced it is
that there is no reasonable scenario where power is suddenly lost on one
bus. Rather by monitoring the voltages on each bus during flight (and
assuming batteries don't suddenly blow up) and paying attention to an
alarm saying a bus has dipped below charging voltage, an alternator or
controller failure will become evident with more than enough time to
tuck it away and cross feed the buses to complete the flight as
planned. The battery will keep everything running in the mean time. An
MFD may fail, but that's a different thing.
Think of the Z-14 like a twin engine plane - you don't plan on dual
simultaneous engine failure - you plan on recovery from a single engine
failure. On a Z-14 you identify which side failed, what failed, tuck it
away and hit the cross feed and proceed to your destination. Unlike a
sudden engine failure, the sudden failure of an electrical system
component just causes the voltage to slowly fall on one side. Your
robust battery is your backup until 'the dead foot and dead engine is
identified, feathered, etc'
>
> Regarding the ubiquitous always-hot battery bus, my hypothesis is that its 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
I totally agree. You may still end up with a so-called battery bus
which is really just one or more fuse protected components attached
directly to a battery. The electric fuel pump is a backup device and
doesn't need to be on a direct battery connection. My GRTs use to have
internal clocks that required an always not batt bus which led to some
battery problems. Fortuanately they got rid of the always on requirement
and picked up the time from the GPS(?). Dome lights or a USB power plug
might be nice to have but lights can be left on and USB devices left
plugged in (like my new Sentry) and threaten to kill a battery. Dual
batteries means that it's manageable even on a remote Bahamian island
airport, but a completely unnecessary risk. I have a battery bus with
only a cigarette lighter/power plug left on it and that is mentally
placarded against use.
>
> 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, youre 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.
The way I manage my Z-14 is turn on bus2 to light up the panel and get
the IFR stuff going. I light up bus1 so both radios work (there's a
reason), then cross connect for the start and takeoff. I could
disconnect the cross feed so that the voltage readouts would make any
electrical failures apparent but I don't. After takeoff I disconnect
the cross feed for the flight. Just before landing (or whenever I
remember since I don't use a before landing checklist), I reconnect the
cross feed to insure both batteries get a full top off before shutdown.
Landing at night with everything on and the engine at idle will tend to
drain one of the batteries a bit so I cross feed to prevent that.
I don't worry about having one side setup at a lower charging voltage
than the other. I set them at the max recommended by the battery
manufacturer - 14.7 volts. Since I fly with them disconnected, both
alternators are doing whatever they have to do and the both batteries
stay fully charged. If anything dips below say 14.4 volts, even
momentarily while in flight, I get concerned. I think I have the low
voltage alarm on my engine monitor screen is set at 14 volts - the low
voltage light on the LR3 are separate.
>
> 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 doesnt isolate power to the fire, opening the Aux Battery relay would
dump the balance of loads and feeds. At that point, Id 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.
Here's where I'm probably missing it because I don't really have a plan
for an electrical fire in IMC short of shutting everything down except
for my backup instruments which currently include an ADI with it's own
backup and some pnuematics. I may need some education here.
>
> So, what I have I missed? Or misunderstood? Or miscalculated? Does eliminating
the battery bus increase risk in some unintended way? Im 100% sure Ive missed
something small, complex, and nuanced. Im also pretty sure Ive 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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