Message-ID: <FDD045B6B83FD311A9880090273053E2CF0B43@CDIM-PTS-MAIL>
From: "Rumburg, William" <william.rumburg@cdicorp.com>
To: "'lancair.list@olsusa.com'" <lancair.list@olsusa.com>
Subject: Backup Electrical Power
Date: Fri, 28 Jul 2000 13:04:51 -0400
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> 
>>
> The thought that backup electrical power is overly complicated or
> unreliable probably scares some builders away from a modern all-electric
> plane. The backup electrical system on my Lancair is uncomplicated and
> reliable. It can be fully tested on the ground at any time by simply
> switching it on and letting it run the connected loads for a few hours.
> Whether or not you like my overall system, you may want to adopt some
> features, such as the battery's location and installation method. 
> The entire system consists of only three components:
> 1. A backup battery installed on the right side of the nose gear tunnel,
> forward of the rudder pedals (check to see how much room is available in
> this area on your aircraft, there's enough to accomodate at least a
> medium-size battery).
> The battery need not be physically large. Should backup power ever be
> necessary, it will be required to operate critical loads totaling only
> 5-10 amps for 2 to 3 hours at most. Even if you had enough gas onboard to
> continue flying for a longer time, it would be foolish to press your
> limits. 
> By definition the A-H (amper-hour) capacity of a battery is it's ten hour
> capability at rated amperes. A 12 A-H battery, for example, will deliver
> 12 amperes continuously for at least ten hours, if it's fully charged. I
> installed a Concorde RG-12 as a backup four years ago, but it's no longer
> manufactured (A Lifeline GPL-1300 (marine grade) is their current
> equivalent). Now, I'm using a Sears Die-Hard. There are many batteries you
> could choose, but I would go with a modern RG (recombident gas) sealed
> lead-acid type
> To install it, I first fastened a 6" length of 2" x 2" aluminum angle
> (purchased at a local hardware) to the side of the nose gear tunnel
> (approximately 4" above the subfloor) using 3/16 flat head aluminum pop
> rivets. In order to successfully "pop" the rivets and for load spreading,
> use 3/16" area washers (AN370-16) on the inside of the nose gear tunnel.
> The battery then rests on the aluminum angle and is secured against the
> tunnel by means of a 1" wide nylon strap (purchased at a local marine
> supply and made for the purpose). The strap is supplied with two small
> stainless steel brackets to allow it to wrap around the battery. Attach
> the brackets to the nose gear tunnel forward and aft of the battery, again
> using pop rivets and area washers.
>  
> 2. A power diode connecting the backup battery to the main (charging) bus.
> It has two functions:
>     a.) Provides a path for charging current from the main bus. 
>     b.) Provides isolation for the backup battery (preventing backup
> current from flowing back to the main bus).
> I used a conventional silicon power diode, which has an inherent voltage
> drop of approximately 0.7 V DC in the forward direction. This lowers
> charging voltage to the backup battery and if your main bus voltage is
> less than 14.0 V DC, the result would be less than full charge on the
> backup battery (Concorde states that their testing shows that as little as
> 13.5 V DC will maintain full charge on a lead-acid battery). However, as
> Ed Armstrong just posted, a Schottky diode, which has a voltage drop
> ranging from 0.2 V to 0.3 V (at the charging currents we would experience)
> can be used, thus eliminating the possibility of undercharging the backup
> battery due to low voltage. By the way, I contacted VMS and was informed
> that the VMS 1000 accuracy is +/- 1% for all instruments, which equates to
> 0.14 V at 14.0 V DC for the voltage instrument.
> As pointed out by Jim Frantz, another possible fault would be shorting of
> the diode, thus connecting the backup directly to the main bus, which
> would drain it in a relatively short time. Although such an occurance was
> reported by George Stevenson in the LNN several years ago, I believe this
> to be a rare event provided that the diode is suitably rated. 
> 
> 3. A DPDT (double pole-double throw) switch rated at least 10A. This is
> about as simple and reliable an electrical device as exists. 
> It has three sets of terminals. 
>    a) The center terminal is wired to the device(s) requiring backup
> power. 
>    b) Power from the main bus is connected to one outer terminal.
>    c) Power from the backup battery is connected to the other outer
> terminal.
> Whichever position the switch is in determines whether power is fed to the
> device (ignition, gyros, etc.) from the main bus or the backup battery.
> Admittedly, it would be impractical to connect several devices (wires) to
> a terminal. I connected directly to the switch, but you could make an
> "essential" bus for connection of several devices.
> 
> Bill Rumburg
> N403WR  (Sonic bOOm)
> 
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>