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EXTERNAL PWR with MASTER/BATT SWITCH?

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supik

Joined: 22 Aug 2018
Posts: 70

Posted: Sat May 16, 2020 5:54 pm Post subject: EXTERNAL PWR with MASTER/BATT SWITCH?

What's the reason, most of the certified aiplanes do not allow connecting the External Power Src without turning the MASTER / BATTERY contactor ON?

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Igor

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supik

Joined: 22 Aug 2018
Posts: 70

Posted: Sun May 17, 2020 1:05 am Post subject: Re: EXTERNAL PWR with MASTER/BATT SWITCH?

..battery protection?

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nuckolls.bob(at)aeroelect
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Posted: Sun May 17, 2020 10:38 am Post subject: EXTERNAL PWR with MASTER/BATT SWITCH?

At 08:54 PM 5/16/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "supik" <bionicad(at)hotmail.com>



 What's the reason, most of the certified aiplanes do not allow

 connecting the External Power Src without turning the MASTER / BATTERY contactor ON?

 	


 That's a kinda long story . . . but the pick-em-up

 truck is off the jacks and running, so I guess I'll

 offer up the long version:



 "Ground Power" is a somewhat nebulous term like

 "automobile", "train" or "boat". It alludes to

 a function but without specifics as to quality

 or safeguards. 



 I've seen some pretty creative pieces of hardware

 for getting an airplane started. I was sold a

 cold-weather assist in Brainerd, MN one really cold

 morning where the energy source was a converted

 engine driven welder that some electron-herder

 had figured out a way to re-regulate down to

 aircraft system voltages.



 On another, not quite so cold morning in Kansas City,

 I got a start from a WWII era engine driven

 ground power unit wherein the line boy shot

 28v to my 14v airplane and had his back to me

 all the time I was waving at him to shut-it-off!



 That experience prompted me to add ov protection

 to recommended ground power systems.



 During the final phases of aircraft certification

 there's a Function and Reliability test phase

 where a carefully carded and monitored series

 of flights totalling 150 hours are carried out.



 During cert on the Premier I at RAC/Beech, I was

 called to ride the test aircraft on one of the

 ICT-SLC-TUC-ICT round-robins to investigate

 the reason that a ground power cart in Tucson

 wasn't allowed to connect to the aircraft. The

 airplane was fitted with some sort of power

 quality monitor that would allow application

 of ground power only after certain 'quality'

 issues were addressed.



 The ICT-SLC-TUC-ICT loop was flown at night.

 Piled all my test gear in the baggage

 compartment and sat in the back waiting a chance

 to look at the TUC conditions.



 In what was a rare rain storm, I stood on

 the ramp, head in the baggage compartment

 staring at my DAS screen. The airplane accepted

 the GPU. I didn't see anything amiss! We

 cranked up and I rode home in wet shoes.



 At the de-brief it was discovered that, "Oh,

 you wanted to check that 'other FBO GPU."

 When you're flying two or three crews 24/7 

 to rack up hours on the a/c, it's a bit of

 a chore to keep all the activities tightly

 coordinated. So . . . back on the airplane

 at 2100 and we took off for SLC.



 This time TUC weather was clear and I got

 some really good data. Seems that particular GPU was

 a plug-in-the-wall device that used silicon

 controlled rectifiers to deliver regulated

 energy to the aircraft. But before being allowed

 to connect to the ship's systems, the non-

 loaded GPU had a terrible output waveform.



 The power quality gizmo said 'no-go'. Now,

 had the power quality monitor not been

 present, the system would have connected.

 With a battery on line all would have

 been right with the universe.



 So, THAT particular GPU's features required

 the ship's battery to be present for operation.

 If the GPU's designers had included some

 fat capacitors in the output filters of their

 product, I would have been cheated out of two

 rides in the B390.  BTW, after the first night's

 experience I was compelled to wear earplugs

 during climb out. The noise levels were pretty

 much okay a cruise but really irritating during

 climb out.



 Nowadays, I believe there is a TSO describing

 the qualities of the best we know how to do in

 GPUs . . . I wouldn't be surprised to see notes

 in operational documents for some ships to strongly

 recommend hooking up to GPUs qualified to "TSO-

 something-or-another.



 But on a cold rainy night when you're trying

 to launch out of Hog Waller International and

 Bubba pulls up to your airplane with his pride

 and joy ground power unit, the thought processes

 involving the go/no-go decision may fall

 victim to a bit of ground fog.



 The short answer is: It probably wouldn't matter

 but it MIGHT . . . and in any case it NEVER hurts.

 Depending on where the current limiters were

 set on that KC GPU, the battery just might

 have saved some electro-whizzies from being

 toasted in the 172XP I was flying.



 

   Bob . . .

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supik

Joined: 22 Aug 2018
Posts: 70

Posted: Sun May 17, 2020 12:41 pm Post subject: Re: EXTERNAL PWR with MASTER/BATT SWITCH?

Bob, thank you for the long story. Actually I used your scheme with the OV protection and the GPU is wired to be plugged in directly on to the battery through the EXT PWR Contactor. 

If I am not wrong, Marc Ausmann's scheme connects the GPU to the battery as well. This is what I am planning for..



But with your scheme there is no need to turn on the MASTER SWITCH..



C172 and SR22 -both require to turn on the MASTER to close the BATTERY and EXT PWR CONTACTORS at the same time. Thus you are not able to charge your battery unless you leave the MASTER ON (stupid?) or you connect your charger directly with the battery poles or take the battery out..



Cirrus POH says NOT to recharge the battery through the EXT Power socket. The only thing which comes to my mind is if one would leave the battery on a GPU overnight which would eventually overcharge it..?

------------------------------------------------------------------------------------

Cirrus POH: 

Application of External Power

An external power receptacle, located just aft of the cowl on the left side of

the airplane, permits the use of an external power unit for cold weather

starting and maintenance procedures.



• CAUTION •

In accordance with the manufacturer’s recommendation, external

power should not be used to start the airplane with a dead battery

or to charge a dead or weak battery in the airplane. The battery

must be removed from the airplane and battery maintenance

performed in accordance with the appropriate AMM procedures.

----------------------------------------------------------------------------------

Dead battery is clear but why not charge a weak battery if your charger is safe..? 

Or is it that the intelligent charger might not provide enough amps to close the EXT PWR Contactor when plugged in through the EXT PWR Socket?

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Igor

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