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Old Mar 13, 2011, 02:37 PM
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What size plane to run separate RX batt?

A recent thread on what type of BEC to buy had me wondering...

What size plane do you scrap the idea of a 7A and up BEC and run separate receiver power.

All my smaller planes use the integrated BECs on the ESC. My medium planes (400 to 1000 watts), use seperate BECs

I have some larger planes, Hyperion Samson and Showtime 90 that are running 6S and 8S 5000 packs. I'm using 7A and 10A BECs. Thinking of safety and redundancy as far as still having servo control IF something in the power system dies, I'm thinking of installing a 4cell nicad pack for RX and servos.

What's the general consensus on when to run a rx pack.

Tom
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Old Mar 13, 2011, 03:33 PM
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Joined Mar 2003
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If you have a wattmeter, plug it in as usual, and 'stir the sticks', (but not throttle). It should show you just how much current the servos are taking. Load up some of the surfaces, ( gentle finger pressure), to see what the flying loads may go up to.

I would be very surprised if you saw anything close to your 7A and 10A BEC limits. At those sort of currents you would need to upgrade all the servo and power wires. Any longish extension leads would have a volt drop along them if the 'standard' gage wire is used.

Also you would need to find small nimh packs, (4 cell), that can deliver at least a constant 10A without much volt drop to better those BECs.
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Old Mar 13, 2011, 03:37 PM
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A BEC is safer.

With a BEC if you have enough power to fly you still have power to the radio.

With a separate Rx NiMH pack you are never sure what capacity you have left. You could install a fresh LiPo and forget to charge the Rx pack and disaster awaits.
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Old Mar 13, 2011, 09:36 PM
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I have been running separate RX packs on models making about 1200 watts or more. A property cared for RX battery is completely reliable and I like the security of knowing that even if the lipo or any of the power system harness has a major failure, if things are smoking, I can land the model anyway.

The best of both words would be a power system with both...a separate BEC and a backup RX battery set up with a diode to prevent current backflow. That way, either one could supply RX power if and as needed.
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Old Mar 13, 2011, 10:03 PM
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United States, NY, Syracuse
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Quote:
Originally Posted by RobC(UK) View Post
A BEC is safer.

With a BEC if you have enough power to fly you still have power to the radio.

With a separate Rx NiMH pack you are never sure what capacity you have left. You could install a fresh LiPo and forget to charge the Rx pack and disaster awaits.
If you are used to the routine you charge your Rx pack before every day of flying. The glow guys have been doing it this way since the beginning of R/C. I'd argue since you discharge LiPos much faster than you would an Rx pack that there's a greater chance you could push the LiPo just too much and it will shut down both the ESC and BEC. I believe that it is actually safer to have seperate packs. You can't factor operator forgetfulness into the factor of safety, that's just bad statistics.
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Old Mar 14, 2011, 03:09 AM
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Originally Posted by ChillPhatCat View Post
You can't factor operator forgetfulness into the factor of safety, that's just bad statistics.
If you want to talk statistics the majority of crashes, both full size and model are caused by pilot error. Present an opportunity to screw up and someone will take it.

All modern ESCs have an LVC that will cut the power to the motor to allow enough time for a safe landing.
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Old Mar 14, 2011, 07:36 AM
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That and a timer make good safety factors.

Gord.
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Old Mar 14, 2011, 08:04 AM
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USA, AZ, Payson
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I have lost a somewhat large plane due to loss of all power using a UBEC. It's repairable but..... nothing is as sickening as just watching your plane fall out of the sky.

On my new Extreme Flight 70" Extra I'm using a simple and reliable two 2200 A123 receiver and flight pack direct with two output leads that go through independent switches so I have switch redundancy also. Two days ago it saved my new expensive plane when I had a problem with the Deans connector to the ESC in flight. I just dead sticked the plane in under full control. If I ever have a 100cc size I'd have two A123 systems for redundancy. As an old glow guy I'll stick with a separate flight battery on anything I really value dearly. On the smaller less expensive planes (40-60 size) I'll continue to use a UBEC. On the really small planes then I'll use the built in BEC. It's nice to keep it simple where it doesn't matter too much if you loose it.

Dave
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Old Mar 14, 2011, 08:40 AM
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I've been using 2s, Lipos & 10amp BECs for my seperate Rx/servo power. As small as a 900mah 20c pack for my Hyperion Mustang, and 1800mah 20c for my 2 60e hanger-9 warbirds. Why is everyone stuck on using NiMh or NiCad Rx packs? Lb. for Lb. Lipos weigh less & produce more power. Blue skies!

Rick
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Old Mar 14, 2011, 09:20 AM
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Originally Posted by RickAvery View Post
I've been using 2s, Lipos & 10amp BECs for my seperate Rx/servo power. As small as a 900mah 20c pack for my Hyperion Mustang, and 1800mah 20c for my 2 60e hanger-9 warbirds. Why is everyone stuck on using NiMh or NiCad Rx packs? Lb. for Lb. Lipos weigh less & produce more power. Blue skies!

Rick
Rick, I've thought a lot about using the 2s LiPo's I have (2200ma) and a BEC but went with the A123's direct to take one more possible failure (the BEC) out of the system plus they will deliver all the amps I'll ever need and have great charge rates and shelf life. I really like the simplicity of it. You're right though no more Nicad or Nimh for me Hobby King has the A123 cells now both 1800 and 2200 real reasonable.

Dave
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Old Mar 14, 2011, 09:26 AM
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They also have ready made 2 cell rcvr packs.

Gord.
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Old Mar 14, 2011, 09:41 AM
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I use a seperate bec for all my planes and depending on how many servos you are using, you would be surprised at how much a digital servo uses. For my planes that have 4 servos (I use JR DS 821's) I use the CC 10 amp bec. For anything more I use the CC 20 amp pro. I just called CC and discussed this issure with them 2 weeks ago and ended up changing out a couple of the 10 to the 20. Am also using a Spekturm DX 8 with the telemetry and it alarms you when your flight pack reaches your predetermined low voltage. I dont push my batteries and stop at 30%. Love the telemetry!

Harry
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Old Mar 14, 2011, 10:47 AM
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Quote:
Originally Posted by av8djc View Post
Rick, I've thought a lot about using the 2s LiPo's I have (2200ma) and a BEC but went with the A123's direct to take one more possible failure (the BEC) out of the system plus they will deliver all the amps I'll ever need and have great charge rates and shelf life. I really like the simplicity of it. You're right though no more Nicad or Nimh for me Hobby King has the A123 cells now both 1800 and 2200 real reasonable.

Dave
Dave, Yes, eliminating another component which could fail is smart. I receiently had a castle 10amp BEC fail to power up. It didn't happen in flight, so no harm. I don't know the chances of one failing while being used? I figured 8.4vdc, (a fully charged Lipo), would be a bit high for the Rx/servos. I have not looked in to A123s. I will though. Thanks & blue skies!

Rick
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Old Mar 14, 2011, 03:49 PM
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United States, NY, Syracuse
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Quote:
Originally Posted by RobC(UK) View Post
All modern ESCs have an LVC that will cut the power to the motor to allow enough time for a safe landing.
I simply don't trust the LVC for anything of worth to me. 36" or smaller I will use a UBEC or built in BEC, anything larger will get it's own flight pack. I can't help people be more careful flyers, but I should think we'd all benefit from a checklist before every flight, just like you would a full scale. Pilot forgetfulness simply isn't a good enough excuse to use the wrong equipment for a given application. Everybody's definition of "wrong" is different, but if you're factoring in forgetfulness into the power system, you're making a compromise where you shouldn't in my opinion.
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Old Mar 14, 2011, 04:31 PM
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Yes, the ESC's LVC is more of a last ditch effort incase your flight has to last longer than expected. You should not be flying to it every flight.

I go up to a separate BEC/power source when the demand is there, otherwise if the ESC has a built in BEC that can handle my servos, then I go for it. Haven't had a BEC fail on me yet.
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