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Old Jan 04, 2003, 09:20 AM
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new rochelle, ny
Joined Dec 2000
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Powering Receiver without BEC?

I am in the process of building a plane that will be useing a battery pack in the 12 to 14 cell range. I have been tinkering with Motocalc to refine my motor/battery/gearing/prop selection. I have run in to a bit of a quandry in that I would like to use a 14 cell pack, but it looks like I would then need to install a seperate receiver battery pack to power the receiver/servos. It seems that once you go above 12 cells there is no BEC circuit available in the speed controllers.

I just read the December Controlling Interest article. Within it they talk about the Ultimate BEC which would certainly solve my problem and is definately an option.

The question I have is this....Since I will be fabricating battery packs for this plane anyway, can I just put in a seperate power take-off within the cell pack to "tap" into 5 cells worth of the pack to supply my receiver with 6 volts (when fully charged and say 5 volts when the pack is dis-charged)? If there is no BEC within the speed controller does the speed controller still "kill" power to the motor when the pack drops below a certain voltage?....all help is greatly appreciated!
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Old Jan 04, 2003, 10:45 AM
f5b-uk
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United Kingdom, Dorset
Joined May 2002
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Generally, what you describe will work and I'm sure many have done this in the past. However, there is a real risk that the 4 or 5 cells you tap into drop below the required voltage and your plane dies. Much better to spend a little and get the UBEC or a separate receiver battery (4x300mAh NiMh will weigh virtually nothing compared to the total in your 14 cell model). I'd go with the receiver battery if you have the ability to recharge it at the field because it is cheaper than the UBEC option.

Mike
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Old Jan 04, 2003, 02:27 PM
Been There! Done That!
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Eugene, Oregon, United States
Joined Sep 2001
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Quote:
Originally posted by Mike Seale
Generally, what you describe will work and I'm sure many have done this in the past. However, there is a real risk that the 4 or 5 cells you tap into drop below the required voltage and your plane dies. Much better to spend a little and get the UBEC or a separate receiver battery (4x300mAh NiMh will weigh virtually nothing compared to the total in your 14 cell model). I'd go with the receiver battery if you have the ability to recharge it at the field because it is cheaper than the UBEC option.

Mike
When I hear someone talk of doing a power tap off of a battery pack I get visions of losing the plane do to a pack failure! Follow Mike's advice... Don't do it!
boomer
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Old Jan 04, 2003, 10:19 PM
BEC
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Not recommended, though it can work.

Some ESCs cut the motor off when the battery falls below a certain voltage whether they have a battery eliminator circuit or not, and some don't. Remember that that cutoff voltage would be for the aggregate voltage of the whole pack - in your case 12-14 cells. Even a conservative one such as a Jeti that cuts off at 0.7V per cell, calculated on startup voltage, would have your tap at 5 cells be 3.5V at that point....... as I say, not recommended.

Now that I've tried it I think that for most applications in your cell count range the best option is the UBEC. I'll disagree with Mike only because even though a UBEC costs more than a small four cell battery for the Rx it completely eliminates the need to worry about/maintain/charge that little receiver battery. If the drive battery is charged, you have radio power.
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Old Jan 05, 2003, 11:39 AM
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Dick Huang's Avatar
Dallas,TX, USA
Joined Mar 1999
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ewhahn,
I tried the tap you are thinking about. Let me tell you what happened: when the taped cells (4) dropped below about 5 volts, all power to the Rx and servo died. Putting the throttle to motor off and max did not turn the system on again. So I was in free flight! Since I was flying an stable old-timer it did not spin-in.
Hope this will help you-Don't do it!
Dick Huang
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Old Jan 05, 2003, 03:40 PM
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Winnipeg, MB Canada
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Pack Tapping

Let's look at this question a bit more objectively.

Boomer, I assume from your response that you never use BEC. Millions do, and quite successfully, I might add. A pack failure is a pack failure whether you've tapped it or are using BEC.

As far as some other points are concerned, tapping a large capacity pack at four or five cells (I would prefer five) should have very little effect on the pack. Face it, a standard issue 500mAh receiver battery lasts most sport fliers for the day, maybe 2 hours of ops with safety. If one were to fly in the E-flight manner that a 10 cell pack would suggest, flight time would be in the area of five to ten minutes. Your receiver mAh draw would be squat.

One can now see that the drain differential between the tapped cells and the rest of them is quite minimal, nothing that an after session C/10 equalizing wouldn't fix.

I use separate packs, UBEC and BEC and am hoping for further discussion on the topic. Tex.
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Old Jan 05, 2003, 04:04 PM
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United Kingdom, Dorset
Joined May 2002
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Re: Pack Tapping

Quote:
Originally posted by Sabrejock
Let's look at this question a bit more objectively.

Boomer, I assume from your response that you never use BEC. Millions do, and quite successfully, I might add. A pack failure is a pack failure whether you've tapped it or are using BEC.

Tex,

I think Boomer's responce is more in line with Dick's - ie. the motor might still be spinning but the tapped cells can't deliver the power.

Think of it this way...a fully charged 4 cell rx battery will measure at least 5.6V (1.4V/cell) and can hold over 4.8V when delivering 200mA if it's in good condition. If you tap into 5 cells from a flight battery you will see at least 7.0V fresh off the charger, but this battery will be delivering 10A, 20A, 30A when the motor is running? You name it. At these currents, the cells will only deliver perhaps 1 volt per cell WHEN FULLY CHARGED. This is only 5V if you've tapped 5 cells and might quickly drop to a value which is not enough to power the receiver. When this happens the receiver cannot switch the motor off and you have a fly-away as decribed above.

If you did insist on tapping into the flight pack, check on the ground to see what happens when the flight battery is low. If you are lucky the ESC will cut power to the motor before you lose the model.

Mike - who loves BEC on small planes due to the convenience, but would not risk it on anything over 4lbs
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Old Jan 05, 2003, 04:33 PM
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Point well taken. That's what this forum is all about. Tex.
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Old Jan 05, 2003, 04:46 PM
EFlight=Fun,Big-T=BIG FUN
San Antonio, TX, USA
Joined Jan 2001
964 Posts
When I built my 7lb electric with 12 cells I had the same issue.
Went with a 4x600 rx pack andan onboard voltage monitor for it. It added almost nothing to the overall weight but proved useful in balancing the plane. Lasts all day.
For convience I might do the UBEC thing next time (yes, I did forget to charge once - the voltage checker warned me BEFORE the flight anf I quick charged it before flying).
Bernard "Crash" Siegel
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Old Jan 05, 2003, 05:31 PM
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It always amazes me how many people state they don't want the "inconvenience" of having to be certain a separate receiver pack is charged.
Why do so many feel that is so "difficult"?

If you're flying a plane with a 12-14 cell pack the additional weight of a small separate NIMH receiver pack is not enough to even worry about.

Although I use a PEC (Plane Eliminator Circuit) in my small parkflyer planes I certainly wouldn't use it in anything large.
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Old Jan 05, 2003, 05:38 PM
Mr Mootsie
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Pepperell, MA, USA
Joined Jan 2001
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Quote:
Originally posted by Ric Duley
It always amazes me how many people state they don't want the "inconvenience" of having to be certain a separate receiver pack is charged.
Why do so many feel that is so "difficult"?

If you're flying a plane with a 12-14 cell pack the additional weight of a small separate NIMH receiver pack is not enough to even worry about.

Although I use a PEC (Plane Eliminator Circuit) is my small parkflyer planes I certainly wouldn't use it in anything large.
exactly. I use a small nimh pack.
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Old Jan 05, 2003, 06:26 PM
I'd rather be Flying
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Nashville, NC, USA
Joined Mar 1999
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Quote:
Originally posted by Ric Duley
.
Although I use a PEC (Plane Eliminator Circuit) is my small parkflyer planes I certainly wouldn't use it in anything large.
Everyone is entitled to his own opinion on the matter. I can only go by my own experience.

BEC: I've never had a problem. Of course I don't operate beyond the capability of the device.

Receiver pack: I've lost two planes due to receiver pack failure. One was a broken wire at a factory solder joint on the receiver pack.
One was the failure of a factory welded tab, it came off of one cell end.

So in my experience, BEC is more reliable.
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Old Jan 05, 2003, 08:12 PM
BEC
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I'm with Dave.

I've personally had two crashes in my R/C career that were due to an exhausted receiver battery - in one case quite unexpectedly and in the other when I knew I was "pushing it".

I've also had one BEC-equipped plane crash when the drive battery departed the airplane in a maneuver because I didn't really seat the battery on the Velcro very well (but the plane is small and light - Wing-E - and there was very little damage).

As for the inconvenience - it's simply one less thing to worry about to not have to deal with a small Rx pack. I'm afraid I'm not as well disciplined as Bernard S. to check the Rx battery before every flight with an ESV since I use them (Rx packs) so seldom anymore. At the moment not a single airplane I have that is flyable or near flyable has a receiver battery and the only one that is running on more than 10 cells has a UBEC in it.

Between family and the weather, my ability to anticipate opportunities to go flying outside of weekend-long events out of town is very small. Now that I'm getting into lithium-polymer batteries that STAY charged for weeks, all I really need to worry about in order to take advantage of an impromptu opportunity to fly is the transmitter battery. Again not having to check and top off small Rx batteries (especially NiMH!) is a benefit to me. I find myself wishing I had a LiPoly pack I could put in my transmitter......
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Old Jan 05, 2003, 08:34 PM
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NorCal
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Quote:
Originally posted by davecee
I've lost two planes due to receiver pack failure. One was a broken wire at a factory solder joint on the receiver pack......One was the failure of a factory welded tab, it came off of one cell end.
So in my experience, BEC is more reliable.
You do realize that both of those failures could have just as easily occured in a pack powering your BEC, right? You would have then also crashed.
A battery failure, whether powering a receiver or a BEC, will still cause a loss of power to the radio.

So how does that make a BEC "more reliable"?

Adding a BEC adds no "reliability" at all. It is one more piece of electronics that can and does fail, causing a crash.
Does it happen? Oh yeah, all the time.

As far as crashing due to an exhausted receiver battery, in all due respect that's pilot error and not an equipment problem.

If you want to say that a BEC is more convenient, then I agree with you. Just be aware that is does have it's risks.
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Old Jan 06, 2003, 08:31 AM
I'd rather be Flying
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Nashville, NC, USA
Joined Mar 1999
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Quote:
Originally posted by Ric Duley
You do realize that both of those failures could have just as easily occured in a pack powering your BEC, right? You would have then also crashed.
The key phrase here is "just as easily". I don't agree. I solder my own packs so I know I have really good inter cell connections, much more reliable than the tiny spot welds on the tabs of a factory receiver pack.

The 22 guage wire soldered to a factory receiver pack is prone to break at the solder joint due to flex and vibration. That is why there is a strain relief provided. I'll grant you this could occur with the same wire soldered to an ESC.

"Adding a BEC adds no "reliability" at all. It is one more piece of electronics that can and does fail, causing a crash.
Does it happen? Oh yeah, all the time. "

The fact is, the failures have happened to me with one system and not the other. So, I'll continue to use BEC where ever possible.
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