Dual Rx battery/Switch Set-up?
 

I notice that in the manual for my "50cc size" (3 meter wingspan) Paulistinha P-56 it shows dual Rx batteries. I've heard of this before and on a model this size it sounds like a good idea. But how is it actually set up? What plugs into what? (Bear in mind I am extremely "electrically challenged.)

Also what counts as a "high torque" servo...for a model that will not be doing anything more than mild, scale aerobatics for a Cub-style aircraft? I've got some Futaba S3305 servos rated at 124 oz. in. at 6.0V. Will those do?

BTW, I've also got some Futaba S3001 but since I purchased them in Japan the torque ratings are in metric: 2.4 kg/cm at 4.8V or 3.0 kg/cm at 6V. Not sure how that compares with the S3305 servos.

(Ah, just did an online conversion. The S3001 servos would be about 41 oz.in. at 6V)

Dual bat is easy.
One battery feeds into one switch which is plugged into one of the ports in th RX. The other battery, with it´s own switch plugs into another port.
If all ports are taken in the RX you just put the battery/switch on a Y-kabel with what ever servo is plugged into the port you want to use.

I own a BUSA 1:3 scale Super Cub. All control surfaces are driven by Hitec 645´s which should be plenty for your needs also. Throttle and choke is operated by FUT 3001 standard servos.

BTW. I normally use port 1 and 12 for the batteries (A123 2300 mAh packs). That way, in theory, the amp rating of the RX is doubled as far as I know. I use a Weatronic Micro 12 in the Cub, and it´s rated at 5 amp, but by plugging in dual batteries as described this rating should be higher.

Another way is a Smart Fly's PowerSystem or Power Expander, one out put to the RX, but uses and regulates the voltage and Isolates the batteries from one another and the RX.

So it is literally just BATTERY > SWITCH > RX on two ports? That does seem simple.

The Futaba S3305s have similar specs. Technically, they are intended for RC cars but Dave Johnson says he has used them on his WWI models. And, yeah, I can use the Futaba 3001 for the throttle. It's nice that I have most of what I need. Just need one new switch, a Y-harness, and an additional Rx (8ch Sanwa/Airtronics).

I guess I also need to work out the servo leads. I'd need five of these meter-long leads (2 wings, 3 for the tail mounted rudder and elevator servos). I have about 7 metes of stock servo wire that I was going to use because such long servo leads might be pricey.

For Rx batteries, I have two 6.0V 2000mAh NIMH packs.

Dumb question: Does the dual battery/switch system (as described above) draw on both batteries equally during the flight (at least in principle)? So would it be equivalent to flying with one 6.0V 4000mAh battery...but with greater protection again single battery or switch failure?

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Isn't the point of the 6.0V packs (rather than the 4.8V packs) to get the full torque rating from the servos. If the voltage is reduces, wouldn't that reduce the torque as well?

Coiled extension cables? I've only seen really short ones. Do they come in 1 meter lengths? BTW, the servo leads in the CI wings are almost a better long (and pass through straws built into the wings). Hmmm...any reason I couldn't just wrap the standard servo wire around plastic straws to get the same effect?

75 Euros (plus shipping) for that regulator. Yikes. :(

You're saying that with this set-up, if ONE battery fails, they will BOTH fail? If so, then I don't see the point of doing it at all.

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Teus, thanks for those clarifications. I see what you mean now by twisted wire...and it is funny how that's what servos back in the 70s were using. :p I was confused because I've also seen some servos being sold with a "pig tail" curl on the lead. I'm going to be at an RC shop in Osaka on Friday so was hoping to pick up most of what I need there.

BTW, I thought "interference" was just no longer an issue with 2.4G radio systems? Or are we talking about a different kind of interference?

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Abu, Not to throw a wrench, but the 6.0 volt issue is really only an issue with JR servos. I only run 6.6 with all of my Hitec stuff. I have not had a problem for years. In fact, I would highly recommend getting away from those Nimh batteries and get some A123's. They are the best thing going. Especially if using many servos, and 2.4 ghz.

Abu, I disagree with everything that Teus has told you so far, perhaps with the exception of twisted wire, but even thats a stretch these days.
The odds of an "in flight" shorted battery cell are extremely unlikely. Your plane is more likey to be struck by lightening first. Failures such as switches, connectors, are more likley to fail. I would rather have 2 of each for my power path to receiver.
There is no problem running servos rated at 6v to run with a 5 cell nimh. Though I agree with Vertical that I would use A123s (LifeP04) batteries instead.
I have run 6v rated servos for years with 5 cell nimhs and now using A123s (6.6V) with no problems. Some of those servos are over 20 years old.
A servo is more likely to burn up due to mechanical binding rather than overvoltage. As long as they are rated at 6v, 6.6v is no problem.
I run a setup with two Life 6.6v batteries through 2 switches and then into 2 receiver ports. Many of us do that and have for years. Power expanders are extra stuff that you don't really need for your application.
I get my servo extensions as well as A123 batteries from here:
http://www.taildraggerrc.com/

Abu, in my opinion, the single greatest threat to your R/C airplane is losing battery power. Things have changed a lot, and we have greater power requirements than we used to. More servos, higher loads, and the 2.4 ghz rx.

I have been running A123 batts now for a couple of years, and they just perform exceptionally well. It seems like I will go out and fly 6-7 times, long flights, 8-10 min. I have never put more than 500 mah back in. On my 50cc planes, I use 2300-2500 mah packs. In my smaller planes with a single batt, I use 1100 mah. Seriously, if you are getting battery packs as well, do yourself a favor and go A123.

I get mine from Valley view. They also have cool double switches, with charging jacks and fuel dot built in. There are several sources for batteries, though. Expect to pay around $35 for a 2300-2500mah pack.

+1 for vertical and pamsters posts.

I have been running A123 four or five years now, different makes and models of servos, dual battery on GS planes, single battery on smaller ones.

Brian

check out hangtimes hobbies/ no BS batteries.they have good resouces for you to read.been using his products and advice for about 3 years now.

I use power distribution units in all my expensive planes. I have Powerboxes and Smartfly and Spectrum AR Evolutions, etc. in various planes. I use these because they have all one fail-safe switch for both batteries and both batteries are isolated from each other (i.e.if one power source circuit fails, the other brings you home.) I also use the Power Distribution units to regulate the power back down to 7.3 or 6.0 VDC from my 2S LiPos, even though I typically use High Voltage Digital Servos. HV Digital servos are faster and even if the regulator fails will still handle the 2S LiPos. Also, these units typically provided an isolated and regulated 5 VDC circuit exclusive to the receiver.

I am not a proponent of LiFe batteries. I use LiPos exclusively, although I have several LiFes laying around that have been relegated to bench use only. LiFes are designed for high Current applications. LiPos are designed for high Voltage applications. In the past, I found LiFes to be a bit heavier, in general, for a given MAH, although they are getting a bit better. They also are/were a bit harder to come by. Seems like there always was a shortage once the government's bailouts dried up. I understand that in many places, they still are hard to get in the right size, weight and voltage, due to cost or availability. In my case, I choose to work with the higher voltages. For example: ...

With long extensions, keeping up voltage is the issue. That is why I start with a higher voltage. That is also why a 6.6 LiFe has worked well for many 6 volt servos. The voltage seen at the end of a long servo lead is lower than the source voltage. In order to combat this voltage loss, I start with a higher voltage and use heavier servo wire. Fortunately for the 6 volt servos with short leads and a 6.6 volt battery, there is a safety margin built into the 6 volt design. Also there is an unavoidable voltage loss caused by the receiver buss and it's connections. However, counting on the use of a safety margin to prevent failure is an increased and easily avoidable risk. Guess it is a matter of how much risk one is willing to gamble on. That is why you see power distribution units in a savvy pilot's expensive planes.

Using twisted wire is a good idea, but there are drawbacks. It is harder for the servo lead to be resonant to stray RF because twisted wire appears very short to RF. It also minimizes stray voltage from a servo being worked very hard or emitting stray RF possibly because a looming failure, etc.. But if all is working correctly, straight wire typically works fine as long as its gauge is heavy enough. Increasing a wires diameter reduces the voltage loss. Another reason for using power distribution units is that all the circuits are buffered from each other. This precludes stray voltages, RF etc that is on the wire from impacting the receiver. Twisted wire is heavier in weight. It takes more wire to go the same distance. Another reason for opting for buffering versus twisted wire.

The trouble with feeding two batteries directly to the receiver is the receiver buss is a limiting factor, when the load goes up, the buss and all its connections, etc. are seen as resistance. Also, your receiver is subject to any voltage sags that your servos, connections, buss and switches create. Remember when digital servos are used, they can all be moving at the same time. Analogs are sequential. When using digital servos the current is cumulative thus you can have many amps being drawn at the same time. A digital hi torque servo can spike up to 5 amps. Get Six or more of these moving at the same time on a circuit constrained by a receiver buss and you now have a sag on the receiver voltage. Most of the power distribution units have a separate regulated circuit just for the receiver just to avoid this type of sag. The voltage on this circuit is "independent" from what the servos are seeing. However, ...

In your case, I would not use a power distribution unit. The current loads are not there to justify its cost in $s and added weight. The use of two batteries and two switches will afford you some redundancy that may compensate for a future bad switch contact or connection. Assuming that all your connections are solid, a bad switch or an improperly charged battery is most-likely your biggest risk. And as luck would have it, these are the most common root causes of crashes caused by loss of electrical control, ... which is why high $ planes have high $ protection. That protection minimizes these specific risks.

So it all boils down to the risks you are willing to accept and the consequences of the risks should one become a certainty. Personally, I avoid these types of risks as I fly high energy planes at 10-12 shows a year. With the risk of inadvertently dropping one of these planes in a crowd, I choose to minimize the avoidable risks.

OMG! After reading this thread if I were abufletcher I would have no clue. Seems every other opinion is the opposite of the other. And way to much information for what was asked.
Abu, simple answer to your question. Two batteries through two switches to two Rx ports directly or through a Y. As for servo torque, look into that particular models recommendations. As for volt, every servo gives it's rated voltage.

X2 on what acerc said. I prefer the power from the second battery going to a Y, but I don't want to complicate the question/answer any further than what's been done already.

Never heard of any probIems with 6 volts on JR servos. I only use JR gear and use unregulated LiFe's (6.6 volts), without any issues at all on both digital and analogue.

I agree to get away from NimH's. With A123's, (2s = 7.2 volts), unless you use HV servos, you would need to regulate. But with LiFe's, there is no need, especially with digital servos.

As for dual batteries, I think it depends on what type of short as to whether it will drain the other battery?? In my pattern models, I run dual batteries straight to separate ports in the Rx. with no isolation and no switches. It's a weight saving thing and I figure that it is better than nothing. What you could do, (and maybe I should), is solder a diode in each battery lead, which would stop a shorted battery from sucking the other one down. This would reduce the voltage by 0.5 volts as well if you were worried about the higher voltage from a 6-6.6 volt pack.

Just some suggestions.

A 123's are 6.6 v. 2s 3.3. JR must have made changes, as their servos could never handle more than 5.5 v without glitching. This is widely known.

Hmm. Google A123 voltage and it says 3.6 volts per cell. A123 batteries are Li ions. I would not use an unregulated Li Ion on standard servos. HV only.