Battery tap for rx?

Greetings, I've been out of RC for a while but am slowly getting back into it. This has probably been been covered but the answer isn't obvious if it has.
I'm awaiting delivery of a speed 400 plane, it runs on a 9.6v 8 cell nimh, and don't see any mention of a seperate place for an rx battery pack. My old airtronics fm reciever has a 4.8 volt pack. The recommended ESC from tower does not seem to have a BEC.
I think I've read up enough to understand the advantages and disadvantages of a BEC, namely: +reduces weight over seperate rx battery system. +simplifies charging. +simplifies knowing when to stop flying. -less efficient, non-switching regulator. -reduced flying time.
But I'm going to be wanting to go when the stuff gets here with or without a BEC. So my questions are: 1. Can I just center tap the battery pack to get 4.8 volts? Or might it somehow short if I do? Will I need to occasionally or even automatically alternate which batteries I'm leaching the rx current from?
2. Should I not be worried about the additional weight the 4 AA batteries will add?
3. Should I go get a BEC from the local hobby store and not worry about the inefficiencies?
I must say that the ease of charging has a large appeal, since this will hopefully be something I can keep at the ready in my car (12 volt charger) and be ready to use whenever. It is essentially a "park flyer", but I do pass an RC field everyday.
Thanks in advance, Steve.
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Its hard to imagine a Speed 400 ESC without BEC but I guess it could happen. Maybe look at a different ESC as you really don't want the extra weight of a RX pack in a Speed 400 size plane.
-- Paul McIntosh http://www.rc-bearings.com

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The recommended ESC most likely does have a BEC. I don't know of any speed 400 size ESC that doesn't. They are actually quite efficient also. Leave it to Tower to not explain it well. They are extremely behind the times when it comes to electric flight. The descriptions they write for many of their e-flight accessories reveal that they don't even understand what they are selling. They also continue to sell absolute dogs like the Fundango and introduce horrid new designs like the Micro Whizz that just reinforce the opinions of some that electric flight doesn't work.
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Get an ESC with BEC. Castle Creations makes good ones. There are other good ones out there. Weight is critical with most park flier type planes, so a separate 4-cell pack is generally not a good idea. You don't want to take your power from the 8-cell pack without a BEC. You could run the pack so low that your servos no longer respond.
John VB

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Steve Banks wrote:

Then it is most unusual. nearly all ESC's except very high power ones do. I suspect it doies have one.

No reduced flying time really. idle currebnt of servos is 25mA opr so. You will be drawing at least 2/5A acvverage out of teht pack, probably nearly 5A average. Even 4 servos all moving togther only clocks and amp ors os whislt they are moving...

No, for 9.6v just use the BEC that is bult in to the controller: All other options are inferior. If it doesn't have one send it back.

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| Greetings, I've been out of RC for a while but am slowly getting back into | it. | This has probably been been covered but the answer isn't obvious if it has. | I'm awaiting delivery of a speed 400 plane, it runs on a 9.6v 8 cell nimh, | and don't see any mention of a seperate place for an rx battery pack. My | old airtronics fm reciever has a 4.8 volt pack. The recommended ESC from | tower does not seem to have a BEC.
It probably does, and if not, you don't have to get what they suggest. It's not a bad idea to get something a bit larger, so it can be used in your next plane if you want. The added weight and cost are usually minimal. | I think I've read up enough to understand the advantages and disadvantages | of a BEC, namely: ... | +simplifies knowing when to stop flying.
Don't underestimate the value of this. With a BEC, your receiver battery will never peter out in the middle of a flight, unless you've got a powered glider and are staying up a long time with the motor off, or if you do something really odd (stupid) like totally drain your battery, charge it for 120 seconds, then fly.
| -less efficient, non-switching regulator.
They're not that bad. Suppose your receiver and servos need 150 mA -- so your BEC draws 150 mA from your battery pack at 10 volts (just a guess) and delivers 150 mA at 5 volts. The other 5 volts is wasted, so thats aout 0.750 watts of heat being produced.
The 150 mA figure came from some testing I did with an 0.40 sized trainer -- average current draw worked out to 150 mA. For a speed 400 electric plane, it's likely to be even smaller.
Let's do some real world math. Assume an average 100 mA current draw from your receiver and servos. Assume an average current draw of 10 A from your motor. All the other figures factor out of the equation, so they don't matter. This means that 1% (yes, one percent) of your power is being used by the BEC.
For a ten minute flight, one percent of that is six seconds. Do you really care?
| -reduced flying time.
Not always. Adding a seperate receiver battery adds weight, reducing performance and requiring more power to stay up. It may very well reduce flying time to add that extra battery.
| But I'm going to be wanting to go when the stuff gets here with or without a | BEC. So my questions are: | 1. Can I just center tap the battery pack to get 4.8 volts? Or might it | somehow short if I do? Will I need to occasionally or even automatically | alternate which batteries I'm leaching the rx current from?
Yes, you could do that. But is it worth the trouble?
No, you don't absolutely need to alternate which side of the pack you tap as long as it's a NiCd or NiMH pack and you always overcharge them a bit (so that both sides are fully charged.) But it wouldn't be a bad idea.
But consider this -- suppose your pack is completely, perfectly matched. Suppose it's made up of 8 1000 mAh cells.
To make this example better, suppose that your motor draws 5 amps and your servos also draw 5 amps. (Not real world values, but they make my point more obvious.) You use a center tap like you're suggesting.
After six minutes, half of your pack is completely flat, and the other half is half full. No more receiver power, and very little prop power, so you need to land. If you're lucky. If not, you crash.
Replace the center tap with a BEC setup. After six minutes, your entire pack is empty, and you land. You won't crash, because the motor will probably cut off with plenty of power left for the BEC.
What have you gained? Nothing except the ability to run your receiver part of your pack dry (and then reverse charge it, very bad) in flight. And without receiver power, your plane crashes.
| 2. Should I not be worried about the additional weight the 4 AA batteries | will add?
For a park flier, absolutely. For a much larger plane (0.40 sized), probably not.
For a park flier, AAAs would be better, but a BEC is better yet.
If you're worried about the inefficiencies, just get a slightly bigger battery pack to cover the BEC usage.
| 3. Should I go get a BEC from the local hobby store and not worry about the | inefficiencies?
Yes. Or get it from Tower Hobbies.
If your ESC truly doesn't have a BEC, you could add one with a 5 v voltage regulator quite simply.
| hopefully be something I can keep at the ready in my car (12 volt charger) | and be ready to use whenever. It is essentially a "park flyer", but I do | pass an RC field everyday.
You want a BEC.
--
Doug McLaren, snipped-for-privacy@frenzy.com
How many pounds are in a gallon? --Homer Simpson
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Thanks All! It does have a BEC. I was assuming there would be a seperate connection for the power lead, but all the red and black leads are electrically connected at the radio. I did have to rearrange the polarity on literally every servo style plug in the system though (old airtronics rx, futuba style ESC and elevon mixer, airtronics servos) but that is a quick operation with a timy screwdriver. Everything seems to check out, except that it looks like I have more elevator throw with the mixer than aileron, I think it's supposed to be the other way 'round. It doesn't look like I have many servo positioning options without fabricating, so any other suggestions/corrections would be welcome?
Thanks again. Steve

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Disregard, I found a couple pots under a millemeter of shrink wrap on the mixer that allow adjustment. Just needed to cut into the plastic to get to them... Thanks again.

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Disregard, I found a couple pots under a millemeter of shrink wrap on the mixer that allow adjustment. Just needed to cut into the plastic to get to them... Thanks again.

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