Rx battery pack for full house E-sailplane ? - RC Groups
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Mar 20, 2002, 05:55 AM
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oded mazor's Avatar

Rx battery pack for full house E-sailplane ?

I've given up the idea of using the BEC in my E-sailplane (optima pro electric) thanks to e-zoners advices.

which Rx battery pack would you recommend for a 2.5M thermal soarer, using 5 mini/micro servos ?

I read that many use the 4.8V 600AE, is this the one I should get (weight is an issue) ?

I'll overnight charge it with my TX charger at 60ma.

thanks, Oded.
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Mar 20, 2002, 06:17 AM
Registered User

Flight pack batteries

On the For Sale forum, a fellow has 700 Mah NIMH batteries for $0.61 each. These seem like a nice alternative to the 600AE's. Probably lighter and smaller, too!
His E-mail address was: rocker@rockinrollstar.com
Other opinions?
Bob Cowgill
Cocoa, Florida
E-mail: bmwbob@mpinet.net
Mar 20, 2002, 10:45 AM
Registered User
I built a 4x 1600mah NiMH pack (AA sized cells) for my Xenath, which has 4x HS-85 and 2x HS-81 servos... I figure about 50ma per servo typical, for 300ma total servo draw, plus the rx. So that should give me about 5 hrs flight. Even if I'm off by a factor of 2, I still get 2.5 hrs, which is plenty for a day.

I built a small 4x 600mah NiMH pack first (AAA cells) too, but decided that 2 hrs was cutting it a bit close for a typical outing. So that'll go in a HLG instead.

Hope this helps... the key is knowing the typical servo draw, which I think is 50-100ma depending on load and size of the servo. (I got this number from BEC servo count recommendations vs stated amperage output, plus timing how long a 110mah pack ran 2x FMA S80 servos installed in a HLG).
Mar 20, 2002, 01:36 PM
Faded away
Will this work?

Currently I have this...
I take the signal/power lead from the ESC/BEC and cut the red cable and plug it into the Rx for speed control only. I plug in a separate 500 mah NiMH battery pack for the Rx power.

Now, what if I take that cut red lead on the BEC and reconnect it with a diode. The solder in a diode in the same direction into the red lead on the separate Rx pack. Would that not allow the strongest power source to feed the Rx until the BEC kicks in or the ESC shuts down (overload) at which point the battery pack is not the strongest source and supplies power to the Rx.

I want to keep the weight down in the plane, of course, so a small Rx pack is best. However, I'd feel more comfortable if I knew there was more power available for the Rx at all times.

Just a idea. Let's hear from our Electronic Techs out there.

Mar 20, 2002, 01:58 PM
Registered User
oded mazor's Avatar
I sent an e-mail to kontronik's service asking what do I have to do in order to use a seperate Rx battery pack with the "sun 4001" esc. I got a quick reply saying : "Put a seperate battery pack into the receiver and don't disconnect anything..."

so I am not going to cut any wires for the meanwhile.

I am not however fully convinced about the safety of using a seperate Rx pack over the BEC. the reason is that when using the BEC you know when your battery is "low on fuel" since you can not use the motor any more, meaning it's time to land soon. when using a seperate Rx pack you get no warning and you may find yourself without any receiver & servos power simply because you misjudged the length of the flight or because the Rx pack was not fully charged (overnight wall charger and not a peak charger), the weird thing is that you may still have plenty of motor power but no radio power...
taking this theory into account, wouldn't it be safer to use the BEC , of course without exceeding the number of servos limitation ?

thanks for your help, Oded.
Mar 20, 2002, 03:13 PM
Registered User

To BEC, or not to BEC

Another factor in favor of using a separate flight pack is that you could be near or at the end of your motor run, and hook a HUGE thermal, and possibly be up there long enough to completely exhaust what's available to run the BEC.
Recall that the BEC uses a 5 volt regulator to step down the motor battey voltage for use by your receiver and servos. This requires that what remains in the motor battery always be above 5 volts with enough current capability to feed however much R/C gear you're running off it.
The common logic is that if you're still up there when you hit the motor cut-off, prudence suggests landing soon!
I know there are arguments on both sides of the issue, but I think that you'd be safer using a separate pack. The weight penalty of a four cell 700mah NIMH pack shouldn't break you.
Bob Cowgill
Cocoa, Florida
E-mail: bmwbob@mpinet.net
Mar 20, 2002, 06:51 PM
Faded away
This is exactly how I turned a Xenath into a Lawn Dart!

I am still considering trying the diode method with one change. I am only going to put the diode on the BEC lead and not the Rx pack. I will draw from the main battery and this will keep the Rx pack topped of until such time as I do it again.

Now, should I hit another thermal and apply and hold full flaps, with this new wiring arrangement can I expect the heavy draw to overload the ESC/BEC again or because I have the now supplemental pack attached, will that prevent the overload?