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Thread: Wanted. Auto switching duel battery to Rx thing a ma jig.

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    Wanted. Auto switching duel battery to Rx thing a ma jig.

    Not sure of the exact item name but think I've found what I'm looking for & it's from Smart Fly. I've built an E powered Lancaster & will be using 2 batteries as follows. Inboard motors on 1 battery (roughly 3S 3300 70C) and the 2 outboards on another identical battery. In theory I should have enough power to fly on 2 motors because I wanted a specific size of motor with extra power. Motors are 35-30 & each has a 40 Amp ESC & according to my tests at full power on a fresh battery 2 motors almost pull 72 Amps for a few seconds & then drop to mid 60 Amp range. The plane needs 600 Watts to fly according to the build notes. I'd like to use power from both batteries to power the Rx too but know I can't just use a Y connection. Is this what I need?

    http://smart-fly.com/index.php?route...&product_id=99

    [/url]7D2_3547 by Larry Ingram, on Flickr[/IMG]

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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    You could use the smart-fly device but will still need a regulator (BEC)
    I would suggest two of these... http://www.greathobbies.com/producti...d_id=CCRBECPRO
    Put one on each battery, set the outputs 0.5V higher than what you want for servos and Rx, then connect BEC outputs together with the smart-fly.
    I would just use a couple of 10A diodes rather than pay $40US for the smart-fly.
    MAAC 12443
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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    Sorry for the slow reply but I've been on vacation & can't remember the password for this site so couldn't sign on. The ESC's I'm using have BEC's but everything I've read warns not to try to tie both sets of batteries together to feed the Rx. Seems the Rx will be confused by even very small differences in the voltage between batteries so I don't see how just adding a diode can cure that. What I want to do is make sure that if for some reason the ESC I have connected to the Rx gets cooked by an over amperage draw the Rx can instantly be powered by the ESC on the other battery. I have 4 motors, 4 ESC's, & will be using 2 ESC's per battery but right now only drawing Rx power from the one battery.
    Last edited by cicopo; 02-16-2017 at 11:52 AM.

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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    Were I in your position, this is what I would do:
    I happen to have a bag full of 1A Germanium signal diodes. For each of the 4 esc's I would cut the positive line and put a few diodes there in parallel. The number of diodes used depends on the current capability of the internal BEC. With all 4 esc's connected, they share the load between their BECs. If any one BEC fails, the diodes prevent current feeding back to the dead source. Using Germanium only introduces 0.3V drop rather than 0.7V for the more common silicon.

    P.S. I have done this on a twin with great success.

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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    Thanks. Sounds easy enough to do. Each BEC is rated 3 Amps if I remember right but I'll check. I have a good electronics store near enough that getting them is easy.

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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    For 3A BEC use 3 diodes. If you can't get them, try using six 1/2A diodes. You get the idea. If all else fails, use a 10A silicon diode.

    Electronics should have 100% safety factor. Meaning, a 3A source should use at least a 6A diode. However, your servos will not be drawing that much current so 3A rating each should be fine.

    Clear as mud?

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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    Clear enough. Although I'm no genius with electronics I did build a lot of Heathkit products back in the day & even further back wired up empty buildings that became telephone offices. I wouldn't even want to guess how many times I stripped & installed a wire with the wire wrapping gun.

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    Re: Wanted. Auto switching duel battery to Rx thing a ma jig.

    Sounds like you're an old pro. So, you should understand why I suggest such a large diode if using silicon. Normally diodes are thought to drop 0.7V. That is a good approximation but if you put a relatively small current through it, the drop will be less. In your case it might be 0.5V or even less.

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