Aug 21, 2019, 03:51 PM
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Discussion

# Where to get power for BEC?

Where are you picking up the input power for BEC's? I've tried getting it from the main battery connection and the balance plug, as in the attached photos.

### Images

 Aug 21, 2019, 04:37 PM Either method should work, but the left-hand photo looks a little suspicious. The balance connector on any battery pack is always wired in cell order, so full battery voltage will appear ONLY on the two outer pins. Your photo shows the JST connector wired to two ADJACENT pins, so you're picking up the voltage of a single cell rather than full pack voltage. For the typical BEC, the minimum input voltage is 2 cells. Also, be sure the polarity is correct.
Aug 21, 2019, 05:00 PM
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Quote:
 Originally Posted by ALL e RC Either method should work, but the left-hand photo looks a little suspicious. The balance connector on any battery pack is always wired in cell order, so full battery voltage will appear ONLY on the two outer pins. Your photo shows the JST connector wired to two ADJACENT pins, so you're picking up the voltage of a single cell rather than full pack voltage. For the typical BEC, the minimum input voltage is 2 cells. Also, be sure the polarity is correct.
Bad photo. If you look close you will see it is picking up from the two outer pins. Sorry about the poor photo.
 Aug 21, 2019, 11:03 PM Registered User How much current is the BEC rated for? The balance connector is limited to only a few Amperes.
Aug 22, 2019, 07:50 AM
FASST flyer
Quote:
 Originally Posted by Wintr How much current is the BEC rated for? The balance connector is limited to only a few Amperes.
I can't remember for sure, but I'm pretty sure it's a 5 amp BEC. That's 5 amp BEC output, at 5 volts. So 25 watts. Input voltage from a 6S pack is ~22 volts, so 25 watts at ~22 volts would be roughly 1.2 Amperes, right?

On aircraft larger than 6S I don't use the balance connector for BEC power. I have used various methods of tapping power from the main battery circuit.

Do you pick up your BEC power at the main battery connector?
 Aug 22, 2019, 08:18 AM Registered User At 5 amps, I would connect the BEC to the power leads instead of the balance leads. Although the input current won't be 5 amps if the output is 5 amps. It will be lower by about the ratio of the input to output voltage.
Aug 22, 2019, 09:45 AM
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Quote:
 Originally Posted by KatManDEW Do you pick up your BEC power at the main battery connector?
Yes. On the ESC side, that is. I solder another set of wires in parallel with the wires that feed the ESC. That additional set of wires end in some sort of connector, usually JST-RCY because they're readily available with pigtails already attached.

Something like the attached picture. This, of course, adds another possible point of failure, and doesn't give any redundancy against main plug failure. But I can live with that.

### Images

Aug 22, 2019, 09:58 AM
Registered User
Quote:
 Originally Posted by KatManDEW I can't remember for sure, but I'm pretty sure it's a 5 amp BEC. That's 5 amp BEC output, at 5 volts. So 25 watts. Input voltage from a 6S pack is ~22 volts, so 25 watts at ~22 volts would be roughly 1.2 Amperes, right?
Just to add, no supply is 100% efficient, so input power would be closer 31W at full load; still only about 1.4A, though. Close enough that surges in power draw by a strong servo could cause a drop in power available to the BEC, so I'd use the main power connector, too.
 Aug 22, 2019, 02:02 PM Registered User I wire my BEC directly to the ESC's battery plug. It means only one plug to connect when I'm ready to fly. I don't like the extra plug in the photo in post #7 because it's one more potential failure point
 Aug 22, 2019, 03:06 PM Registered User Slightly off-topic: remember it's best to have only one ground/negative wire going to each component. For example, I have a battery-voltage monitor connected to my RX. The RX get's its power and ground from the BEC in the ESC via the 3-pin connector, so I have just one wire from the battery positive connection to the RX's voltage sense pin. It's not instantly fatal to have multiple ground connections, but "ground loops" like that can generate some of the most difficult-to-troubleshoot bugs.
Aug 22, 2019, 05:31 PM
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Quote:
 Originally Posted by lametec Yes. On the ESC side, that is. I solder another set of wires in parallel with the wires that feed the ESC. That additional set of wires end in some sort of connector, usually JST-RCY because they're readily available with pigtails already attached. Something like the attached picture. This, of course, adds another possible point of failure, and doesn't give any redundancy against main plug failure. But I can live with that.
That's basically how I do it, except that I do it on the battery lead, as in my photo. That allows me to easily power up the BEC and aircraft, without the motor, to check out everything safely without the motor armed. The BEC's are usually located somewhere such that would not be possible to do that with the BEC wire connected to the ESC connector.

I use Mini Deans for the BEC connector, as in my photo.

Thanks for the feedback.
 Aug 22, 2019, 05:45 PM pull up -- PULL UP!!! I attach to the esc power wires, but not to the connector pins because I find that a pain. Instead, I shave away some insulator jacket an inch or two back from the plug and solder the bec power leads to the exposed copper. And then of course cover the splices with HST.
Aug 22, 2019, 07:36 PM
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Quote:
 Originally Posted by hard line I attach to the esc power wires, but not to the connector pins because I find that a pain. Instead, I shave away some insulator jacket an inch or two back from the plug and solder the bec power leads to the exposed copper. And then of course cover the splices with HST.
Yes it is a pain. I was wondering if there was a better way that I overlooked.

Thanks for the suggestion!
Aug 23, 2019, 11:46 AM
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Quote:
 Originally Posted by KatManDEW Yes it is a pain. I was wondering if there was a better way that I overlooked. Thanks for the suggestion!
https://graysonhobby.com/deans-bec-lighting-tap.html

I've used these connectors on a couple of planes to hook up a BEC and they worked great.
Aug 23, 2019, 03:31 PM
FASST flyer