BEC wiring question / thoughts

[dillwhacker]

Hi peeps, just been discussing this with a mate...
If you have a 12S system with the BEC wired onto the 'lower' 6S, like in the attached pic, which is very common.
1.JPG
Then if you connect only the top 6S pack, the BEC is in a strange state, the black of the ESC is connected to the black of the BEC, and the red of the BEC is connected to the black of the LiPo, the most negative point.
So the ESC is experiencing reversed voltage on it's input wires....
I've never heard anyone say that you should / must connect the LIPOs in a certain order..so I can only assume the BEC is designed to cope with this with a chunky diode on its input wires to protect it?
Anybody know for sure? Or any comments?
Cheers!

[treff]

I think you will find that when the top battery is connected you then have 2 negative potentials on the bec so no voltage is developed across this point and no
current may flow.

[dillwhacker]

I think you will find that when the top battery is connected you then have 2 negative potentials on the bec so no voltage is developed across this point and no
current may flow.

Nope. I don't find that.
I find one negative - the black wire of the only LiPo connected and the most negative part of the whole circuit.
What I find is that the motor bleeps 6 times, so the ESC must be alive and seeing 25 Volts.
Which means the return current to power the ESC is flowing backwards through the BEC.

[Grumpy]

Then if you connect only the top 6S pack, the BEC is in a strange state, the black of the ESC is connected to the black of the BEC, and the red of the BEC is connected to the black of the LiPo, the most negative point.
So the ESC is experiencing reversed voltage on it's input wires....

But the black wire of the ESC itself isn't connected to anything because the 'low' battery that it's supposed to connect to isn't installed yet. So the BEC is actually only connected on it's red wire, the black wire not connected to any battery terminal so there is no circuit completed.

I've the same arrangement on one of my helis and can confirm that connecting only the 'top' battery does nothing. If yours does then something is wrong with your wiring. However it does work if you only install the 'bottom' battery, possibly that's what you are doing when you get the beeps?

If you dont believe me then try marking the diagram with what you think is a closed circuit and you will find there isn't one until the bottom battery is installed.

[trillian]

Nope. I don't find that. That's cobblers.
I find one negative - the black wire of the only LiPo connected and the most negative part of the whole circuit.
What I find is that the motor bleeps 6 times, so the ESC must be alive and seeing 25 Volts.
Which means the return current to power the ESC is flowing backwards through the BEC.

Yeah the BEC is effectively becoming a jumper to close the circuit. It should work fine though when connecting the BEC side first.

[dillwhacker]

Agreed Trillian.
Grumpy, that's not true - the ESC black is connected to the BEC black, so there is a circuit back to the battery through the BEC, but the current is flowing into its black and out of its red, back to the LiPo.
Yes it works as expected with only the lower batt connected (BEC powered and not ESC), and with both connected.

[Grumpy]

ok, I see what you are getting at now, penny took a while to drop!

Connecting the 'top' battery only does nothing in terms of energising the BEC, output from BEC is zero. But it does energise the ESC (which counts 6 cells) which indicates that there must be a circuit back through the BEC.

[xygax]

I think you will find that when the top battery is connected you then have 2 negative potentials on the bec so no voltage is developed across this point and no
current may flow.

That is false. during the charging cycle of the main bulk capacitor on the ESC the supply as seen by the bec is -25v ( reversed ) you should always connect the most negative battery first or risk damage to the BEC do the circuit analysis when the capacitor is discharged view it as a short circuit this will show you where the voltages are.

S

[KevDavies]

^^^ What Steve says. On my BEC equipped helis I always connect the Battery that supplies the BEC first, this allows the fbl unit to power up and self test. The ESC will not arm until I plug the second battery in

[dillwhacker]

Thanks Steve & Kev.
That was my point - this seems potentially () a nasty surprise for the BEC, but I have never seen a warning from the BEC manufacturers, or even a discussion recommending connecting the bottom pack first.
Maybe everybody just does it that way instinctively ?

[ragge1]

this is how I do it

[dillwhacker]

Thanks Ragge1, that's the same wiring as my original post. I think we're all agreed on the wiring.
The question is not "how to wire it", but "what happens if you connect the top pack first".
Cheers!

[Si Goodchild]

Trev, if you're using the Gryphon Quasar I sold you, just run that sucker on 12s!

[ragge1]

Thanks Ragge1, that's the same wiring as my original post. I think we're all agreed on the wiring.
The question is not "how to wire it", but "what happens if you connect the top pack first".
Cheers!

cheers , just did a test , nothing special happens , when you plug the top pack first you just get some pips from the esc ( assume gonna be different pips according to which esc your using ) the when you plug the other pack the FBL will do its intiation and you have controll your heli as normal
I filmed it but will maybe upload it tomorrow

[Grumpy]

Yes, that's what i got when i did the same experiment. ESC counts the cells but does not arm, there is no output from the BEC so the Rx/FBL does not activate.

I measured voltage drop over the BEC input wires with only the 'top' battery fitted and there was 0.75v drop. That sounds like what you would expect if there was a diode of some sort in there.