BEC powered via balance lead

[simon170]

The other day I was watching a quadcopter fly in my local park and I noticed that some of its systems were powered from the battery via the balance lead plug.
This got me thinking, could you power a BEC from the same plug if you didnt want to solder on to the ESC/battery wires?

[rangercat23]

Originally Posted by simon170

The other day I was watching a quadcopter fly in my local park and I noticed that some of its systems were powered from the battery via the balance lead plug.
This got me thinking, could you power a BEC from the same plug if you didnt want to solder on to the ESC/battery wires?

Or maybe some LED lights???

[simon170]

Yeah I geuss you could run those of them to... I just wondered if anyone here had done it?

[Charlie-III]

Yes, you could.
No, you shouldn't.

Why?????

Unless you add in a LVC or LiPO alarm, you can overdraw 1 or more cells in the pack. Since the pack is not through the ESC, there is no way to shut stuff down before a potential fire issue.
Also, if you're running 2S or more, are you drawing off a single cell and not full pack voltage? If so, you can overdraw that cell which will kill the whole pack (unless you like soldering on a LiPO pack...... )

Your choice.

[Olle P]

I think Charlie is overreacting and looking at the wrong problems...

As long as the balance lead is used by draining the red and black cables only (full pack voltage) there's no voltage issue (note below) as long as some LVC is in place at the ESC.

But...
Balance leads pretty much can't handle more than 1A current! So it works for feeding the Rx and a couple of LEDs, but not a high power servo.
Back to the voltage issue. The more current being drawn off the balance plug the more responsive you need to be about the battery running low. How responsive you need to be of course depends on the battery capacity. A scaler sporting a 5Ah battery will give you plenty of time to react with LVC set to 3.0V/cell and <0.5A drawn from the balance lead.

[simon170]

So maybe run some LED's off one, but not a BEC...

[madkawi650]

If you gear up the plug right, you'll be evenly drawing from both cells. Also.. If the esc has LVC then it will detect when the battery is low.
It sounds like an awesome way to connect the BEC. Makes it portable..
I've been using a servo plug soldered to the esc wires and the male servo end on the BEC which also makes it portable..


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[Charlie-III]

Originally Posted by madkawi650

If you gear up the plug right, you'll be evenly drawing from both cells. Also.. If the esc has LVC then it will detect when the battery is low.
It sounds like an awesome way to connect the BEC. Makes it portable..
I've been using a servo plug soldered to the esc wires and the male servo end on the BEC which also makes it portable..


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The ESC LVC will shut down the motor, but not a high draw servo running off the external BEC.
As Ollie mentioned, the balance plug wires are rather small, not able to carry much power through them.

[Calderwood]

Originally Posted by Olle P

I think Charlie is overreacting and looking at the wrong problems...

As long as the balance lead is used by draining the red and black cables only (full pack voltage) there's no voltage issue (note below) as long as some LVC is in place at the ESC.

But...
Balance leads pretty much can't handle more than 1A current! So it works for feeding the Rx and a couple of LEDs, but not a high power servo.
Back to the voltage issue. The more current being drawn off the balance plug the more responsive you need to be about the battery running low. How responsive you need to be of course depends on the battery capacity. A scaler sporting a 5Ah battery will give you plenty of time to react with LVC set to 3.0V/cell and <0.5A drawn from the balance lead.

I balance charge my lipos through the balance tap at 5 amps. That's how my Hyperion charger does it and it's certainly not a cheap charger. So they definitely can handle more than one amp.

Originally Posted by Charlie-III

The ESC LVC will shut down the motor, but not a high draw servo running off the external BEC.
As Ollie mentioned, the balance plug wires are rather small, not able to carry much power through them.

Regardless of wether you are pulling power from the main wires or the balance wires, the bec leaves the servo powered up even after the lvc hits.

Some guys are running jst plugs on 2.2 comp rigs. Ever compared a jst pin to a jst xh (balance plug) pin?

When ever I find the time and ambition I have been meaning to remove the main plugs/wires from my indoor 2s 3d plane packs and convert the escs to balance plugs.

[simon170]

If I read this right, some of you are saying a BEC will over discharge the battery once the ESC LVC cuts in...? Surely you'd just unplug it once the LVC trips?

[Calderwood]

Originally Posted by simon170

If I read this right, some of you are saying a BEC will over discharge the battery once the ESC LVC cuts in...? Surely you'd just unplug it once the LVC trips?

Yes you have to unplug any battery after the lvc hits since the bec will continue to draw.

[SINister]

Nice idea

If you unplug it when the lvc on the esc trips it won't hurt the batt. Most people set the lvc a little higher then the minimum so there's a little buffer there too. I run the jst plugs on my batts and it's never been an issue. If a wire that small can handle two swm, bec and a 7950 it should work with lights or bec.

[JohnRobHolmes]

Totally possible, I suggest using the outermost leads so full pack voltage goes to the BEC and the pack stays evenly balanced.


A servo does pull more average current than a motor in a crawler, so you will have to be very careful about overdischarging stuff. Once the motor cuts off, you will need to stop right then and there.

[madkawi650]

Originally Posted by simon170

If I read this right, some of you are saying a BEC will over discharge the battery once the ESC LVC cuts in...? Surely you'd just unplug it once the LVC trips?

+1 that's what I was getting at.
BEC and servo wires are tiny too, and they handle the 6 volt 5 amp draws just fine.


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[k7zpj]

Using the balance leads to power the BEC/servo could cause the battery to become unbalanced when not using all of the cells. Helicopters using high voltage packs power the receiver and camera from the balance leads. These are low power devices so they really don't unbalance the battery. Servos require more power and have a higher chance of unbalancing the pack. This could lead to a situation where you have low cells but the total pack voltage is higher the LVC in the ESC.


Larger capacity packs have 22 ga wire which are rated for a continuous 7A current. The JST-XH connector is rated for a continuous 3A. The wire and connector has enough capacity to power a BEC and 1 servo.


Bottom line is that you can do it, but it is more complicated than connecting the BEC to main power leads. I use a T-connector made out of a male and female Deans connector to power the BECs in my rigs.

If you really want to do it, here is a good link to an article that will help you identify which leads to use to get the right voltages.

https://sites.google.com/site/tjingu...ce-connectors/

Here are some references about the current capacity the JST-XH connector and wires.

http://www.jst-mfg.com/product/pdf/eng/eXH.pdf

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire strength

[simon170]

Sir John Holmes has spoken... his word is holy and final

[Olle P]

Originally Posted by nigels_world

I balance charge my lipos through the balance tap at 5 amps. That's how my Hyperion charger does it...

Then you're not following the instructions.

Manual for Hyperion EOS 720i, page 2:
"... Connect your Battery Main Connector to... the Output Cable set. Connect your Lithium Polymer battery Balance Connector to the EOS Balance Multi-Adapter. ..."

You should really use the main connector. My charger won't even start a balance charge, even at 0.2A, unless the main connector is attached.