Just posted this for a beginner in electrics. i found it useful information for myself

When ESC HAS NOT built-in BEC function
No change is needed for ESC, just connect the input cables of UBEC with the battery, and plug the UBEC's output cable (connector) into one spare channel of receiver.

   
When ESC HAS built-in BEC function
You must disable the BEC function of ESC, i.e. you must first cut the red wire in the trio of receiver wires. Simply use a pair of wire cutters to remove a short section of the red wire near the receiver connector, and insulate the cut wire with a bit of electrical tape.

or it can be removed from the connecter with a sharp screw driver and insulated

I am unable to understand why anyone would want to use an additional battery when power from the in-built ESC is available.

Its always good idea not to fiddle with the ESC/BSC connector themselves.

Better still, get a Servo Extension cable (with Male-Female connectors on either ends) and cut the middle wire at both ends and remove the metal connector stub from the barrel. Use this cable in such situation between the receiver and ESC. You can reuse the ESC in usual way later if needed, if its connectors are intact.

hi guys.
seperate ubec has a double function.
most lower price esc have a beck that drops currant to 5 or 6 volt by 'generating heat'  ..........an unwanted side effect.....
if your esc has a 'switching' bec then this  heat is minimal as the currant is sent in pulses and generates negligable heat.
If you run a seperate 'ubec' then you have taken any unwanted heat away and let the esc power the motor quite happily without the risk of 'overheat power cutout' caused by the bec section.
as for a seperate battery for the ubec this is not nessesary as your second battery would be 6volt and conected directly to your rx leaving all the main battery for power (as would be done on gas etc).
as said above if running a 'second' ubec then the built-in one 'must be dissabled' or the esc will still generate heat and cause problems.
regards
don

Quoteif running a 'second' ubec then the built-in one 'must be dissabled' or the esc will still generate heat and cause problems.

Not true. If there is no current being drawn there is no heat dissipation in the BEC. It can be left alone.

Quoteas for a seperate battery for the ubec this is not nessesary as your second battery would be 6volt and conected directly to your rx

Isn't the "second battery" the same as the "separate battery" referred to? Perhaps, what you mean is that you do not need another 2S - or more - S LiPo.

BTW, even in a linear (non switching) BEC the heat generated would not be much if a 2S battery is used. Certainly not worth the added cost of another battery and BEC.