In the Hellcat (from another thread) I'm using the cheapo HK radio that you have mentioned having.  Those receivers are spec'd at 6.5 volts max., and I was using a 2 cell lipo with a BEC.  Decided to try the 2 cell, 700 mah LiFe battery which would be a nominal 6.6 volts while running.  Long story short, I got rid of the BEC and just use the LiFe straight into the receiver.  The servo is plenty fast and there is no heating or glitching to the receiver that I can tell.  Of course, if your receiver is spec'd at much less, then you may (probably "will") want to keep using a BEC, so no point in changing to the LiFe's.

Out of curiosity, why did you get a 3.3 v. BEC?  I guess if it works, then it works. But I have 4.8v and 6.0v stuff pretty much ingrained in my head after all these years and can't even conjure up a reason to design a 3.3 volt receiver unless its for some indoor RC that uses a single LiPo for a battery.

Mike
I can't detect any difference in servo speed. The servo is a cheap generic, no specs for it, but no problem driving three throttles at the same time. Apparently there isa wide tolerance range with these things.

John

So you're probably running the receiver over the specified voltage, and just lucking out as far as not damaging it (or, you're shortening the life of the thing and don't know it).

Yes, by about 1.5% IF the batteries are at rated voltage.  By my measurements, all three that I bought are at or under an actual 6.6 volts after removing from the charger and letting them sit for an hour.  Depending on the voltmeter I use and the temperature of the room I'm in and how long they have actually been sitting and .... and .... and ....

My point is that the "6.6 volts nominal" and the "6.5 volts maximum" are BOTH computed values using averages and engineering deratings.  In the real world, it's close enough.  Those packs are sold as receiver packs and come with the warning to check your receiver spec's.  That is why I told John that if his receiver was spec'd at much less than 6.5v, to stick with the UBEC.

The receiver is going to have an internal voltage regulator anyway, that provides an operating voltage to its circuitry.  I seriously doubt that that regulator is going to go up in smoke from providing a 1.5% overvoltage on Vin.  And if it does, well it's a $5 receiver. 

These are sold by Hobbyking as receiver packs   (for like $2.65 @)  :

http://www.hobbyking.com/hobbyking/store/__14075__ZIPPY_Flightmax_700mAh_6_6V_5C_LiFePo4_Reciever_Pack.html

This is the description given:

A perfect reciever size battery to replace those warn out old fashioned NiCD and NiMh reciever packs. Although only 700mAh LiFePo4 batteries have a supeioir useable voltage range when compared with NiXX batteries.

This pack will work with most receivers on the market without the need for a UBEC*

Specs:
Capacity: 700mAh
Voltage: 2S1P / 2 Cell / 6.6V
Discharge: 5C Constant / 10C Burst
Weight: 41g (including wire, plug & case)
Dimensions: 53mm x 30mm x 15mm
Balance Plug: JST-XH
Discharge Plug: Standa JR Servo plug

* Always check the suggested input voltage of your reciever before using LiFePo4 batteries.


The receiver specs are just given as:

"Power   4.5-6.5 VDC (1.5 volts x 4)"


It isn't given as an "absolute maximum" as you often see in semiconductor data sheets.

I figure there is some room for a fudge factor somewhere in there.    

Also remember that Ni-Cd's come off the charger at about 1.4-1.5 v per cell. You wouldn't think twice about using a 5 cell Ni-Cd on a radio, so a 2 cell Li-Fe isn't going to hurt anything.

Mike
Your right, I actually got the ubec for using with an opto esc, just found by accident that it also drives the servo.

Since it works so well I will probably use a 1 cell lipo. Weight =almost nothing.