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[DickyD]

This will probably seem like an incredibly daft question, but could someone tell me why a seperate electrical supply would be needed by the Rx ?
I dont use this system on any of my boats and was wondering if I am missing out on something here.
Thanks.

[Colin Bishop]

I like to have a separate supply to ensure voltage stability etc. but I think it's just a matter of preference these days.

[banjo]

Doesn't it help if you have heavy duty servos?
I have a separate supply for the servos on my Tito.

[Shipmate60]

DickyD
The BEC is usually limited to a MAX of 1 amp (submarine ESC's now have 1.5amps).
This limits the receiver to about  3 Servos, if you run more servos you will need a separate battery supply.
Also the BEC usually supplies at 4.8 to 5 volts, 4 x AA disposable batteries give 6 volts a 20% increase which does have an effect on servo speed and strength.
Obviously if you main battery is 6 volt, as it runs down to 4.8 to 5 volts the BEC cuts out leaving model stranded.

Bob

[DickyD]

Right thanks chaps, I have 1 servo and the speed controller in all mine. 2 have JJC sound modules and run on 9.6v.  2 run on 7.2v . and 1 will eventually run 1 on 6v LA.
As I haven't flattened a battery yet I will probably be alright as is.
Thanks was just wondering. 

[malcolmfrary]

Funny things, voltage regulators.  A typical 7805 needs maybe a couple of volts headroom to work in, but dropping down into this headroom does not stop it working.  It still tries to stabilise, but at whatever the supply is less a variable bit.  It is surprising how low the voltage supplied to the RX and servos can go and still have them working.  The important thing is that the voltage is reasonably stable, as the timing components in the electronicery do rely for stable operation on the supply voltage not fluctuating greatly. 
If using a 6V SLA I would not use a BEC as such because the model radio control gear was designed around using this in the first place. 
Using a 6 cell NiCad pack I would use the BEC as this is a bit near the top rated voltage for the chips in the RX & Servos, and very definately with 12 volts. 
Should I want to use several servos or other bits n pieces I would probably use external BECs, but a separate RX battery is a very valid option.