I am about to set up my first electric model that uses a seperate reciever battery. This may be a dumb question but I would like to try and find out what size / capacity battery to use.

 I plan to run

3 x HS-645 MG   servos, on for rudder and elevator and one for the flaps

3 x HS-225 MG . 2 seperate for aileron, and one on the air up / down retracts.

(Plus throttle on ESC of course)

 Radio gear is 2.4 futaba 6exp

Given that I could run at 4.8 to 6 volts what size battery would give good duration, say 4 flights on a 70 inch warbird, bearing in mind I would be trying to stay within scale type flying.

 I understand I could wait and check the servos on the bench but this is difficult with respect to representative loads and usage, and I have not been able to find stall current for the servos which would give some idea of maximum current draw under the very worst conditions.

I have been considering a lipo and realise I need the appropriate regulator but I do not want to run the risk of a power out on the first flight!!

Hi david ( small d , but the big O )

I would use a LiPo but thats just me ..... I am totally LiPofied these days because I am very familiar with them and all my chargers are LiPo etc. The thing to remember with using a 2s LiPo is that your flight duration ( not that the radio gear alone takes much juice anyway ) will be greater due to the way the regulated system works. Lets say for example you choose a 5V system ( like a "normal" flight pack ). Say the radio gear needs 1A to operate. At 5V @1A is 5watts, however if you use say a 2 cell LiPo at nominal 7.4V feeding a decent switching regulator, then to provide 5 watts the input current needed is only going to be around .6A ( 5 / 7.4 ) so your 2s LiPo will last ages. Use 3s and the duration gets even longer, although not all switch regs designed for this purpose will take a 3s input. I run a 65 " 8.5lb warbird on a 2s 1500 LiPo via a powerbox systems digiswitch regulator combo ( brilliant bit of kit incidentally - heres a link Als do 'em in  the UK if you dont want to go direct) and get at least 4 average flights, and the LED is still full green at the end of the day. Remember even when a 2s LiPo i snearing "flat" at 6V ...its still higher than the input needed to maintain your radio requirements...3s even more so of course.

Take a look at this piece I did some time ago.

If you want to stick with good 'ol niXX and need to keep weight down then a small pack of around 600-900 mah should be fine for an average flying session. Receivers alone take vey little ( although the spekky 2.4Ghz is relatively thirsty ...not sure about the fuby? ) and servos take surprisingly little if control surfaces are well hinged, free moving etc. Stalled of course they take a lot more, and under flight pressure the current of a standard analogue is around 1A or so, digitals tend to take more juice. HTH

Thanks for the info so far.

The futaba doesn't seem too thirsty as far as I can tell but haven't actually done any measuring.

Definitely LIPO then.

I like the idea of the digiswitch but it reckons on a max of 6 servos and I am running seven having just found out the warbird uses two flap servos instead of one. Mind you the digiswitch does handle up to 5 amps and not all the servos will be running at once, or pulling max current all the time. Do you reckon it would do the job?

Well with the usual disclaimers I would reckon it would probably be ok.

You could maybe use a smaller analogue servo for the retract valve ?

In my Spit I am using 4 digital servos, 1 small analogue for valve, 1 x analogue full size for throttle and driving two electronic switches for ancillary lighting /gun equipment .

Timbo, thanks 

Yes I have already gone with a smaller servo for the air valve.