Voltage depends on servo. Look inti life or lipo + reg.

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If using normal servos stick to a 4 cell pack. I wouldn't use AA. NiMhs give more capacity than the old NiCd packs but they don't give as much current as the NiCds. It is also difficult to know how long they will keep their charge. The eneloop types are better for keeping charge. A LiFe or Lipo battery and a regulator is more predictable. A 4 Cell pack will be about 5.6V after charging and a 5 cell or 6 volt pack will be up to 7V. I'd go for 4 NiMh D size eneloops personally. You do need to keep a check on the off load voltage and the load voltage when drawing say 5 amps on a bigger heli and make sure that no servos are binding or stalling as that will push the current right up. I think that 4 NiMh D size is probably more reliable than using a BEC. Only because it is difficult to know how reliable a BEC will be. If running off a low voltage like a 2S Lipo it could be a linear BEC which has less to go wrong than a SBEC. In general though with current standards of quality being hit and miss, it is a good idea to power things up for a few hours on the ground before flying them.

I'm thinking of testing some NiMhs for voltage drop at a few amps. I'll do it sooner if anyone wants some answers. Most people can do it themselves using their charger on discharge.

Make a small investment and go 7.4v 2 cell Lipo & Regulator, you'll be amazed how much better your system will perform.You'll have bags of power to your servo's, longer runtime and less hastle in the long run.

I've got a Raptor 30 NiMH Battery Pack 4.8v 3300mAh Twin Connector | eBay battery in my Rappy 30. I've only got standard Futaba S3001 servos in it as it's just for learning to fly and not 3D and the battery lasts a good days flying, as long as you don't keep it plugged in all the time and spend ages setting things up. I've got a LED battery indicator on the heli too so I can see when it's getting low similar to this one RC Model 7 LED Battery Voltage Indicator Monitor for Car Helicopter Truck Plane | eBay and this set up works for me!

For the 700 I've got a LIPO and regulator.

Thanks for the advice folks