so i am going to be using mg996r servos for my first project, and i wanna use li-po batteries on it.
the servo specs for voltage are 4.8-7.2V
all li-po packs are made from 3.7 cells so the closest for me is 7.4volts…
is there a way i can drop the voltage to 7v and make sure its not gonna go over 7v?
thanks 
You might could put 9 or 10 diodes in series across your battery terminals to keep the voltage from going greater than ~7v. Below are some available from Radio Shack.
Try a 7.1v Zener diode and a resistor instead.
Or better still, get BEC.
Alan KM6VV
A BEC is the correct answer… people seem to have good results with the Turnigy one and the Castle Creations one as well.
Be aware that a fully charged 2S lipo pack can approach 8.4 volts. This falls off kind of quickly to a more nominal range of 7.6 or so after roughly the first 10% of the pack capacity is depleted. YMMV but this is pretty basic RoT using common 2200-2400mah 18650 type Li-ion cells. LiPO are very similar but (probably) not exactly the same due to the different geometry. What I mean by this is the amount of charge that will represent the voltage step between 3.7V/cell and 4.2V/cell will vary from the rough 10% approximation depending on cell and manufacturer.
Series dropping elements like diodes or resistors will drop varying amounts of voltage as the load current changes. it is a lot like not having a big enough filter capacitor on the output of your DC bench power supply… you get ripple and noise on your power supply output. this can cause performance issues with your servos or other equipment. series elements are also horribly inefficient since the voltage they drop is converted to heat and wasted.
Because a BEC is doing a power conversion the heat generated is mostly losses in switching elements and I-squared-R losses of certain components. It is considerably less wasted energy than any series requlation approach unless the source voltage is very close to the output voltage. Using a BEC will also let you use an even higher pack voltage of 11.1 and possibly as high as 14.8 volts (3S and 4S respectively) which will make the BEC even more efficient because the input current drops even more.
I totally agree with Eddie here. I’m using this Turnigy UBEC together with a Polyquest 4900mAh Lipo for some time now. The UBEC got a swith that allows it to drop it’s voltage to 5 or 6 volts. Since it’s a switched regulator it doesn’t get hot and got a much better effectivity.
I just received my second set yesterday so this says enough about me beeing satisfied with the UBEC. 
Xan
Xan,
I didn’t see a 4900mAh battery, but then I probably only need 2500mAh on a 4DOF 'quad. I know that you can run the servos at 6V from the BEC, but how are you powering the BB2 and SSC-32? 11.1v sounds a little hot for the electronics. < 9.0 would be better, I think. I could of course set up a T220 regulator for 8 or 9 Volts, but that’s extra circuitry.
I’m considering your approach for my new 'quad, and possibly retrofitting to my 'hex as well.
Cheers!
P.S. I just re-read the specs on the BEC, does it have other taps for ~7.8v?
Alan KM6VV
How come you are still using the little lipo? 
Yup, this UBEC is good, and very well price. Bought 2 for my quad, though I have yet to test it.
If BB2 and SSC-32 doesn’t draw much current from the 9v supply, just use a VR to step down the voltage from 11.1v to 9v.
For ~$16 not a bad deal.
I’m using the “oldâ€
Oh, I just looked at quite a few LiPoly batteries on the page the first link went to, and found a 2800 mAh battery. I’m not familiar with all of this LiPoly stuff yet, so I mistakenly got data for a 3S (3 cell?). I’ll look again, and see if I can find a 2S that fits my available space. No reason to go to 11.1V I suppose. I’m still trying to figure this stuff out!
Yeah, I know how well it works with a “non- low-dropout” regulator on the processor board, and trying to run a 6V battery! I’m currently just using a 9V battery on the creepy hybrid. Guess I’ll do the same (at least initially) for the 4DOF quad.
the ~7-8V was supposed to mean approximately 7 to 8V; which I’d want to feed the 5V regulators.
Thanks for the battery tips!
Alan KM6VV
Hi,
I do also want to drop in and say that the Turnigy UBEC’s (continous 8A and 15 A burst) works very fine. For my LM Phoenix, A-Pod and Felix I’m using two UBEC’s on each. They all work very fine. What I like with the SSC32 board is that it has two power banks (VS1 and VS2), that makes it very easy when using two UBEC’s. I do hope the NG SSC32 also has two VS terminals…
On Felix I tried to use only one LiPo (2s 4000 mAh) for both servos and all electronics. And that works very fine! Without any resets. The electronics (BB2 and SSC32) are supplied directly to the battery. Since the voltage on a 2S LiPo can be from 8,4v to ~5,6v (most LiPo’s should be disconnected at 6v and lower) the BB2 and SSC32 work very fine.
I’m thinking of dropping the two 9v batteries on my LM Phoenix to. Much easier to replace only one battery too.
Hi KÃ¥re,
That’s good to hear! I’m learning. Your Felix is 4DOF x 4, same as I’m doing, so that’s probably my best course of action. Do you have a URL for that battery, or is it at HobbyCity?
Thanks!
Alan KM6VV
P.S. I’m cutting a Polycarb chassis for my 4DOF 'quad today!
I did buy it from HobbyCity but they don’t sell the 2S 4000 mAh LiPo I’m using for Phoenix and Felix now. But you should be able to find alot of other higher amps LiPo’s there.
Sounds great! Good luck with that, looking forward to see the end result.
Hi KÃ¥re,
Do you really need 4000 mAh? My hexapod only has a 2800 mAh battery (although it might be able to use more). The larger capacity batteries are large! I’ve only got room for a 6.0vdc Ni-MH 1600mAh battery pack in my current chassis. Pack measures 2.0625" x 2.875" x 0.625", and I’m currently slinging it below the chassis. I suppose I could move it up to a second deck. I’ll have to think about that.
Alan KM6VV
EDIT:
Got the Polycarb chassis cut: