Background: ProModeler servos are rated at 7.4V (nominal voltage for a 2S LiPo pack under load), However, it's widely known that fresh off a charge, the output of a 2S LiPo may approach 8.4V. Thus, it's not illogical to wonder; is it OK to use 8.4V as derived synthetically via a voltage regulator (i.e. the BEC on an ESC), or the voltage out of a power expander/box? In fact, we're often asked variations of this same basic question - like this;
Q. Is it OK to set the output-voltage of my XXXX-device to 8.4V with ProModeler servos?
A. The short answer is yes . . . but we recommend using a battery instead of a BEC - even for our own line of ESCs, some of which include a BEC!
Long Answer: Many hobby shops sell BEC equipped ESCs (BECs are part of an Electronic Speed Control), power expander devices (often used with multiple servos per control surface on large model airplanes), as well as standalone BECs. In fact, most ESC manufacturers include the BEC feature for free because it's extremely cheap to implement (and this fact alone should tell you all you really need to know). As background, BEC means Battery Eliminator Circuit, and the heart of the idea is to use FETs (Field Effect Transistors) to derive a lower voltage from a higher voltage source.
A well known example involves outputting 4.8-8.4V (user selectable) from a 3S (11.1V) LiPo pack, then using the BEC to power the receiver/servo (instead of a separate receiver pack). This is where the battery eliminator part of the name comes in. Thing is, BECs (and built-in BEC on an ESC) are purely a cost savings measure for manufacturers. And the problem with cheap solutions is that while a $10 voltmeter may show 8.4V, if you use a tool like a $1000 oscilloscope, you'll see a different truth. Basically, that 8.4V isn't clean. Not clean like when it comes from a battery. In fact, it's downright dirty and fuzzy . . . like this.
- When viewed on an oscilloscope, the voltage supplied by a BEC is fuzzy - yuk!
And this also holds true for pricey power expanders, and standalone BECs (even the best ones) and as for why the output voltage is fuzzy, This is because of how FETs work. Basically they're fast switches. Remember when you were a kid and discovered you could make the lights in the hallway dimmer by flipping the on-off switch really fast with your hand? FETs work the same way! Anyway, without getting down into the weeds with technical stuff, just remember this; the BEC came into existence solely for manufacturers to save money (or add features cheaply). They have absolutely 'nothing' to do with high performance.
Bottom line? The output from a hobby-grade BEC is – at best – suitable for a $200 toy truck, perhaps for the driving lights, or to power a winch, or maybe for some cheap foamie model airplane . . . but it sure as Hell isn't something we'd recommend for powering good quality electronics!
Of course, this isn't exactly breaking news amongst experienced modelers and electronic engineers (but few of our customers are EEs). We're of the opinion hobby dealers who continue to traffic in BECs these days are either ignorant, cavalier in their responsibility toward trusting users, or maybe just in it for the money.
But there's another issue. Performance. This is because when you nail the throttle, the propulsion battery voltage is drawn down. Basically, the BEC circuit is fighting to provide stable voltage to the servo - but it can't. And this happens 'exactly' when you need the most performance from your servos . . . like when a wheel is wedged in a crevasse, or during a waterfall, during the hover, or while performing a torque roll and you're moving all the flight controls at the same time (while also varying motor thrust with throttle).
Bottom line? When the propulsion motor is making its strongest demands is often exactly when your servos aren't seeing anything like 8.4V, and this means the servos aren't developing the torque or the speed you've paid for! Cutting to the chase; an inexpensive 2S LiPo battery isolates your pricey electronics from all this nonsense.
Anyway, the trick to selecting a battery is first, size it for how long you operate your model between charges (an 800-1000mAh 2S is usually plenty for an hour, or more with servos of 180oz-in), and second, to get one with a sufficiently high C-rating to withstand the power demands of a stalled servo (without browning out the receiver).
For our part, while we offer packs ranging from 2S 250mAH LiPo through 5000mAH LiFEPO4 packs as used in giant scale 104" models, our advice isn't due to this. It's because even the cheapest battery you can buy online is a million times better than any BEC output on the planet at any price.