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, a BEC on an ESC, or via a power expander/box? In fact, we're often asked variations - framed in various ways - all asking the same basic question;
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.
Long Answer: Many hobby shops sell BEC equipped ESCs (battery eliminator circuits as 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). It's where the battery eliminator part of the name comes
from. 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 $500 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 holds true for power expanders, and standalone BECs (even the best ones that are made in America) and as for why it's fuzzy, it's because
of how FETs work. Basically they're fast switches. And they function
pretty much the same as making an overhead hallway light look dimmer by flipping
the on-off switch really fast with your hand. Anyway, without getting
down into the weeds with too much technical stuff, just remember this; the BEC
came into existence 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 driving
lights, a winch, or some cheap foamie model airplane . . . but it sure as Hell isn't something we'd
recommend for powering good electronics! Since this isn't
exactly breaking news, we're of the opinion hobby dealers who
continue to traffic in BECs are either ignorant, cavalier in their
responsibility toward trusting users, or maybe just in it for the
But there's another issue. Performance. This is because when you nail the throttle, the propulsion battery voltage is drawn down, the BEC is fighting to provide stable voltage to the servo - but it can't. And this often happens exactly when you need the most performance from your servo, like when a wheel is wedged in a crevasse, or when torque rolling and you're moving all the flight controls as you vary motor thrust with throttle. Bottom line? That's when your pricey servos aren't seeing anything like 8.4V, and this means the servos aren't developing the power you've paid for. A separate LiPo battery isolates your pricey electronics from all this nonsense.
Anyway, the trick to selecting a LiPo is first, size
it for how long you operate your model between charges (an
800-1000mAh 2S is plenty for an hour, or more), 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, we offer 60C 2S2200mAh packs (probably good for 3 hours). 60C may be rightly considered overkill but the delta between it and a cheapo 35C pack is minimal. Moreover, we have them built with two receiver plugs instead of one to double the available current to the servo bus. Anyway, they weigh about 3-1/2
oz, which experienced modelers know is negligible for a 10 lb
crawler, or a model with a 60” wingspan.
Last thing; since most guys get tricked
into buying a cheapo battery charger when they first get into the
sport, use that one to charge the electronics-pack while at the same
time using the good charger for the propulsion-pack. Frankly, dealing with
two packs isn't that big a deal and with clean voltage your expensive electronics won't be subject to glitches, spikes (as a motor
unloads), draw down (depressed voltage because of the onrush of current due to opening the throttle), or the ripples and noise from the FETs.