Q. The guy at the hobby shops say ProModeler servos won't work with my 2700mAh NiCd battery pack. Is this true?
A. No offense but it sounds to us like someone's trying to blow smoke up your skirt because ProModeler servos operate fine on NiCd batteries and there's absolutely no huhu! Let me explain a bit more. If you look at the specifications tab on the website (there's one
for every servo, perhaps you've seen it) you'll see we disclose performance at various
levels of voltage like 5.0V, 6.0V, 6.6V, 7.4V, and 8.4V.
Here's the thing; 'how' you
make that voltage doesn't matter. The chart below is for the DS360DLHV servo. The specs for it disclose a lot but take special note of the 5 columns showing its performance (torque/speed) at various voltages.
But take note of what's not mentioned in the specs above. There's nothing there whatsoever about
how you get the voltage. E.g. whether it's with NiCds . . . or with a hamster running in a wheel connected to a DC motor generating power that gets cleaned up with a BEC and fed to the control avionics (receiver and servos). Nothing about the how, just the levels, e.g. voltage!
- Hamster wheel powered light bulb? Not as cray-cray as it might seem!
For example, let's say you want to use
nickle chemistry. If so, then you have two choices; NiCd (nickle-cadmium) or NiMH (nickle-metal-hydride). With either, because individual cells operate at 1.2V/cell, then you can make various levels of voltage by putting them together in series (nose-to-tail), thus, creating a battery pack (a 'battery pack' consists of individual cells).
- 4S = 5.0V - 4S is 4-cells in series, or 4x1.2V/cells=4.8V (5V nominal)
- 5S = 6.0V
- 6S = 7.2V
- 7S = 8.4V
- Summer afternoons shooting touch-and-goes until dark calls for a big pack
Similarly, lithium-chemistry works the same way but because it begins at a higher state of voltage, it requires fewer cells to make a given level of voltage. But the actual chemistry behind getting there - voltage level? That simply doesn't matter. Not in the slightest little bit!
Anyway, there are four popular lithium-based chemistries, which in increasing order of volatility are LiFePO4 (Lithium Iron Phosphate), LiFe (Lithium-Iron), LiIon (Lithium-Ion), and LiPo (Lithium-Polymer). This means the LiFePO4 are the most stable and the LiPo are the least stable (when you read about battery fires inevitably it seems to involve LiPo packs).
- 2S = 6.4V (2 cells in series of 3.2V per cell)
- 2S = 6.6V (3.3V cells - slightly higher voltage = more volatile)
- 2S = 7.4V (3.7V cells - most volatile)
BEC - Battery Eliminator Circuit
- Synthetically derived - you set a range from between 5-8.4V
- a big ol' 2S2P 5000mAh LiFePO4 battery pack is perfect for many models!
Bottom line regarding battery packs used to make the voltage to operate your servos? The chemistry itself doesn't matter. Basically, whether you use NiCd, NiMH, LiFePO4, LiFe, LiIon, LiPo,
or synthetically derive it via a BEC the servos don't care! Above we listed the most popular combination of cells and chemistries to make packs for powering control avionics (with the number of cells in series and parallel as required to
reach a certain voltage level). The math is easy!
For example, the 5000mAh LiFePO4 pack above is a 2S2P, which means 2-cells in series to get to the desired voltage, and two of these sets in parallel to boost capacity. Here's the math . . . two 2500mAh cells of LiFePO4 chemistry (at 3.2V each) in series gives you 2500mAh at 6.4V (3.2V + 3.2V = 6.4V) and then putting these two together with another set just like it, but wired in parallel then gives you 5000mAh (2500mAh + 2500mAh = 5000mAh). This is how we get to 6.4V with a whopping 5000mAh of capacity using 2500mAh cells. And believe me, this is easily enough for a Cub to fly a dozen times between charges . . . easy peasy!
- Modeling is the fulfillment of many dreams - and cheaper than therapy!
However, the important thing to take away from reading this is;
as long as you operate your ProModeler servo within the range of 5-8.4V, the how
regarding getting there simply doesn't matter. Do you take my meaning? When you give it 7.2V, a servo doesn't care whether it comes from a 7S NiCd vs. a 2S LiIon vs. even a BEC because 7.2V is 7.2V is 7.2V pretty much no matter what!
Same thing when you want to run a 6V pack because a 5-cell nickle-based chemistry battery (for example, composed of five 1.2V 1650mAh cells - as shown below) gets you there beautifully. And take note of the two-lead construction of a ProModeler pack, which permits you to run two switches for enhanced redundancy (safety) and deliver 10A of current (because each connector is rated at 5A and using two are additive, so 5A + 5A = 10A total into the receiver).
- Rugged, reliable NiMH construction, this 5-cell pack delivers 1650mAH
Anyway, since batteries fail gracefully over time (while switches fail
like light bulbs, e.g. suddenly), then two switches are how pros
approach redundancy (because the odds of both switches failing on the
same flight are astronomical). This works better than using systems with
two battery packs because toting the dead weight of a spare battery
pack makes as much sense as towing a spare car with you everywhere (take
note; cars only have one battery, so too do real airplanes, as do cell
phones, laptops, and many other mission critical applications).
Regarding how fast/powerful a given servo is within a certain voltage range? We disclose that
in the specs, also! Just scan down the voltage column to find what you're looking for - it's easy!
- Built in a pouch, a prismatic pack are cells taped together to form a brick
Anyway, when weight doesn't really matter, the nice thing about old school battery technology using nickel-based chemistry is it's cheap, reliable, rugged, and is capable of delivering very high amperage (current) which is useful when you're running a passel-full of servos in a model. Thus,
there's still a lot to recommend nickle technology . . . even considering they were first invented in 1899, and went to the moon with Apollo in the 1970s!
Bottom line? Your hobby dealer is mistaken about not using NiCds to power ProModeler (or any other brand) of servos!