Castle Creations has a wide range of ESCs with clearly labeled ratings.
A 12S pack was made with two 6S packs in series for the Ag
Wagon. Note that the label shows they are 6S and 5,000 mAh
packs capable of 60C discharge.
thing to remember is that we determine power in our models
by multiplying volts by amps and get the total watts our
systems produce (P = IV).
• Resistance: This is expressed in ohms and abbreviated as
the Greek letter “Ω,” but in our equations we use “R.” Think
of resistance as a crimp in a hose causing a restriction. Low
resistance is always our goal to make sure we get the most out
of our systems.
• Gauge: This refers to the size of our wires in the system.
It’s measured in American Wire Gauge (AWG), and the bigger
the diameter, the lower the AWG number. A 10 AWG wire
is thicker than the 22 AWG we see on servos. A bigger wire
means lower resistance.
• Power loading: When discussing the power in our electric
airplanes, we refer to it as how many watts per pound are
being produced. If our airplane weighs 5 pounds and we have
500 watts of input power, we have 100 watts per pound (500
watts/5 pounds = 100 watts/pound).
• Kv: This is a term commonly referred to when people
discuss motors and is known as a motor constant, specifically
voltage constant. It indicates how quickly the
motor would turn at a given voltage if there
were no internal resistance. It’s expressed as
rpm/volt. When you see motors listing a Kv
of 500, for every volt applied to the motor
without a load (no propeller), the motor will
turn 500 revolutions per minute.
• Efficiency: Being efficient is better, but
when applied to our electric airplanes, it’s
bantered about like the Holy Grail. Nothing
is 100% efficient, but we do our best to
reduce things such as resistance that affect
our systems. I referenced input power earlier
and efficiency is the ratio of input power to
output power. For all purposes, we determine basic power
requirements using input power.
• ESC: This is the electronic speed control that connects
between the battery and motor. It is also connected to the
receiver-throttle channel and controls the motor. It is generally
rated by the number of cells and current the system can
If you are using LiPo batteries, which have become the
standard in electric flight:
• 3S, 4S, etc: Battery packs are made up of a number of cells
in series and this number represents that. If the pack is listed as
a 3S pack, then it has three individual cells connected in series
within the pack, each with a nominal voltage of 3. 7 volts. The
pack’s total will then be listed as an 11.1-volt pack. A 4S pack
would be 14. 8 volts, etc. ( 4 cells x 3. 7 volts = 14. 8).
• Pack capacity: This is the capacity in either milliampere-hours (mAh) or amp hours (Ah) of the pack. A typical 3S
pack might be listed as either 2,200 mAh or 2. 2 Ah. A large
pack with more capacity might be shown as 5,000 mAh.
• Discharge rating: “C” represents the capacity of the LiPo
pack. Labels will typically show the discharge rating of the
pack as 25C, 30C, or whatever the manufacturer believes
the pack will handle during discharge without degrading
the pack. Discharge ratings, sometimes mistakenly referred
to as C-ratings, are often overly optimistic. A 2,200 mAh
pack rated as a 30C pack, however, could be discharged at
66 amps (30 x 2. 2 = 66) without being damaged. This is
optimistic, but the number is a guideline. Packs with higher
discharge rates have lower internal resistance, which is a good
• Charging: The important thing to remember is to buy
a balancing charger. This ensures that each cell within the
pack matches the others. Get one that will do the maximum
number of cells you expect to charge at the rated pack capacity.
A 2,200 mAh pack would be charged at 2. 2 amps, a 5,000
mAh pack would be charged at 5 amps, etc.
I Thought It Was Simple!
It is! Electric power can be as complicated as you want to
make it, and some pilots like to get into the details. But it can
also be simple. You’ll see and hear the terms listed often and it’s
important to have a basic understanding of them, but a degree
in electrical engineering is unnecessary. You will appreciate
knowing them if you call for technical support and the
manufacturers begin asking questions.
If you want to try an electric airplane or helicopter and learn
30 Model Aviation FEBRUARY2014 www.ModelAviation.com