Lithium-Polymer (Li-Po or Li-Poly) Battery Characteristics
Lightweight lithium ion polymer (Li-Poly) batteries have made a huge impact on the performance, and subsequently
the acceptance of electric systems as a replacement for the traditional nitromethane (nitro) and gasoline power
systems in model airplanes and helicopters. Their energy density (Wh/kg is the most common unit of measure),
combined with the relatively new and extremely powerful brushless motors, electric power systems are rivaling the
internal combustion systems in terms of both energy and duration.
Development of both the brushless motors
and the advanced battery technologies has been, both officially and unofficially, a joint venture between
government and civilian research and development efforts. If you keep up with the news headlines for NASA, defense
contractors, green energy researchers, and similar organizations, you have witnessed the plethora of new vehicles
that have been built tested, and in may cased deployed in the field. These range from micro air vehicles that
carry surveillance equipment for the military, to hybrid and fully electric passenger vehicles, vastly improved
power sources for portable devices and equipment, and a whole lot of new products for the model enthusiast.
the right is a scary sight - a severely bloated Li-Po battery pack (next to an unbloated one). These two
packs arrived in the same undamaged shipping container, but one looked like it was ready to explode -
Scary! I took the photo and then put the pack outside in case it decided to self-destruct.
Li-Poly battery packs (red) in my scale model of the Piper J-3 Cub (covered in the L-4 color scheme). Each
pack contains two, 3.7 V Li-Poly cells wired in series internally (7.4 V for the pack). Externally, they
are plugged into a "Y" harness to double the energy rating from 1,500 mAh to 3,000 mAh. This particular
model originally used an OS 0.25 in³ nitro motor, and was converted to electric power using a geared,
brushed motor and these batteries. Total flying weight increased by only a couple ounces.
(right) holding my Great Planes J-3 covered in the U.S. Army Air Force L-4 Grasshopper with the D-Day
Invasion color and trim scheme.
Hogs remote control helicopters come ready to fly. Plug one into the charger that is built into the
transmitter for a couple minutes and get about 45-60 seconds of flight. Price: A mere $30.
image shows the totality of the electronics from my Air Hog. Main rotor shaft is in upper left corner,
battery (probably about 25 mAh) in lower right.
Events tailored specifically to electric-powered models have been created at the major contests worldwide, and
some "electrics" are taking top prize positions in events where internal combustion engines are used in the same
events as electrics. Energy densities of four to eight times as great as the older nickel metal hydride (NiMH) and
nickel cadmium (NiCad) cells have been realized. Lithium technology has markedly improved flight performance and
duration even with maintaining the existing brushed motors in the airplanes and helicopters. Some reports indicate
that the use of electric power systems in model aircraft now exceeds the use of nitro and gasoline power. Model
boat and car enthusiasts were easier converts because weight concerns are not as great.
One reason that
electric power has gained so much momentum is the huge number of battery powered models that can be found in
retail stores like Wal-Mart, Target, and Radio Shack. The Cox and Testors control line models of yesteryear using
glow fuel (nitro) and the little 0.049 in³ internal combustion engines that once lined the shelves have given way
to dozens of free flight and radio controlled airplanes and helicopters made of Styrofoam and come ready to fly
out of the box - just charge and toss them into the air. Success is virtually guaranteed. For $30, you can buy a
remote control helicopter that is more controllable by the novice flyer than the radio controlled helicopters I
had back in the 1970s and even the 1990s. Many more varieties of electric powered airplanes and helicopter models
(with differing degrees of complexity and cost) can be purchased at hobby shops and through online distributors.
Another motivation for the adoption of electric power is the nearly unperceivable noise emitted by them. As areas
become more and more densely populated, finding and keeping an adequate flying site is increasingly difficult.
Noise ordinances and nearby residents unwilling to tolerate the noise of the internal combustion engines have run
off many modelers who used to enjoy being able to fly their craft. Now, with the "quite revolution" firmly in
place, nobody even knows that you are flying unless they happen to see your plane in the air. The mostly
irrational fear associated by the ignorant with the noise emanating from a nitro engine isutterly assuaged by the
lack of noise coming from an electric. I say mostly irrational because in fact even the electric powered models
can inflict a substantial amount of harm to a body or an obstacle upon contact. You can bet that I will not be to
one to enlighten anyone (other than you, dear reader) as to the remaining potential for damage. Ignorance on the
part of the layman is bliss on the part of a modeler desperate for a convenient place to fly.
As a side
note, I will point out that responsible fliers of models are members of organizations like the Academy of Model
Aeronautics (AMA), and therefore carry accident liability insurance. In the case of the AMA, I am covered for up
to $2,000,000 (that's right two-million dollars) per incident. Another side note: Back in the early 1970s, whilst
still a pup (about 14 years old), I managed to "land" one of my RC airplanes (an Andrews S-Ray, for those of you
in-the-know) inside the local elementary school library one weekend. My radio system (on 27.195 MHz) and piloting
skills were very bad at the time, and when combined with a gust of wind, my craft broke through the glass window.
Even in my freaked-out mood, I found the courage to ride my bicycle over the principal's house and report the
incident, and the AMA covered the cost of repair. My AMA member number is 92498, BTW.
OK, now that the very long and some might say unnecessary introduction to Li-Po batteries is out of the way, let
us get down to the business of presenting some of the relevant technical facts.
video of an overcharged lithium-polymer (Li-Po) battery will make you worry just a little about the one
sitting inside your cellphone, camera, or laptop computer. I use them in my R/C airplanes. Li-Po cells are
generally considered safer than lithium-ion (Li-Ion) cells, but they can - and do - still catch fire in
the wrong conditions.
Lithium-Polymer, abbreviated Li-Po or Li-Poly, evolved from lithium-ion (Li-Ion) chemistry. Li-Po cells
are rechargeable, and if treated carefully, can have a lifetime of many years and can be recharged hundreds of
times. The major distinction between Li-Ion and Li-Po is that the lithium-salt electrolyte is not suspended in an
organic solvent as it is in the Li-Ion cell. It uses a solid polymer composite such as polyacrylonitrile (hence
the "poly" for the electrolyte) as a physical separator, which reduces, but does not eliminate, the opportunity
for internal shorting that can cause fires and explosions. Lithium, at atomic number 3, is a silvery white/gray
metal that is highly reactive. Having an atomic number of 3 also accounts for its light weight. It is first in the
series of alkali metals, and is the first solid element. Lithium reacts with and can catch fire in water and water
vapor in the presence of oxygen. It also reacts with nitrogen, and since our air is 79% nitrogen, there is ample
opportunity for a reaction in everyday use if the packaging is breeched either by puncturing or overheating.
The video link above illustrates what can happen can happen to an overcharged Li-Poly battery pack. Pictures have
been published in modeling magazines of the burnt remains of airplanes and helicopters whose owners did not
properly recharge their batteries while the packs were still in the aircraft. The recommended procedure for
charging the Li-Po battery packs is to remove them from the aircraft and place them in a fireproof container
during the actual charging cycle. I use a clay flower pot as a container, with a metal plate below to cover the
water drain hole, and a metal plate above with a weight on top of it. The charging cord rests in a notch that was
made in the lip of the pot. Fortunately, after many recharges, I have never had a fire incident.
lithium polymer (and lithium ion) are so sensitive to overcharging, the individual cells that make up the battery
pack are charged independently. With NiCad and NiMH packs, the standard charging method is to apply a voltage and
current across the series-connected cells so that the same current passes through each cell. The voltage across
the entire pack is the sum of the individual cells voltages, but the cell voltages are not necessarily all equal.
In fact, it is almost certain that they are not. Since nickel-based cells are relatively tolerant of some
overcharging, there is usually no danger. It is not uncommon for one cell in a NiCad or NiMH battery pack to be
dead, and yet the rest of the cells charge normally. What that means is in the case of an 8-cell pack, with the
nominal voltage of each cell being 1.2 V (9.6 V total), if a charging voltage of 10.4 V is present, each cell
would ideally receive 10.4 / 8 V = 1.3 V. If one cell is dead, however, the voltage across each cell would be 10.4
/ 7 V = 1.5 V. That 15% increase can be handled by most NiCad and NiMH cells, but a 15% overvoltage applied to a
lithium cells would eventually result in a failure, and likely a fire. Li-Po batteries are also intolerant of over
discharging, and tend to die if discharged below around 2.5 V. In operation, controller circuitry should prevent
the cell voltage from dropping below 3.0 V. Cell temperature should never exceed 90 °C in order to prevent the
internal separator polymers from melting and allowing plate shorting through physical contact.
Here is an "outrunner" brushless motor by E-flight, model 450, rated at 890kV (890 rpm/V), installed
in an electric-powered radio controlled sailplane (note the folding propeller). It is a slotted 12-pole
motor, and draws 14 A of current at full load. It uses an electronic speed controller that output a
3-phase voltage. Although commonly referred to as a DC motor, it really is a miniature 3-phase AC
permanent magnet synchronous motor, although induction types are sometimes used in
the right is my Mini Pulse XT aerobatic airplane. It uses the 450 motor and a 3-cell, 11.1 V, 2100 mAh,
Li-Po battery. Futaba 4-channel radio.
Early lithium batteries had a rather high internal resistance, and had rather low discharge rates. As with all
technology that is doggedly pursued, significant improvements have been made to the point that the contemporary
Li-Po batteries may be substituted in most systems for the original NiCad or NiMH batteries. Discharge rates of
20C or 25C are commonly available on the commercial market. In battery discharge terminology, each "C" is a
discharge current equivalent to the value of the energy capacity of the cell (it is not the abbreviation
for the Celsius degree unit). In the case of a 1,200 mAh rating, 1C is equal to a discharge current of 1,200 mA,
or 1.2 amps. A 10C cell can deliver a continuous current to a load of 10 x 1.2 A = 12 A during its discharge
cycle. The E-flight 450 motor shown to the right would require at least a 12C battery to deliver full rated power.
This motor, by the way, is the one I use in my electric-powered sailplane (2-meter wingspan) and in my 4-channel
aerobatic airplane (an E-flight Mini Pulse XT).
In my humble opinion, lithium batteries are grossly
overpriced, given the huge production volumes in effect. When you consider that almost every mobile electronic
device and cordless power tool uses these batteries, the cost should be a lot lower. Maybe it is partially product
liability insurance that manufacturers have to purchase that keeps the prices high, but I just paid $50 each for
two 3-cell (11.1 V), 2,100 mAh packs. That is at least 2x to 3x what is should be costing at this point in the
lithium cell evolution.
Here is the charging chemical reaction (from Quest
LiCoO2 + 6C --> Li1-xCoO2
Here is the discharge chemical reaction (from
Li1-xCoO2 + LixC <----> Li1-x+dxCoO2
Here are typical charge and discharge cycle charts for Li-Po batteries at room
temperature (20°C). These particular charts are based on Sanyo data. Sanyo is currently the world's largest
producer of lithium polymer batteries. As you can see, the cell voltage is reasonably constant out to about the
20% capacity state, and then falls off precipitously, similar to NiCad and NiMH cells.
There is a table of parameters for the most common battery types on the
Battery Types & Specifications page here on RF Cafe.