This page is constructed to help answer questions about different aspects of modeling. If you have a question, submit it to John Gill and I will do my best to condense the information gathered from the web into a understandable article.
PLEASE NOTE: The following is an excerpt from a Navy Postgraduate School publication entitled “Safety & Usage Procedures for Lithium Polymer Batteries.” The section on Quality Control Procedures was pub-lished last month. This month I am publishing Appendix A & B with C, D, E and F to appear in the March newsletter. That will complete the series. Thanks once again to Kevin Jones for providing this valuable in-formation. The Editor
If charging at a field or flying location, batteries shall not be charged inside of an aircraft or any other vehi-cle. Exceptions are: systems with built-in battery packs as shipped by the manufacturer, which must be charged using charging equipment shipped with the system, and packs that have built-in cell protection equipment to prevent cell under/over voltage and/or cell balancing during charge, often referred to as Battery Management Systems (BMS).
You must check the pack voltage after each flight before re-charging. Do not attempt to charge any pack if the unloaded individual cell voltages are less than 3.2 V. For example: Do not charge a 2-cell pack if below 6.4V.
Upon retrieval of a battery after a flight, the battery shall be inspected for any signs of physical damage such as punctures, broken heat shrink, bared wires, dents, scratches, and swollen or ruptured cells.
After confirming that the battery’s physical condition is acceptable, the battery shall be connected to a LiPo battery pack checker to ensure that each cell is in the range 3.2 V to 4.2 V and that the cell imbalance is less than 0.1 V.
If the battery does not meet any criterion in section 2.1.1, it shall be disposed of immediately in accordance with the disposal procedure in section 1.3. Any minor damage meeting the criterion of section 2.1.2 shall be repaired in accordance with section 3.1.4 only if all disposal criteria of section 2.1.1 are met.
An ABC fire extinguisher shall be kept on hand wherever LiPo batteries are being charged, stored, or uti-lized. For field operations the fire extinguisher shall be kept near the batteries during charging, and on the flight line during flights.
Aircraft Crash (battery ignition)
Although rare, LiPo batteries can ignite as a direct and immediate result of physical damage alone. In the event of a battery ignition subsequent to an aircraft crash, the following procedure shall be followed:
1. The battery shall be allowed to burn to completion within the aircraft.
2. If there is any danger to surrounding structures or property, contact emergency services immediately.
3. The burn shall be monitored and the fire extinguisher used to ensure that surrounding materials do not catch fire.
4. After the ignition has subsided, continue to monitor the battery for at least fifteen minutes.
5. After fifteen minutes use the extinguisher to douse the remains of the aircraft and battery.
6. Once doused, extract the battery from the aircraft using a shovel and place in a safe place.
Leave the battery in a safe location for at least 24 hours prior to disposing of battery remains.
Aircraft Crash (no ignition)
The great majority of aircraft crashes involving LiPo batteries will not result in battery ignition. In the event of an aircraft crash involving LiPo batteries that does not result in immediate ignition, the following proce-dure shall be followed:
1. The Pilot/operator shall immediately cut power to the motors.
2. The aircraft shall be left in place and monitored for a fifteen minute period after the crash to ensure that ignition is not going to occur.
3. After fifteen minutes, the aircraft can be recovered and the battery removed.
4. The battery shall then be inspected in accordance with section 2.5.
5. The exception is crash sites which are in the midst of highly combustible materials. In this case, approach the crash site with caution, with an extinguisher, and upon inspection, determine if the aircraft can be moved without imposing additional hazards. If so, move the wreckage to a safe site for the cooling-off period.
If an airplane containing LiPo batteries is involved in a crash, motor power shall be turned off immediately. The aircraft shall be left in place and monitored for a fifteen minute period after the crash to ensure that igni-tion is not going to occur. The exception is if the crashed aircraft is located in the vicinity of highly combus-tible materials. In this case the owners should approach the crash site with caution, with an approved fire ex-tinguisher, and attempt to relocate the wreckage to a safer location. Use of a LiPo bag or ceramic container for the batteries is encouraged.
After fifteen minutes, the aircraft can be disassembled and the battery removed. Upon retrieval of a battery after a crash, the battery shall be inspected for any signs of physical damage such as punctures, broken heat shrink, bared wires, dents, scratches, and swollen or ruptured cells.
After confirming that the battery condition is acceptable, the battery shall be connected to an open circuit voltmeter and the pack voltage shall be confirmed to be a minimum of 3.2 V per cell.
If the battery does not meet any criterion in section 2.1.1, it shall be disposed of immediately in accordance with the disposal procedure in section 1.3. Any minor damage meeting the criterion of section 2.1.2 shall be repaired in accordance with section 3.1.4 only if all disposal criteria of section 2.1.1 are met.
Appendix B: Multi-Pack Wiring/Usage Requirements
Many systems require LiPo packs to be used in parallel or serial configurations in order to achieve the needed voltage and capacity requirements. Individual packs that include serial and/or parallel connections are not considered in this section, but they are discussed in previous sections.
Typically multiple packs are connected in parallel when increased endurance is required, increasing the sys-tem Amp-Hour capacity at the same voltage as a single pack. This poses a significant risk if an operator inad-vertently plugs one fully charged battery in parallel with a second battery at a significantly lower charge state. Without safeguards in the circuitry, the full pack and the low pack will immediately attempt to balance, with the low pack getting charged at a very high rate, controlled only by line resistance in the wiring. There is a significant chance of ignition occurring in the low pack.
In the radio control community, this is typically mediated through operator training. However, in campus labs with potentially untrained students working with these systems, some form of electrical safeguard is required to prevent or limit back current into the packs. The simplest scheme is through the use of a diodes or a Schottky rectifier. Use of an inline diode will prevent current to flow from the system back into the battery. In a system with parallel packs, each behind a diode, in use the system will draw power from the pack with the highest voltage until the packs are balanced, and then power will be drawn equally from the two.
Note, some precautions may need to be taken to protect other electronics if diodes are used in this way. For example, many electronic speed controls (ESCs) have a brake capability built in to stop a propeller from spin-ning at zero throttle. Many of these brake systems work through regenerative braking (as on the Toyota Pri-us), where the freewheeling motor acts as a generator, and feeds power back into the battery. Since the pro-peller typically doesn’t have much inertia, the total energy spike is quite small. However, with the diode in place, this small surge cannot reach the battery, and other circuitry in the line may be subjected to a very brief power surge with voltages roughly twice the nominal battery voltage. Systems should be capable of handling these surges, use a fly-back diode to isolate the surge, or disable the brake.
Typically series connections are used when a higher voltage is required, but for mechanical or other reasons, smaller packs were used. The voltage is increased to the sum of all the packs in series, and the capacity is equal to the weakest pack in the system.
In principal, series connections are less dangerous, as the possibility of high-rate pack-balancing is removed. However, some care must be taken to insure that the energy is removed from the packs uniformly. For exam-ple, as packs age, their internal resistance increases, and they are less willing to release power. If a new pack is mated with an old pack in series, the new pack will end up draining more quickly than the old pack. The net result can be that the new pack is drawn well below the 3.2v/cell safe limit while the old pack remains at a safe voltage level. Monitoring the total system voltage will not catch this, as the full system voltage may be above 3.2v/cell.
To mediate this, packs that are to be used in series should be of identical size and manufacture, and should remain as a matched pair throughout their useful life. The unique identifier on the packs should reflect that it is part of a series group.
The forces. Propeller blades are constructed using aerofoil sections to produce an aerodynamic force, in a similar manner to a wing. Consequently the blades are subject to the same aerodynamics – induced drag, parasite drag, wingtip vortices, lift/drag ratios at varying aoa, pressure distribution changing with aoa etc. There is a difference in application because, in flight, the propeller has rotational velocity added to the translational [forward] velocity thus the flight path of any blade section is a spiral – a helical flight path.
The diagram at left represents a blade section in flight and rotating around the shaft axis. Because of the different application it doesn’t serve much purpose to express the resultant aerodynamic force as we would for a wing, with the components acting perpendicular (lift) and parallel (drag) to that flight path, as in the upper figure. So we represent the aerodynamic force component acting forward and aligned with the aircraft’s longitudinal axis as the thrust force, and that component acting parallel to the direction of rotation as the propeller torque force.
As you see in the lower figure the component of the lift acting in the rotational plane has now been added to the drag to produce the ‘propeller torque force’ vector. The remaining forward acting portion of lift is then the thrust. That is why propeller efficiency is usually no greater than 80 – 85%, not all the lift can be used as thrust and the propeller torque force consumes quite a bit of the shaft horse power. The propeller torque and the engine torque will be in balance when the engine is operating at constant rpm in flight.
There are other forces acting on the blades during flight, turning moments that tend to twist the blades and centrifugal force for example. The air inflow at the face of the propeller disc also affects propeller dynamics.
Blade angle and pitch
Although all parts of the propeller, from the hub to the blade tips, have the same forward velocity, the rotational velocity – and thus the helical path of any blade station – will depend on its distance from the hub centre. Consequently, unless adjusted, the angle of attack, will vary along the length of the blade. Propellers operate most efficiently when the aoa at each blade station is consistent (and, for propeller efficiency, that giving the best lift drag ratio) over most of the blade, so a twist is built into the blades to achieve a more or less uniform aoa.
The blade angle is the angle the chord line of the aerofoil makes with the propeller’s rotational plane and is expressed in degrees. Because of the twist the blade angle will vary throughout its length so normally the standard blade angle is measured at the blade station 75% of the distance from the hub centre to the blade tip. The angle between the aerofoil chord line and the helical flight path (the relative airflow) at the blade station is, of course, the angle of attack and the angle between the helical flight path and the rotational plane is the angle of advance or helix angle. The aoa and helix angle vary with rotational and forward velocity.
The basic dimensions of propellers for light aircraft are usually stated in the form of number of blades, diameter and pitch with the latter values given in inches. e.g. 3 blade 64″ × 38″. The pitch referred to is the geometric pitch which is calculated, for any blade station but usually the 75% radius position, thus:
Geometric pitch = the circumference (2 π r) of the propeller disc at the blade station multiplied by the tangent of the blade angle. Thus it is the distance the propeller – and aircraft – would advance during one revolution of the propeller if the blade section followed a path extrapolated along the blade angle.
e.g. For a blade station 24 inches from the hub centre [0.75r] and a 14° blade angle, the circumference = 2 × 3.14 × 24 = 150 inches and tangent 14° = 0.25. Thus the geometric pitch is 150 × 0.25 = 38 inches. Propellers are usually designed so that all blade stations have much the same geometric pitch.
Designers may establish the ideal pitch of a propeller which is the theoretical advance per revolution which would cause the blade aerofoil to be at the zero lift aoa; thus it would generate no thrust and, ignoring drag, is the theoretical maximum achievable aircraft speed.
The velocity that the propeller imparts to the air flowing through its disc is the slipstream and slip used to be described as the difference between the velocity of the air behind the propeller ( i.e. accelerated by the propeller) and that of the aircraft. Nowadays slip has several interpretations, most being aerodynamically unsatisfactory, but you might consider it to be the difference, expressed as a percentage, between the ideal pitch and the advance per revolution when the the propeller is working at maximum efficiency in conversion of engine power to thrust power. Slip in itself is not a measure of propeller efficiency; as stated previously propeller efficiency is the ratio of the thrust power (thrust × aircraft velocity) output to the engine power input.
Pitch and velocity
The performance of aircraft fitted with fixed pitch or ground adjustable propellers is very much dependent on the chosen blade angle. Fixed pitch propellers limit the rpm developed by the engine at low forward velocity, such as occurs during the take-off ground roll and may also allow the engine rpm to exceed red-line maximum when the load on the engine is reduced, such as occurs in a shallow dive. Fixed pitch propellers operate at best efficiency at one combination of shaft power and airspeed. Blade angle is usually chosen to produce maximum performance at a particular flight condition, for example:
• Vy climb i.e. a climb propeller
• Vc cruise i.e. a cruise propeller
• High speed.
The climb propeller is usually chosen when the aircraft normally operates from a restricted airfield or in high density altitude conditions. The climb propeller will produce maximum efficiency at full throttle around the best rate of climb airspeed and will perform fairly well at take-off, but during the initial take-off acceleration even the climb propeller may restrict the engine rpm to less than 75% power. The cruise propeller will achieve maximum efficiency at 75% power at airspeeds around the design cruising speed but aircraft take-off and climb performance will not be the optimum. The cruise propeller usually has a little more pitch than the standard propeller fitted to the aircraft. A high speed propeller might be fitted when the aircraft is intended to be operating at, or above, rated power for short periods – in speed competition for example.
“How to setup an electric aircraft” or “Come on guys, it’s not rocket science”
Posted by: Fly RC Staff July 25, 2011
It is now summer 2011 and still there are two questions that crop up in my emails. Can you equate an electric motor to a glow equivalent? How do you go about converting a glow model to electric? The answer to the first question is technically no, but manufacturers do and they are doing the motor a disservice by doing so. The answer to the second question is the main thrust of this article.
Converting a glow-power aircraft to E-power starts with a 10-step process to help you choose the right equipment. This article guides you through these steps. Next time, we will cover the specific physical steps to converting a Hobbico Avistar.
Unlike a glow engine, a motor does not have a narrow operating range. When you buy an ordinary .40 cu. in. glow engine, you generally put a 10×6 pop on the shaft and it turns around 12,000 rpm. If you are lucky, it develops about one horsepower. The first thing we should be learning when flying e-powered models is that if we are swinging a glow size prop at typical glow rpm on our models we are doing something wrong.
The biggest advantage to electric flight is to choose a motor/prop combination that swings a larger prop that better matches not only the way you want to fly, but the ability of the model to fly in the manner intended. Calling a motor a 46 and using it on a 40 glow size model may work well for a sport model that, because of the design, cannot swing a much larger prop. However, it would not be the best answer for a lumbering 40 glow trainer with lots of room for prop growth. That 46 motor can be run on three LiPo cells effectively with a large prop and fly a lightweight (but large) model and it can be run on 5-6 LiPo cells on a fast moving sport model with a much smaller prop. This electric motor is capable of running effectively at 1/3hp as well as it is at one horsepower! This is why we should not give the motor a glow equivalent designation. It makes things easier for the marketing manager, but not for the modeler.
I have put together a 10-step method for converting a glow aircraft to e-power. I would like to begin by making two statements. If the model in question is considered overweight as a glow model, do not even start the process. It will not fly any better as an electric model. In addition, most ARF and RTF glow models cannot be lightened enough to make a difference, so dont start putting holes in everything (especially balsa!) thinking that the performance will improve. All you may be successful in doing is making the model weaker.
I will be using a Hobbico Avistar for the purpose of this article. Although the glow to electric conversion presented here is rather specific, the steps involved are applicable to any type model of any size from .049- .90 cu. in. displacement. The 40-size Avistar is one of the most popular trainers still sold today and I dont see that changing any time soon. In each of the 10 steps presented, I will describe the process and then how it relates to this particular model. Next month, I will describe the physical process of converting the model from glow to electric.
Ten steps for electric conversions:
Determining the weight of the model is a simple process and should start by assuming the model will weigh no more with e-power than with glow-power. The pictures show the glow components weighing 5 ounces less than the e-power components, but the tank is empty and will easily weigh 5 ounces more when full.
ESTIMATE THE WEIGHT OF THE MODEL
How can I do this if I dont know what I am putting in it? A simple way to estimate the weight of the model is to consider that after you take out the glow motor, mount, fuel tank, throttle servo and pushrod, you will find very often that when you add in all the electric components (if using LiPo bat-teries) that the weight of the model will only be a few ounces more than the glow model with an empty tank! Add the fuel weight and the glow model can actually be heavier!
The estimated weight of the Avistar is 5 pounds, giving us our first data point.
With the model level, measure the distance from the old glow engine prop shaft to the table. Then subtract 1-2 inches based on your experience level.
CHOOSE A PROPELLER
How do I choose a prop when I have not chosen a motor yet? As I mentioned earlier, we desire to swing as large a prop as possible to reap the benefits of electric flight. The only exception to this rule is if you plan on racing your model (that would be the subject of a different article). Before you remove the engine, measure distance from engine shaft to ground with model level and subtract experience.
Experience is a number that is directly related to your skill level. A skilled pilot can subtract one inch or so. A less skilled pilot might subtract 1.5-2 inches. If you fly off a very rough field, even a skilled pilot may choose a higher number to avoid prop breakage. The measured distance on the Avistar is 7.5 inches or a 15-inch diameter prop. As this model is supposed to be a trainer, I will subtract 1.5 inches on the radius, giving me a 12-inch prop. The Avistar flew with a 10-inch prop when glow-powered.
Now we need to choose the pitch of the prop. Though this number can be changed later after some flight testing, typically we calculate the pitch of the prop to be about 50 percent its diameter which should be good for sport aerobatics with a good climb rate. We can go with 60-75 percent of the diameter if we want more speed and less climb. For the Avistar, I chose 50 percent which gives me a 6- inch pitch. I now have my second data point; 12×6 prop (Use APC Eseries props whenever possible).
ASK YOURSELF THESE THREE QUESTIONS
The next step is all up to you. I cannot help you with the answers. How was this model intended to be flown? How do I want to fly it? What is my skill level? Answering these questions (honestly) will help us choose an appropriate power system for the model.
DETERMINE POWER LOADING
Determining how much power we need to fly our model depends on the answers given in Step three. We express power loading in W/lb. The higher this number, the more energetic the model will fly, however, at two costs; weight and expense.
Use 75 W/lb. for mild sport models and trainers. Use 100 W/lb. for nonaggressive aerobatics. For aggressive aerobatics, use 125 W/lb. and you can use 150+ W/lb. for 3D and wild flight modes. As I consider the Avistar an advanced trainer and would like it to do a little more than putt-around, I will choose 100 W/lb., giving me my third data point.
SELECT MAX CURRENT
To help define the electrical parameters of the model, I will use a fire hose analogy. The electrical current is a measurement of the flow rate of electrons through the wires. Consider this the same as the gallons per minute in a fire hose. Voltage can be compared to the pressure of the water in the fire hose.
The greater the push (voltage x amps) the more power will be received by the prop. Unfortunately, more current increases heat. Because of this, we always like to use more voltage (pressure), a larger wire (hose) and lower current to get the work done. It has been my experience to use the following wire relationships for my models:
10-20 amps for glow models from .049-.15cu. in. with 18-14 gauge wire
20-30 amps for .15-.30 cu. in. with 14 gauge wire.
30-45 amps for .30-.90 cu. in., and 12-13 gauge wire.
50+ amps, I suggest staying away from these high-current setups.
Since motor, ESC and battery manufacturers generally supply the appropriate gauge wire on their units, we do not have to concern ourselves with wire gauge unless we have to make changes. We are planning a 35-amp current draw for the Avistar.
To determine the watts required to fly our model, we multiply the power loading from step four by the weight of the model. 100 watts/lb. x 5 lbs. = 500 watts
DETERMINE CELL COUNT AND ESC SIZE
The cell count, or voltage, when using LiPo cells, is determined by simply dividing the watts required by the amperage chosen. 500 watts / 35 amps = 14.3V
Since LiPo cells are normally 3.7 volts, we need to round off to the nearest cell count. 14.3/3.7 = 3.86 cells. So we round up to 4. Our Avistar is going to require a four cell LiPo battery and a speed controller that can handle at least 35 amps.
Choose an ESC with enough capability in the battery eliminator circuit (BEC) to handle the number and size of servos installed in the model. I would choose a Castle Creations ICE 50 or ICE Lite 50 for the Avistar.
DETERMINE MAX DIMENSIONS IN BATTERY BAY
It has been my experience that most models will require the LiPo battery pack to be installed in the fuel tank compartment to obtain proper CG. Measure the height and width of that compartment and choose as large a battery as you can fit. The Avistar is easily able to accommodate a 4S 3000 to 3800mAh battery pack. At this point, I know I need a 12×6 prop, 50 amp ICE ESC, and a 4S 3200 mAh battery pack.
The maximum size of the battery that can be used is basically a function of how big the fuel tank area is, however, in the case of the Avistar, the largest battery was determined by what would tip in through the new battery hatch made in the top fwd section of the fuselage. This allowed the wing to remain in place while swapping out batteries.
DETERMINE THE BATTERYS C RATING
I have found manufacturers of battery packs very optimistic about the ability of their batteries to deliver the power required. This is commonly referred to as C rating. The C rating is the ability of the battery to give up its stored power without damage. If a battery pack is listed as 20C and the capacity is stated as 3200mAh, then the maximum amount of current one should draw out of that battery pack is 20 x 3.2AH or 64 amps. DO NOT use this as an acceptable number, especially from inexpensive, unbranded batteries! My recommendation is to use 50 percent to 60 percent their number to be safe. A 20C advertised battery will be used by me at no more than 10-12C. This means a 20C 3200mAh battery will be fine at 35 amps for my Avistar.
PICK THE MOTOR
Finally, we need to choose a motor. From the data determined above, we need to find a motor that will swing a 12×6 prop on a four-cell LiPo battery at around 35 amps. You can research different motors on the Internet or use an electric aircraft performance emulation programs such as Electri-Calc or Moto-Calc.
On the Internet, you might find a generic motor company with model xxyy-400, that specs it will swing a 13×6 prop with 14.8V (4 cell) at 5500 rpm drawing 40 amps. The fact is that if you use a 12×6 prop on this motor that the current will drop substantially. It is not necessary to be concerned with rpm ratings; you just need to find a motor that will spin a 12 x 6 prop on 4S at 35 amps. If an exact match cannot be found, get as close as you can.
Emulation programs such as E-Calc and Moto-Calc can get you a more specific answer, but they assume that the manufacturer lists all the motor constants on their website. Kv (RPM/volt constant), Io (no load current) and Ra (armature resistance) are required inputs for these emulation programs.
There are a number of brushless outrunner motors that I could have picked for the Avistar, but I chose the BP Hobbies BL- 4120-7. Other motors that may have been suitable for the Avistar would be: Scorpion HK-4025-550Kv or Model Motors AXI 4120-18.
I hope this article helps you in any E-conversion projects you plan to do. Next time I will cover the physical steps to convert an Avistar to E-power.
APC Propellers, distributed by Landing Products
www.apcprop.com, (530) 661-0399
AXI Motors, distributed exclusively by Hobby Lobby International, Inc.
www.hobby-lobby.com, (866) 512-1444
www.bphobbies.com, (732) 287-3933
www.castlecreations.com, (913) 390-6939
E-flite, distributed exclusively by Horizon Hobby Distributors
www.e-fliterc.com, (800) 338-4639
www.hobbico.com, (800) 682-8948
www.motocalc.com, (519) 638-5470
Scorpion Motors, distributed exclusively by Innov8tive Designs
www.innov8tivedesigns.com, (760) 468-8838
LiPo info to put you to sleep!
I here people saying all the time, “I’m not smart enough to understand that,” WRONG; experience in a subject simply means you grasp the subject a little faster since your familiar with the terms and concepts, ANYONE CAN LEARN IF THEY WANT TO!
With my rant complete, enjoy the article presented in this months “Model Aviation.”
LiPo Battery Basics
Understanding the technology and its safe use
There are two items everyone entering electric flight has
to deal with: batteries and connectors. On the surface, it’s simple, but it causes more confusion and questions than
practically anything except motor designations. I will shed some light on these and I hope to clarify things. This is for the electric power newcomer who wishes to understand and make the right choices for his or her requirements.
Everyone is familiar with the batteries that he or she uses around the house. Most are alkaline in AA, AAA, C, D, or 9-volt
formats. Others are rechargeable Nickel Cadmium (Ni-Cd) or Nickel Metal Hydride (NiMH). In the dark ages of electric flight, we used Ni-Cd and NiMH batteries, but now the standard is Lithium-ion Polymer (LiPo) and that will be my focus. These batteries are sometimes referred to as battery packs or simply packs. There is another type of battery chemistry called Lithium Iron Phosphate (LiFePO4) that some use for flight packs, but are more often found in receiver and transmitter packs. They are usually referred to as LiFe packs or A123 packs, referencing their makeup and brand name.
Each type has its advantages and disadvantages, but because LiPos are the de facto standard in electric flight, I’ll concentrate on them.
Common Electric Terms
When discussing electricity, two terms often come up:
Voltage and amperage (or volts and amps). You don’t have to be a scientist to understand what these terms mean. If you’ve ever
used a water hose, you already know all you need to make informed decisions about batteries. Think of the hose as an electrical wire, and the water as electricity.
Voltage is the force or pressure of the electricity. With a garden hose, voltage is like the water pressure. For this article, think of a battery as a water pump providing pressure for the water.
Amperage is the unit of measurement of the amount of electrical flow or current. With a garden hose, this is the amount of water that can flow through the hose in a period of time. Amperage is similar to the electrical equivalent of “gallons per minute.” Most of our equipment is measured in milliamps, which is 1/1,000th of an ampere. So a 3,300 mAh battery pack is 3.3 amperes.
Ohms is a measurement of resistance to the flow of electricity. With a garden hose, consider the flow of water through a large-diameter hose compared to that of a small-diameter hose. Water flows more freely through a larger hose than a smaller one.
Electrical resistance is measured in ohms and is related to the ability of electrical components to let electricity travel through them. As do water hoses, electrical wire comes in different sizes. Larger-diameter wire lets electricity flow more freely than does smaller-diameter wire. With electricity, the by-product of resistance is heat. The more power that is pushed through a wire, the hotter that wire will become. Heat in a wire indicates that you don’t have a large enough wire size for the amount of current that you are pushing through it. Too much heat can start a fire.
AWG stands for American Wire Gauge, sometimes referred to as simply “gauge.” It is nothing more than a long term to describe
the diameter of a wire. As you learned from electrical resistance, the more electricity you want to move through a wire, the bigger the wire must be to minimize resistance and control heat. AWG measurements are whole numbers—for example, 8 AWG or 12 AWG. A smaller number means a bigger wire. In the same way a larger hose allows more water to pass through it, each gauge of wire is rated for a specific maximum electrical throughput measured in amperes.
On a typical LiPo battery such as this, the label identifies this pack as having a 1,300 mAh capacity (C) and 25C discharge rating with 50C burst. The battery’s voltage is shown in the bottom right corner where the white circle corresponding to the voltage has been dotted with a marker from the factory. The battery is equipped with an XT60 connector (yellow) and a balancing connector (white).
Rumors abound about safety, or lack thereof, when using LiPo batteries. Much of that is left over from the early days of LiPo packs and the lack of information available to the user at the time. Incorrect chargers were used, incorrect voltage cutoffs were used, and they were being discharged at levels that the packs couldn’t support. As chemistries, protective
circuits, and information improved, LiPo batteries have become a safe and suitable source of power. Here are a
few simple rules for increasing your safety:
• Always store batteries in a fire-safe container.
• Always charge with an appropriate charger designed for LiPos.
• Always follow the manufacturer’s instructions for charging and discharging rates.
• Always size a pack according to its usage.
• Never overcharge.
• Never overdischarge.
• Never use a puffed pack.
• Never use a pack that has visible damage (dents, cracks, etc).
• Never charge a pack unattended.
• Never disassemble or reconfigure a damaged pack.
Most accidents involving LiPo packs are the result of not following one of these rules. Understand the charger
you’re using and follow the manufacturer’s guidelines and they will serve you well. Charge safely.
Understanding the Labels
Labels contain plenty of information, but understanding them is often confusing. A few simple definitions will help you.
• 3S, 4S, etc.: Battery packs are composed 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. (four cells x 3.7 volts = 14.8).
• Capacity: The capacity rating of a LiPo battery tells its output potential, or how long you can take power from the
battery at a given rate before it reaches the cutoff voltage or is discharged. The faster you take power from the battery, the
less time it will last. Many times, our batteries’ capacities are listed in milliampere hours (mAh) instead of ampere-hours (Ah). This is merely a metric conversion to a smaller unit—1 ampere hour = 1,000 milliampere hours, so 2.2 Ah is 2,200 mAh.
• Discharge rating: “C” represents a measure of the rate at which a battery can be discharged relative to its maximum
capacity. If the battery is discharged at a rate higher than the discharge rating, the battery may be damaged, or worse, could pose a safety hazard, like a fire. If a battery’s discharge rating is 15C, it means that the most power that can be drawn from it at one time is equal to 15 times its capacity. Using the example of a battery which has a capacity of 2,200 mAh, this means that greatest flow of electricity you can safely get from the battery is 15 x 2,200 = 33,000 milliamperes (or 33 amperes). The discharge rating listed on the battery’s label is based on what the manufacturer believes the pack will handle during discharge without degrading the pack. These discharge ratings, sometimes mistakenly referred to as C ratings, can be optimistic and are best used as a guideline. Packs with higher discharge rates have lower internal resistance (IR), which is a good thing. Many batteries provide two discharge ratings such as 30C/60C. These represent the continuous and burst ratings. The first number means that it will continuously support a 30C discharge, and for short bursts (typically less than 15 seconds) it should support 60C. This allows for spikes during rapid throttle changes, but shouldn’t be something you use regularly. If you need higher current levels, buy a higher capacity/rated pack.
• Internal Resistance: This represents the internal resistance of a cell or pack. Some chargers will test the IR for each cell within a pack during the charge cycle. As internal resistance increases, the battery efficiency decreases. So as a general rule, the lower the resistance, the more punch a battery will provide. It’s nice to know, but not something to get hung up over as a beginner. As a rule, packs advertising a high discharge capacity will have a lower IR. Battery pack labels are often the manufacturer’s attempt to put its product in the best light. A pack rated as a 65C pack and sporting small-gauge wires to the connectors won’t really handle that amount of current. Sometimes packs come with large-gauge wires, but they’re soldered to tiny tabs inside the pack, which negate the benefit of those monster wires. Shop carefully and use the best battery you can afford.
If you’re beginning to fly electric-powered aircraft and your only experience has been with Ni-Cd or NiMH packs, you’re
probably wondering about memory effect. Older Ni-Cd and NiMH batteries suffered from an effect termed “memory” in
which the way the battery had been discharged in the past would affect its performance in the future, even after being fully recharged. The good news is with LiPo and LiFe packs, there is no such concern.
Sizing Your Battery Pack
If you’re new to electric-powered models, you will probably follow the manufacturer’s recommendation for an appropriate pack for your aircraft. That’s what you should be doing. As you expand your hangar, you may decide to add a
bigger battery or need something that isn’t specified. You need to do enough research to get a feel for what type of
current the setup will pull under full throttle and size your pack accordingly. If your airplane requires a 3S setup using a typical 2,200 mAh pack and you change to a “hotter” motor—meaning one that is more powerful and will pull more
current—you need to see if your current packs can handle it. If your current power system is pulling 20 amps with your 2,200 mAh 15C pack, but your next motor upgrade will pull 35 amps, that pack won’t be happy. Let’s look at why.
The 15C pack is technically capable of pulling 33 amps (2,200 mAh x 15 = 33,000 mAh or 33 amps), so your 20-amp
requirement was well within its limits. Now looking at the new setup with the motor requiring 35 amps, you see that the pack is undersized, if only by a couple of amps. That’s enough to cause problems that can be costly in the long run.
I recommend buying a quality LiPo pack that is well beyond the projected requirements of the setup. Running a pack at
its limit will guarantee a short life and wasted money. Pay attention to the label and notice if it gives two ratings such as
30C/60C. These represent the continuous and burst ratings as previously mentioned.
Charging and Storage
Always balance charge when you can. Balance charging evenly distributes the energy stored in the battery across
the multiple cells inside. This will prolong your pack’s life and ensure better service from it. You can get away with fast
charging at the field without balancing if your regular routine is to balance charge at home. There are debates about charging and storage levels, but the safe bet is to store batteries at something other than fully charged or fully discharged. Most good balancing chargers offer a storage mode that takes them to a level of approximately 3.8 volts per cell. The important thing is not to leave them fully charged or discharged for long periods of time.
The Secret to Long Life
The secret, at least for your batteries, is to charge to 4.1 volts per cell as opposed to the full 4.2 volts per cell, and never
discharge them to full discharge level. Working your packs in between the two ends of the charge/discharge levels will
greatly increase their lifespan. Engineer/charger/ESC designer Doug Ingraham described it this way: “There are several things that cause degradation of lithium batteries. One is heat and for the purposes of RC modeling, this is most likely the one that causes the greatest degradation. The others have to do with the effects on the materials at both ends of the state of charge. “The lithium ions are forced into the carbon material on the plates at both ends of the state of charge. This causes a breakdown in the material, and in future charge cycles less ions can be held, causing degradation in capacity. It is mostly at the ends (full and empty) that this damage occurs, so staying away from the ends even a little can help
extend the life of the cells.” Several chargers offer a charge cutoff labeled “Long Life” or something similar, and they stop the charge at 4.1 volts per cell. From Doug’s explanation, you can see that using the 4.1 volts keeps you off the top end and setting an ESC low-voltage cutoff above the traditional 3 volts per cell will keep you off the bottom end. Unless
you’re a competitor trying to squeeze every last bit out of your flight, this will serve you well and save you money.
When your batteries get to the point that they need to be disposed of, one of the simplest options is using a no-cost
used rechargeable battery and cellphone collection program offered within a network of more than 34,000 collection sites
throughout North America. Call2Recycle accepts NiMH, Lithium Ion (Li-Ion), LiPo, and Ni-Cd batteries weighing up to 11 pounds. Simply visit the program’s website, www.call2recycle.org, and enter a ZIP code to find a collection center near you. If you don’t have Internet access, call (877) 273-2925. Drop-off centers are located at corporate offices, healthcare facilities, manufacturers, military bases, and at major retailers such as The Home Depot, Lowe’s, Staples, and Best Buy.
Tips for Happy Batteries
Keep them cool.
Don’t over charge.
Don’t over discharge.
Size them appropriately.
Use quality connectors.
Balance them whenever possible.
Charge at 1C.
Thanks to “Model Aviation” for this insightful article.