PRESIDENTS CORNER“Da Prez Sez”The July meeting has come and gone, the attendance was good. Our second trip to Cochiti lake was a good one, it was a repeat of the first trip  with warm weather and a nice calm morning with a gentle wind. Tundra's were the pick of the day as many had new ones with floats to try out. All who flew had very good flights. A date and place for the Christmas party

has been set so that we will have a location without waiting till the last minute and not having a place to party. Further plans will be taken up when we start meeting indoors in the fall. Will see you all when I return from  vacation in August.

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TURBULATORAugust 2017 Volume 28 Issue 8

Newsletterof the Rio Rancho Radio Control Flying Club

AMA Club #2770

Drone view of downed plane

recovery

WATERMAN FIELD ELEVATION 5840 FEET 35° 17.2'N 106° 44.8'W

Coming Events1. Next Meeting Saturday August 5th 10am at Waterman Field.2. Family day this month date to be determined at the meeting.

3. Next Float Fly determined at the meeting.

LiPo BatteriesWhat Do All the Numbers Mean?They way we define any battery is through a

ratings system. This allows us to compare the properties of a battery and help us determine which battery pack is suitable for the need at hand. There are three main ratings that you need to be aware of on a LiPo battery.

So what does it all mean? Let's break it down and explain each one.

Voltage / Cell CountA LiPo cell has a nominal voltage of 3.7V. For the

7.4V battery above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a "2S" battery pack - it means that there are 2 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.

About Nominal VoltagesI thought (mistakenly) that this was common

knowledge, but after a handful of emails on the topic, it was clear I needed to clarify what nominal voltage is.

Nominal voltage is the default, resting voltage of a battery pack. This is how the battery industry has decided to discuss and compare batteries. It is not, however, the full charge voltage of the cell. LiPo batteries are fully charged when they reach 4.2v/cell, and their minimum safe charge, as we will discuss in detail later, is 3.0v/cell. 3.7v is pretty much in the middle, and that is the nominal charge of the cell.

In the early days of LiPo batteries, you might have seen a battery pack described as "2S2P". This meant that there were actually four cells in the battery; two cells wired in series, and two more wired into the first two batteries in parallel (parallel meaning the capacities get added together). This terminology is not used much nowadays; modern technology allows us to have the individual cells hold much more energy than they could only a few years ago. Even so, it can be handy to know the older terms, just in case you run into something with a few years on it.

The voltage of a battery pack is essentially going to determine how fast your vehicle is going to go. Voltage directly influences the RPM of the electric motor (brushless motors are rated by kV, which means 'RPM per Volt'). So if you have a brushless motor with a rating of 3,500kV, that motor will spin 3,500 RPM for every volt you apply to it. On a 2S LiPo battery, that motor will spin around 25,900 RPM. On a 3S, it will spin a whopping 38,850 RPM. So the more voltage you have, the faster you're going to go.

CapacityThe capacity of a battery is basically a measure of

how much power the battery can hold. Think of it as the size of your fuel tank. The unit of measure here is milliamp hours (mAh). This is saying how much drain can be put on the battery to discharge it in one hour. Since we usually discuss the drain of a motor system in amps (A), here is the conversion:

1000mAh = 1 Amp Hour (1Ah)I said that the capacity of the battery is like the

fuel tank - which means the capacity determines how long you can run before you have to recharge. The higher the number, the longer the run time. Airplanes and helicopters don't really have a standard capacity, because they come in many different sizes, but for R/C cars and trucks, the average is 5000mAh - that is our most popular battery here in the store. But there are companies that make batteries with larger capacities. Traxxas even has one that is over 12000mAh! That's huge, but there is a downside to large capacities as well. The bigger the capacity, the bigger the physical size and weight of the battery.

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Another consideration is heat build up in the motor and speed control over such a long run. Unless periodically checked, you can easily burn up a motor if it isn't given enough time to cool down, and most people don't stop during a run to check their motor temps. Keep that in mind when picking up a battery with a large capacity.

Q: Why do we use voltage, and not capacity, to determine how charged a battery is?

A: The reason we use voltage, and not capacity to determine how charged a battery is stems from our difficulty in measuring capacity. Voltage is simple to measure — if you've ever used s voltmeter to measure a AA battery, you understand how trivial it is to measure voltage.

Capacity, however, is nearly impossible to measure accurately. We can measure how much energy is going into a battery (at least somewhat accurately), but we can't measure how much is actually in the battery.

Think of it like beakers of water. For voltage, the beaker is transparent, and we can easily see the amount of water in the beaker in the same way we can measure voltage whenever we like. On the other hand, we have the beaker representing capacity, and it's opaque — we can't see through it, and so the only way to know how much is inside is to empty it and measure the water (energy) as it's leaving the beaker (battery).

Because amperage and voltage are intertwined, as we will discuss later in detail, the voltage of a battery does correlate, approximately, to the capacity left in the battery, and while there are times when the voltage can deceive you, in general, it's okay to rely on voltage as our primary measure of how full a battery is.

This question was asked by Donald via email, and made complete sense to include my answer to him on the guide. Thanks, Donald!

Discharge Rating ("C" Rating)Voltage and Capacity had a direct impact on

certain aspects of the vehicle, whether it's speed or run time. This makes them easy to understand. The Discharge Rating (I'll be referring to it as the C Rating

from now on) is a bit harder to understand, and this has lead to it being the most over-hyped and misunderstood aspects of LiPo batteries.

The C Rating is simply a measure of how fast the battery can be discharged safely and without harming the battery. One of the things that makes it complicated is that it's not a stand-alone number; it requires you to also know the capacity of the battery to ultimately figure out the safe amp draw (the "C" in C Rating actually stands for Capacity). Once you know the capacity, it's pretty much a plug-and-play math problem. Using the above battery, here's the way you find out the maximum safe continuous amp draw:

50C = 50 x Capacity (in Amps)Calculating the C-Rating of our example battery:

50 x 5 = 250AThe resulting number is the maximum sustained

load you can safely put on the battery. Going higher than that will result in, at best, the degradation of the battery at a faster than normal pace. At worst, it could burst into flames. So our example battery can handle a maximum continuous load of 250A.

Most batteries today have two C Ratings: a Continuous Rating (which we've been discussing), and a Burst Rating. The Burst rating works the same way, except it is only applicable in 10-second bursts, not continuously. For example, the Burst Rating would come into play when accelerating a vehicle, but not when at a steady speed on a straight-away. The Burst Rating is almost always higher than the Continuous Rating. Batteries are usually compared using the Continuous Rating, not the Burst Rating.

There is a lot of vitriolic comments on the Internet about what C Rating is best. Is it best to get the highest you can? Or should you get a C Rating that's just enough to cover your need? There isn't a simple answer. All I can give you is my take on the issue. When I set up a customer with a LiPo battery, I first find out what the maximum current his or her application will draw. Let's look at how that works.

Let's assume that our example customer is purchasing a Slash VXL R/C truck. That motor, according to Traxxas, has a maximum continuous

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current draw of 65A and a burst draw of 100A. Knowing that, I can safely say that a 2S 5000mAh 20C LiPo will be sufficient, and will in fact have more power than we need. Remember, it has a maximum safe continuous discharge rating of 100A, more than enough to handle the 65A the Velineon motor will draw. Similarly, the Burst Rate of 150A easily covers the 100A the motor could draw.

However, the ratings on the motor aren't the whole picture. The way the truck is geared, the terrain the truck is driving on, the size of the tires, the weight of the truck... all of these things have an impact on the final draw on the battery. It's very possible that the final draw on the battery is higher than the maximum motor draw. So having that little bit of overhead is crucial, because you can't easily figure out a hard number that the truck will never go over.

For most applications, a 20C or 25C battery should be fine. But if you're driving a heavy truck, or you're geared up for racing, or you have a large motor for 3D flying applications, you should probably start around a 40C battery pack. But since there is no easy way to figure this out, I encourage you to talk to your local hobby shop to have them help determine which battery pack is right for your application.

Internal Resistance: The Mystery NumberThere is one very important rating we haven't

talked about yet: Internal Resistance (or IR). Problem is, you won't find the IR rating anywhere on the battery. That's because the internal resistance of a battery changes over time, and sometimes because of the temperature. However, just because you can't read the rating on the battery doesn't mean it isn't important. In a way, the internal resistance is one of the most important ratings for a battery.

To understand why the IR is important, we have to understand what it is. In simple terms, Internal Resistance is a measure of the difficulty a battery has delivering its energy to your motor and speed control (or whatever else you have a battery hooked up to). The higher the number, the harder it is for the energy to reach its preferred destination. The energy that doesn't "go all the way" is lost as heat. So the internal

resistance is kind of a measure of the efficiency of the battery.

Internal Resistance is measured in milliohms (mΩ).1,000 milliohms is equal to 1 Ohm (Ω)Measuring the IR of your battery requires a special

toolset. You either need a charger that will measure it for you or a tool that specifically measures internal resistance. Given that the only tool I have found for this (at least in the hobby world) is almost as expensive as a charger that does this for you, I'd go with a charger for this process. Some chargers measure each cell's IR separately, and some measure the entire battery pack as a whole. Since internal resistance is a cumulative effect, and the cells are wires in series, if you have a charger that does each cell independently, you need to add up the IR values of each cell, like this:

Suppose we have a 3S (3-cell) LiPo battery, and the measuring the cells independently yields these results.

Cell 1: 3 mΩ     Cell 2: 5 mΩ     Cell 3: 4 mΩTo find the total internal resistance for the battery

pack, we would add up the values for the three cells.3Ω + 5Ω + 4Ω = 12 mΩFor a charger that measures the pack as a whole,

all you would see is the 12 mΩ - the rest would be done for you - behind the scenes, as it were. Either way, the goal is to have the IR for the entire pack.

The first reason internal resistance is important has to do with your battery's health. As a LiPo battery is used, a build of up Li2O forms on the inside terminals of the battery (we'll go more in depth on this later in the Discharging section). As that build up occurs, the IR goes up, making the battery less efficient. After many, many uses, the battery will simply wear out and be unable to hold on to any energy you put in during charging - most of it will be lost as heat. If you've ever seen a supposed fully charged battery discharge almost instantly, a high IR is probably to blame.

To understand how Internal Resistance works in R/C applications, first we have to understand Ohm's Law. It says that the current (Amps) through a conductor between two points is directly proportional

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to the difference in voltage across those two points. The modern formula is as follows: Amps = Volts / Resistance. In the formula, the resistance is measured in Ohms, not milli-ohms, so we'd have to convert our measurements. If we use our previous 3S LiPo, and plug it into the equation along with a 1A draw, we can find out how much our battery pack's voltage will drop as a result of the load. First, we have to change the equation to solve for volts, which would look like this:

Amps x Resistance = VoltsSo plugging in our numbers and solving the

equation would look like this:1A x 0.012 Ω = 0.012VSo our battery would experience a tiny drop in

voltage when a 1A load is applied. Considering our 3S LiPo is around 12.6V when fully charged, that's not a big deal, right? Well, let's see what happens when we increase the load to 10A.

10A x 0.012 Ω = 0.120VNow we see that when we increased the load

10X, we also increased the voltage drop 10X. But neither of these examples are very "real world". Let's use the Slash VXL from the previous section and plug those numbers in. If you recall, our Velineon motor has a maximum continuous current rating of 65A. Let's assume we manage to hit that mark when driving and use that.

65A x 0.012 Ω = 0.780VWow, more than 3/4 of a volt! That's around 6.2%

of the total voltage of our battery pack. Pretty respectable, but it's still a reasonable drop in voltage.

"So, yeah, the voltage drops. But so what? What does that actually mean? How does it effect my R/C vehicle?" Well, let's continue on with our example to show you.

The Velineon motor our Slash VXL uses has a Kv rating of 3500. That means it spins 3,500 RPM per volt. On a fully charged 3S LiPo we'll see this (assuming no voltage drop):

12.6V x 3500RPM = 44,100 RPMNow, assuming we can hit that 65A draw on our

unloaded motor (which we can't in real life, but for the

purposes of demonstration we can), here's the RPM on the same motor with our voltage drop from before:

11.82V x 3500RPM = 41,370 RPMDifference of 2,730 RPMSee the drop in performance? That's the effect

Ohm's Law has on our hobby. A lower internal resistance means your car or truck or airplane or boat or helicopter goes faster and has more power.

This begs the question: how low should it be? Unfortunately, there's no easy answer for this. It's all dependent on your use case and battery. What is great for one battery may be terrible for another. Based on my online research, combined with my own experience and findings, I would say, as a general rule, a per cell rating of between 0-6 mΩ is as good as it gets. Between 7 and 12 mΩ is reasonable. 12 to 20 mΩ is where you start to see the signs of aging on a battery, and beyond 20mΩ per cell, you'll want to start thinking about retiring the battery pack. But this is only a guide - there is no hard rule set here. And if your charger doesn't give you the per cell measurements, you'll have to divide your total count by the number of cells in your battery to get an approximate per cell rating.

Internal Resistance and C-RatingThere are many people out there that believe a

higher C-Rating will make their vehicle perform better. We know from our previous discussion on C-Ratings that you need to account for the power draw your motor has when picking out the right C-Rating for your battery, but does more equal better? Many people say yes.

But there isn't anything intrinsic to the C-Rating that substantiates their claims. It's simply not true that a higher C-Rating makes your car or airplane faster.

However, there is a correlation between the C-Rating of a battery and the internal resistance of that battery. In general, batteries with a higher C-Rating also have a low internal resistance. This isn't always the case, as there are always variances in manufacturing, but the general idea seems to hold true, and a lower IR will make a car or airplane faster.

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This is a case of correlation not equalling causation. It's really the internal resistance making a battery faster, not the C-Rating.

FOR SALEHanger 9  Pawnee D  powered by a 4cycle OS

FS-70  with Spektrum  servosAsking 550.00.

Hanger 9   25% J 3 Cub  powered by 4 cyl OS 160  Gemini twin engine. with Hitec servos (Floats are not included. but plane has the hard points )

Asking 700.00

If you are interested in either plane please contact Rudy Caceres. @ 510-914-9195 or email him @ arcaceres@comcast.net

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AMA Sanctioned event, AMA membership required.

CD: Rick Linden, ralinden@comcast.net, 505-268-1881

See our website at www.arcc.club for a map to the field.

Maloof Air Park features a smooth newly paved 800 ft runway,

Heli/Drone area, and shade covered tables. Lunch concession will be available both days.

Limited camping, no hookups. Please call first.

The event will have lots of open flying. Jets welcomed but please no profiles!

Landing Fee: $30 including BBQ

September 9 & 10, 2017 Sponsored by the Albuquerque Radio Control Club.

Held at the George Maloof Memorial Airpark in Albuquerque, an AMA historic flying site.

There will be 4 prizes given: the best built-up Civilian & built-up Military aircraft, and the best ARF Civilian & ARF Military aircraft.

A “Pilots Choice” award will be given on Sunday. (aircraft must fly to qualify for prizes)

An ongoing pilots raffle will be running each day. We will have a Saturday night barbecue with a special aviation prize.

There will be a Swap Meet Friday night the 8th beginning at 4 PM. (Please bring your own table if you want to sell. No fee. )

ARCC Scale Classic

MEETING MINUTESMinutes from the July 2017 Club Meeting

The meeting was called to order at 10:00am with 15 members present.Minutes were accepted as published.Treasurers Report was accepted as presented.Membership Report: 37 2017 Members.Field Report: 1. Crack repair is continuing in a repair as you fly mode. The repair tools and filler are in the Storage Shed. If you’re flying and the wind comes up take a bit and fill a crack.Safety: 1. Use spotters if there is more than 1 plane in the air. 2. Be aware of your surroundings. One of our members was taking off and as he watched his plane accelerate down the runway he noticed 3 large dogs on the runway. The take off was aborted and in avoiding the dogs the plane ran off the runway into the cable breaking the prop, bending the landing gear and breaking the landing gear doors. We’re all watching our planes on take off but don’t look more than about 15 feet in front of the plane. The runway was clear, take off started and poof the dogs walked onto the runway. Try to be aware.2. Rick Svitzer a former club member reported an accident that occurred at his new field in North Carolina. One of their members bumped their throttle and the propeller came to life and cut into his fore arm. The prop cut through 3 muscles to the bone ! Fortunately, no nerves or tendons were cut. The result was a trip to the hospital and surgery to reconnect the muscles. The club doesn’t routinely use the

“Throttle Kill” safety switches that we recommend. Rick is in the process of teaching the club the value of the Throttle Kill safety switch. If your radio isn’t set-up for throttle kill then see Don and he’ll get it programmed in your radio.New Business: 1. There was a discussion on reserving the room we used last year for the Christmas party again this year. Ken recommended we take action on this now to ensure we get the date we desire. The dates that are good with the club were decided Saturday 2nd or 9th of December. Bill Ryan took the job of going to the rental agency and seeing which date was open and reserving the room for our Christmas party. If both dates are open then Bill will decide which date and reserve it. The motion to give Bill the authority for this was unanimously approved.2. The next Float Fly will be Thursday 13 July. Don will email the club.3. The club historian would like club members to email him with pertinent information on club members that have passed. If you have information for Andy his email is edyem@live.com. Don will send and email to the club.4. Family Day will be scheduled for Late August or Early September this year. The club will set the final date at the next meeting.The meeting adjourned at 1033AM

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Turbulator:Editor Don McClelland We are always looking for articles, pictures and your input!

For comments, or suggestionsPlease Email Don at macmoke1@gmail.com

Please support our sponsors:HJobby Proz2225 Wyoming Blvd NE # JAlbuquerque, NM 87112-2638(505) 332-3797

RIO RANCHO RC CLUBAMA Charter #2770

www.rioranchorcflyers.org

Next Club MeetingAugust 5th, 10:00am at Wallen field