“NiMH Charging”
snagged from: http://www.powerstream.com/NiMH.htm
CRCM TIMPA . . . . . Capt.Kill4Fun
NiCAD |
NiMH |
LiPoly/LiIon |
1.2 V |
1.2 V |
3.6 V |
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What
I’ve learned from all this: |
Setting a charging rate of C
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Basics The
coulometric charging efficiency of nickel metal hydride batteries is
typically 66%, meaning that you must put 150 amp hours into the battery for
every 100 amp hours you get out. The faster you charge the worse this gets. The minus
delta V bump that is indicative of end-of-charge is much less pronounced in
NiMH than NiCad, and it is very temperature dependent. To make matters worse,
new NiMH batteries can exhibit bumps in the curve early in the cycle,
particularly when cold. Also, NiMH are sensitive to damage on overcharge when
the charge rate is over C/10. Since the delta V bump is not always easy to
see, slight overcharge is probable. For this reason PowerStream does not
recommend using minus delta V as a termination criterion for nickel metal
hydride batteries. As the battery
reaches end-of-charge oxygen starts to form at the electrodes, and be
recombined at the catalyst. This new chemical reaction creates heat, which
can be easily measured with a thermistor.. This is the safest way to detect
end-of-charge during a fast charge. Overnight
Charging The cheapest way to charge
a nickel metal hydride battery is to charge at C/10 or below (10% of the
rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for
15 hours. This method does not require an end-of-charge sensor and ensures a
full charge. Modern cells have an oxygen recycling catalyst which prevents
damage to the battery on overcharge, but this recycling cannot keep up if the
charge rate is over C/10. The minimum voltage you need to get a full charge
varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even
though continued charging at C/10 does not cause venting, it does warm the
battery slightly. To preserve battery life the best practice is to use a
timer to prevent overcharging to continue past 13 to 15 hours. Examples of
this kind of charger are shown at http://www.powerstream.com/NiMHWM.htm.
This charger uses a microprocessor to report the state of charge via an LED
as well as performing the timing function. Faster Charging Using a timer
it is possible to charge at C/3.33 for 5 hours. This is a little risky, since
the battery should be fully discharged before charging. If the battery still
has 90% of its capacity when the timer starts you would have a good chance of
venting the battery. One way to ensure this doesn't happen is to have the
charger automatically discharge the battery to 1 volt per cell, then turn the
charger on for 5 hours. The advantage of this method is to eliminate any
chance of battery memory. PowerStream does not currently have such a charger,
but the microprocessor board used in the C/10 charger http://www.powerstream.com/NiMHWM.htm
could easily be modified to do the discharge. A power dissipating package
would be needed in order to dissipate the energy from a partially charged
battery in a reasonable amount of time. Another example of a 3 hour charger
is http://wwww.powerstream.com/9vnmh.htm
. This is a very inexpensive microprocessor based 9 volt "transistor
radio" battery charger that that drops to low current when the battery
voltage indicates a full charge. Fastest Charging If a
temperature monitor is used NiMH batteries can be charged at rates up to 1C
(in other words 100% of the battery capacity in amp-hours for 1.5 hours). The
PowerStream battery charge controller shown in http://www.powerstream.com/product3.htm
does this, as does the battery management board shown in http://www.powerstream.com/product5.htm. This board also has the
ability to sense voltage and current for more sophisticated algorithms. |
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http://groups.msn.com/rcmodelsflying/parkflyers.msnw De-Mystifying Electric Power by DOUG BURT (EAVIATOR) Doug is an old friend from Vancouver, British Columbia, Canada and a long time Group Member. An expert in the field of electric powered flyers, Doug writes a monthly column for MAAC Magazine and has very kindly given us permission to reprint his articles for you. Very many thanks Doug! ********************************************************************* Part 1 In
order to get a model aircraft to fly, certain basic elements must be kept in
mind. With electric power, it’s a matter of figuring out how many watts of
power you need to fly your aircraft. To do this you’ll need basic electrical
tools; an ammeter and a shunt to measure current and a basic voltmeter. Even
better would be a Wattmeter, available from AstroFlight. Although MAAC
doesn’t endorse any manufacturers product, this is such an invaluable tool I
think it bears mention and is an item that should be in every electric
modelers possession. It measures running current and voltage it will also give
you figures for output wattage. There
is a simple formula to follow. Measure your wing-area and weigh the finished
airframe to get an idea what kind of wing loading you have to work with (wing
area in square feet divided by weight in ounces). The airframe should weigh
less than 50% of the final weight so double what you get to estimate a rough
all up weight (AUW). From the estimated AUW we can figure out different
performance levels. Trainers capable of ROG should have between 35 - 50 watts
per pound and have a wing loading roughly 10-20oz. per sq. ft. Sport plane’s
capable of minor aerobatics and general good flying characteristics should
have wing loadings of 15 -20 oz. per sq. ft. and between 50 - 65 watts per
pound. Aerobatics planes should have between 75 and 100 watts per pound and a
wing loading of 15-25 oz. per sq. ft. Full dress heavy metal aircraft will
require 100 watts a pound and over and have an even heavier wing loading.
Next issue we will go over how to arrive at a proper power system and fine-tune
it for your application. Until
next time get out there and Fly-It…. Quiet Part 2 Now
as promised, the follow-up to the article started on De-mystifying Electric
Power systems. In that article (Aug 2001) we determined that we had to have a
certain amount of power available for different types of aircraft i.e. Trainers
capable of ROG should have between 35-50 watts per pound, sport planes
approximately 50-60 watts per pound, aerobatic aircraft from 75-100
watts per pound and war birds over 100 watts per pound. It should be
mentioned that this is a guide only, as many applications out there will
prove to be exceptions to the rule. Fun, huh! Well
this is all wonderful you might say but how does one calculate the power
output for a given model aircraft? Without going into a dissertation on how
electrical power is calculated it will suffice to do measurements that are
slightly less complex for our purposes. In order to do these measurements
we’ll need an ammeter capable of reading up to 40 amps. The first step in
figuring this out is you will need to add the number of cells used in your
pack and multiply that number by 1.2 (the accepted voltage at full charge)
i.e. for an 8-cell pack you would calculate it as 8 X 1.2 = 9.6v. Then,
observing all safety precautions install the ammeter between the motor and
battery pack, (with the pack of cells you are using) and take an amperage
reading from the ammeter while running the motor with the appropriate sized
propeller, gearbox etc. This can be done in a static condition with the motor
mounted to a test bench or while in an aircraft. Take the number from the
ammeter display and multiply it by the voltage of your pack and
"Presto" you have the watts your system is producing with the
combination you’re using. For
instance the very popular reverse wind Magnetic Mayhem motor by Kyosho,
driving a 2.5:1 gearbox spinning a 12 X 8 APCe prop will read about 28 amps
on 8 cells. In order to get the number of watts you have available you would
multiply 28 X 9.6v and get 268.8 watts. Take the weight of your aircraft (in
pounds) and divide by this number to get your total. For instance a plane
weighing 58 ounces or 3.6 lbs would have approximately 74.15 watts per pound,
more than enough to fly adequately as a sport plane, but the power system
would be borderline for aerobatics. An easier way to do this is to use a
Super Wattmeter, available from AstroFlight, as it will calculate amps,
volts, watts and mAh used from the pack in real time. Following
the Ezone (http://www.ezonemag.com) would certainly be
helpful in determining where to start with a particular motor and gearing.
Because of many variables it would be literally impossible to provide a list
of all combinations and the appropriate ratings for each of these systems in
this column. Just remember, amperage can be controlled by changing prop sizes
or gearing and voltage adjusted by changing cell counts. It is also important
to remember to follow the manufacturers recommended voltage specifications
and not to overpower an electrical motor. Otherwise it will end up having
very poor performance and a much shorter lifespan…. Until next time remember
Fly It…Quiet Part 3 Now I’m
sure you’ve all been waiting for the latest installment of De-Mystifying
Electric Power that I manage to get to every second issue or so. If you’ve
been following along, you’ll now know how to calculate the approximate range
of power for your given aircraft, and know how many Watts it will take to get
the plane airborne. Where do you go from here? Well,
with the information you now have, you should be able to confidently purchase
the appropriate motor, gearbox and propeller for your new e-powered plane.
You’ll still need an electric speed controller (ESC), battery pack and some
other goodies to get you up and away. Speed controllers or ESC’s are like a
throttle servo in the sense that they control the amount of power that goes
to the prop through the motor. There are so many controllers on the market I
wouldn’t even think of making a suggestion as to which one to use. They range
in size from miniscule to fairly large, depending on their job and amperage
rating. Some are equipped with a special type of circuitry called a BEC
(battery eliminator circuit), which allow you to eliminate your onboard
receiver battery (and excess weight). This type of controller actually uses
your flight pack to power the ESC and the receiver. Some speed controllers
require the use of an additional battery to power your aircraft receiver. One
of the better ones I’ve found recently that work for modestly powered
aircraft are the Great Planes Mini’s, available in various amperage ratings
from 5 - 30 watts and are competitively priced. Most have BEC circuits too,
making them an even better value for cost conscious modelers. Most magazines
catering to electric flight have good articles on the various and latest
speed controllers. Another source of information would be Bernard E. Cawley’s
column on the Ezone, "Controlling Interests" (www.ezonemag.com).
Just be sure to deal with a reputable dealer when purchasing an ESC (or any
equipment for that matter!) and you shouldn’t have too much difficulty. The
next step is to purchase a battery pack or packs. Since you’ve already
managed to figure out how many volts you need to get the watts required you
can figure out how many cells you’ll require in your battery pack. I’ve had
some very good luck with packs made up from Panasonic 1700 mAh cells and
Sanyo RC 2400’s, but there are a lot of different types of batteries out
there. For quick charging purposes the best bet is still nickel cadmium
(NiCad) and are usually referred to as SCR types (designated for fast
charging). These batteries have a different internal composition from your
receiver and transmitter packs and are designed for high-powered, quick
charging. Make sure that the packs you purchase are designed for quick
charging. If you don’t you’ll waste a lot of money and throw away lots of
battery packs. The next best power source are nickel metal hydride (NiMh).
These are usually charged at a much lower rate, but the duration is better,
weight of the pack lower for the same amount of cells. The one drawback some
flyers have complained about is the fact that the NiMh doesn’t seem to have
the same oomph as NiCad’s. One way of getting around this is to add another
cell to the pack’s configuration, but then you are adding more weight...Hmmm.
Just doesn’t seem all that easy some times does it. Never mind, once you
start getting the hang of it, it becomes a breeze. The
only other thing you’ll need initially is a charger, but hang on. The charger
is one of the most important pieces of equipment you’ll buy if you’re going
to fly electric power for any length of time and as such, deserves a column
specifically for it. Until next time remember Part 4 Well I
promised I’d get back to "Demystifying Electrics" this issue. I
realize that there are getting to be more and more people involved with
electric flight and questions about chargers seem to always crop up.
Therefore I’m going to dedicate the rest of the space this month to helping
you select that most important piece of gear, the charger. As
everyone has different styles and types of flying there are certain factors
to bear in mind when starting to look for a charger. How many cells do you
intend to charge? Standardizing pack size and configuration will help out
somewhat, but everyone doesn’t just fly one type of plane then do they. What
type of cells are you going to charge i.e. NiMh (nickel metal hydride) NiCad
(nickel cadmium) Li (Lithium Ion)? Some types of cells, especially the
lithium ion ones require a specialty charger; they can be extremely dangerous
when handled improperly or charged wrong. Refer to the cell manufacturers
instructions when in doubt. Most
of us however are using NiCad or NiMh cells. These do not require extremely
expensive or complex chargers. The one thing to bear in mind is the highest
number of cells you will be charging at any one time. Earlier on the electric
R/C aircraft community was relying on ready-made battery packs from the R/C
car folks. Most of these packs were 7 cells and the chargers of the day
typically reflected this. Now, commonly referred to as the 7-cell trap, most
of today’s aircraft use much higher cell counts to power our planes to newer
and greater heights. Remember chargers are the most important piece of gear
you own! How else are you going to pack every last electron into the pack
that you want? The most
common chargers in use out here on the west coast at our two electric clubs
are HiTec’s CG 335 or AstroFlight’s 110D or 112D. They’re all really good
chargers, capable of tackling most common cells (all will charge from 2 – 24
cells and the AF 112D will charge up to 40, either NiCad or NiMh). These all
represent good value for the money (starting around 170.00 CDN). The only
drawback to these chargers is that none of them will discharge your packs
when you’re finished for the day. So if you don’t fly out all your packs, you
might be setting yourself up for a letdown in the future, as both NiCad and
NiMh cells will form a memory if they aren’t taken care of. Yes, that’s
right! Batteries need maintenance and care too, but we’ll get into that
eventually. Some
of the better chargers will not only do all of the above, but they will also
discharge and cycle your packs too! These get considerably more expensive
though but if you get serious about E-Flying you will realize the benefits of
proper pack maintenance. Better performance and longer lasting packs. One
really good all around charger/discharger is the FMA "Super Nova"
available for about 220.00 CDN. Some like the Schulze Chameleon and the Orbit
MicroLader will also plot a graph to your PC for use in tracking how your
cells/packs are doing. These get quite a bit more expensive with the Orbit
starting at about 239.00 CDN (plus software, extra charge) and the top of the
line Schulze around 600.00 CDN. Don’t forget also that most of these chargers
won’t be found in your local hobby shops (except for extremely well-stocked
ones), but are available from a plethora of on-line suppliers. All of
these chargers all work off a 12-volt DC power supply, whether it be a car
battery or a regulated power supply such as are found in computers (most
manufacturers also have specialized power supplies for their particular
chargers, but these usually cost more). Most E-Flyers prefer to charge at the
field as the charge is fresher (both NiCad and NiMh cells charge will
deteriorate over a period of hours). So using a car battery at the field is a
good way of supplying the current necessary for your charger. Just be sure
that you don’t overdo it and drain your auto battery! I’ve seen some very
experienced flyers get into a flying session and completely forget about how
the heck they’re going to start their car afterwards. Some of the more
serious E-Flyers keep a 12v deep cycle battery (either marine or RV use) for
use as a charger supply source that they bring with them to various meets and
events (most fields I’ve been to don’t have 110v AC power close enough to the
flight line to permit us to use a regulated power supply on site). Have fun
flying and don’t forget until next time Fly It...Quiet These articles are
copyrighted to the Author and are not to be reproduced in any manner without
the specific written permission of the Author. |
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Radical RC.com Lithium Ion Polymer Applications If they are 4.2 volts a cell when full, how do we apply that voltage? The first thing to understand is how many volts you'll need to fly your model. Lithium cells are 3 volts per cell when empty and 4.2 volts per cell when full. Generally a 2 cell lithium polymer pack will best simulate a 7 cell NiCad or 7 cell low resistance NiMH pack or a 8 cell typical NiMH pack. A 3 cell lithium is much like a 10 cell NiCad/HV NiMh or 11 cell typical NiMH. So, if your replacing an 8 cell 720 NiMH pack in your S-280 or S-300 Bird, we're going to need a 2 cell pack. If your replacing a 10 cell 600 AE NiCad pack in a 7.2V S-400 Ship, we're going to need a 3 cell pack. With NiMH or NiCad cells we can make nice small incremental jumps in our voltage. A bird that doesn't fly properly on a 7 cell NiCad 600 AE pack can often be fixed by adding a cell. How much current (amps) can I pull from a Lithium? Rather than trying to learn the individual discharge amp limits of every cell, it's a good idea to learn the "formula". It's very simple really. All of the Kokam cells listed with Radical RC can handle continuous discharge rates from 3 to 4C. "C" is capacity. So, if we pick a cell out, lets say the 2500, we know it should perform well if we limit long full throttle current to 7.5 to 10 amp. (please note, there are 1000 mah in 1 amp) In other words, 3 or 4 X it's capacity (shown in mah). Learn the formula, then you can look at any cell and know what to expect. An example to look at is a ZAGI, it is very common for these to be flown at or near full throttle for the full duration of the flight. If we need to support 10 amps on a constant basis we need to find a cell that when it's capacity is multiplied by 3 or 4 results in 10 amps. So, the 2500x4=10000 or 10 amps, the 3300x3=9900 or 9.9 amps, 3300x4=13200 or 13.2 amps. You might ask, which is the better number to use when I multiply the capacity, "3" or "4"? Generally, if you know the model will be at absolute full throttle ALL the time, lean heavily towards the cell that when multiplied by 3 equals the current you need. If the model will see mostly high throttle, average of 80% throttle, then you can use slightly smaller cells and use the factor of 4X capacity. Can they deliver more current for shorter periods? Yes! This is the good news. Most models fall into the "Fun-Fly" or "Stunt" category. When the model gets an extreme amount of throttle control (very little full throttle and then only very short bursts) we can use a factor of 7C. An example of this is our Edge 540. On the BD301 it pulls about 8 amps from a 2 cell Li-Poly pack. Our favorite pack for this model is in the 2000 mah range. However, we've flown in many times on an 1120 2 cell pack. Some quick math, 8amps/1.120(amp capacity of pack)=7.14. So, were flying this particular pack just over 7xCapacity or 7C! The model will still hover for 30 seconds at a shot a time and provides a very satisfying aerobatic flight. The pack works fine and no noticeable drop off. But, keep in mind this is a close in high power to weight 3D fun-fly model. The throttle is moving all the time and is seldom at full throttle for very long. Another example is an E3D. At 40 amps, we can get away with a pack in the 5700 to 7500 mah range easily. This type of model naturally is not held at full throttle for long periods. What about my Heli? This is an interesting challenge. Unlike a fun-fly airplane, a heli has a long hard drain and the battery never gets any off time or very low throttle time. We need to pick a lithium capacity that fits in the 3-4C range for best results here. If your flying 10 cells now and hover current is 20 amps (20,000mah), we need a pack of 5000mah to 6600 mah range to support this. If we want to do some more aggressive flying we need a larger pack. Bear in mind that your hover current with the Lithium pack will be reduced somewhat due to lowering the weight of your machine. If we design the pack to handle your machine at it's heavy NiCad or NiMH current then we've also built in a little buffer. How much current is my Heli or Airplane pulling? Nothing beats an Astro Watt meter for taking actual static amp readings. Short of that, there is another way to figure your "Average" current. First, understand you don't get the full capacity of your pack in a flight with the only exception being very slow floaters like thermal sail planes. Lets presume first that we only get about 90% of out capacity out most of the time. You can adjust this number if you've measured differently in your setup. Our example model has a 1700 mah pack in it. It performs well for 8 minutes. We need to get 90% of our capacity as a starting place. 90% of 1700(1700X.9) is 1530. Now, the pack is rated in Milli-Amp-Hours. Hours is the "KEY" word here. We need to convert our 8 minutes of flight time to it's fraction of hours. 60/8=7.5. So, we're flying on 1530 mah of energy for 1/7.5th of an hour. 1530 mah X 7.5=11,475mah or 11.475 amps of average current. If we're flying 6 minutes on 3000 mah cells how much is our average current? 3000 X .9=2700 mah energy used. 60 minutes/6 minutes flying time=10 or 1/10th of an hour. 2700x10=27,000mah or 27amps average current. I've personally owned a 10 cell fun-fly model propped to 50 amps at full throttle on a 1700 size NiCad. The model would fly pretty easily (with lots of aerobatics) for 8 minutes. My average current as figured from above would have been 11.475 amps. The same model on a 3000 NiMH pack would fly about 12 minutes. Average current with the heavier pack is 13.5amps. The model pulled more current on average due the the heavier pack. You might also assume I was more "careful" with the 1700 because I knew it was in the airplane and could be depleted faster. Can you figure the size and cell count of the Li-Poly pack I would need to fly the above example? Max current 50 amps on 10 cells. 50/7=7.14Amp-Per-Hour or 7140 mah minimum pack size. This would be absolute minimum! We could make that up from 3 2500 3 cell packs in parallel (7500 mah total capacity). We'd have voltage similar to a 10 cell NiCad or NiMH pack and would weigh about 15.75oz. About 71% of the 3000 NiMH weight. The "Average Discharge Rate Approximator" formula is (Capacity X .9)X(60 / Flight Minutes)=Average Discharge Rate in Mili-Amp-Hours. Don't be Greedy, Look for about 80%. Near the top I mention flying my Edge 540 profile model on an 1120 pack at 8 amps as a good 7C example. In truth I like the model better with about a 2000 mah pack in it. Performance is the same but flying time is more satisfying also the high power part of the pack is much longer and overall it suits the model and "my tastes" better. Generally the best packs work out to be around 80% of the weight of the best NiCad and NiMH packs for a given application. 80% weight is just a rule of thumb, if your best pack is 70% or 90% of the best NiMH pack then all is well. If however, you are considering a pack that is 50% or 120% of the NiCad or NiMH pack you've got a solid indication to double check your math and be sure your not overlooking or miscalculating something. How do I understand Parallel and Series packs? An example of Parallel packs that probably everybody has seen is a heavy duty pickup with two 12V batteries in it. This is used to extend the electrical work capacity of heavy duty work trucks such as plow trucks and other trucks that run hydraulics or cable winches. Often a pleasure boat will have two batteries in it to make it more certain that it will start when your 10 miles off shore. The two batteries are connected negative to negative (black to black or - to -) and plus to plus (red to red or + to +). Rather obviously we know big trucks and nice boats are still 12 volt systems. When you connect batteries up negative to negative and positive to positive you increase the capacity. This is to say we can do the same job 2 X as long. The voltage stays the same. Below is a drawing of a typical lithium pack that is 2S-2P (2 in series, 2 in parallel) making the 1020 cells into an 8.4V pack of 2040 mah capacity.
An example of Series is your transmitter pack. It is probably made up of cells that are 600 mah in capacity. If you strip off the cover you can read the actual capacity printed on the cells. These cells are connected to each other Positive to Negative to Positive to Negative and so on. When cells are connected thusly the capacity stays the same and the voltage increases. A TX pack is usually 8 cells (1.2V per cell) and 9.6V. The savvy readers will have already figured out that our truck example above also includes a "series" battery. The 12V lead acid battery in your car or truck is really 6 each 2 volt cells in series. Series and Parallel battery packs must contain all cells of exact same capacity and they should be of the same brand. If not they won't run down equally. How long does it take to charge a Lithium battery? Roughly you can figure Empty Capacity in mah/charger output + 1/2 hour. So, if we're charging an empty 2 cell 2100 (empty is 6V) pack at 400 mah it will take about 5 3/4 hours. If your charge at the maximum rate of Capacity X 1 then it will take about 1.5 hours. At Capacity X 1 (2100 mah in this example) it will take about 1 hour to fill the pack to 90% and an additional 30 minutes to pack the last 10% of the charge. There is no good way to charge them faster. Lithium batteries are slower to charge than NiCad and NiMH cells then. :-( NO! Striking statement but the answer is no. It takes longer (or as long) to charge a NiMH or NiCad of similar weight and job. Remember, in electric flight, most of our successful lithium packs fly the models for roughly 3 - 10 minute flights. Our Edge 540 flies about 5 minutes on a 350 NiCad which takes 15 minutes to recharge. About 10 minutes on a 720 NiMH pack taking about 35 minutes to recharge. It flies about 30 minutes (with the same vigor!) on the 2100 Lithium which takes 1 1/2 hours to recharge. In the "flying" time it takes to run down the lithium, you've got to charge the NiCad 6 times taking about 1.5 hours, You've got to charge the NiMH 3 times taking about 1 hour and 45 minutes. So, in the real word, it takes no more time to charge a Li-poly pack than it does to charge any other type of common pack as long as the weights and jobs are similar. Yes, it takes 3x as long to charge a 720mah lithium cell as it does to charge a 720mah NiMH cell but this is manousha as the lithium and NiMH cell of the same capacity can't do the same job. We're typically running a Lithium of 3X the capacity as the best NiMH cells. Even though we can charge a NiMH cell at capacity X 2 giving a 35 minute charge and we can only charge a lithium at Capacity X 1 giving a 90 minute charge what you must remember is the Lithium pack is 3X the capacity often. In the Case of the 720 (Edge 540 example above) the max charge rate is 1.4 amps, the 2100 lithium cell can accept a charge rate of 2.1 amps! Both do the same job but the Lithium is being charged at HIGHER amperage due to it's being 3X as big. So, the lithium pack will accept 10minutes flying time worth of charge faster! I wish I could add less than 4.2 volts to a Lithium Pack. How do I match my 8 cell NiCad machine with a Lithium? Your correct in remembering that a 2 cell Lithium is more directly related to a 7 cell NiCad. If we run a 2 cell Lithium "AND" change nothing else we'll have less power presuming the pack weight is the same. If we run a 3 cell lithium then the voltage will be so high that our current will be more than we had planned for the setup. How do we unravel this riddle? First, understand that you're probably getting a small weight reduction. This will make up for some of our reduced prop RPM form the lower voltage of the 2 cell lithium pack. What we are left with is learning to become better students of gear ratio and prop selection. It's likely you'll need to reduce the gear ratio for example, drop from 4:1 to 3.5:1 to get your RPM back where it was with the 8 cell NiCad. Another alternative (simpler) is to choose a prop with a little more pitch. If you were running a 6 pitch prop, you might get your performance back with a 7 pitch on the lithium pack. It's really not that complex but it is a concern you'll have to address more with the limited number of voltage selections in lithium packs. Improve your skills. Most of our electric models do not fly at full throttle the whole time. In fact, it is a mark of an experienced pilot to be able to make the model look good and do many different maneuvers at many different speeds. Learn to fly with throttle control. The throttle is variable. At times when I watch people I wonder if their speed controls are really like the Space Shuttle Boosters, once you turn it on it goes at full blast till it is empty. ;-) Use only what you need and you'll have much better flying models, lighter battery packs and longer flights. Some simple examples are: don't use full throttle on the down side of a loop, when your flying down or lowering your altitude do it with the motor off or at a reduced RPM. Take some time in your flights to do some low speed aerobatics. Practice these things a little at a time and soon you'll be showing the full range of your model's capabilities in every flight. I think you'll find it enjoyable and it will certainly enrich the quality of your electric experience. |
Lithium
Battery Fires
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AMA Safety Committee |
Lithium Battery Fires
Lithium batteries are becoming very popular for powering the control and power systems in our models. This is true because of their very high energy density (amp-hrs/wt. ratio) compared to NiCads or other batteries. With high energy comes increased risk in their use. The, principal, risk is FIRE which can result from improper charging, crash damage, or shorting the batteries. All vendors of these batteries warn their customers of this danger and recommend extreme caution in their use. In spite of this many fires have occurred as a result of the use of Lithium Polymer batteries, resulting in loss of models, automobiles, and other property. Homes and garages and workshops have also burned. A lithium battery fire is very hot (several thousand degrees) and is an excellent initiator for ancillary (resulting) fires. Fire occurs due to contact between Lithium and oxygen in the air. It does not need any other source of ignition, or fuel to start, and burns almost explosively.
These batteries must be used in a manner that precludes ancillary fire. The following is recommended:
Store, and charge, in a fireproof container; never in your model. Charge in a protected area devoid of combustibles. Always stand watch over the charging process. Never leave the charging process unattended.
In the event of damage from crashes, etc, carefully remove to a safe place for at least a half hour to observe. Physically damaged cells could erupt into flame, and, after sufficient time to ensure safety, should be discarded in accordance with the instructions which came with the batteries. Never attempt to charge a cell with physical damage, regardless of how slight.
Always use chargers designed for the specific purpose, preferably having a fixed setting for your particular pack. Many fires occur in using selectable/adjustable chargers improperly set. Never attempt to charge Lithium cells with a charger which is not, specifically, designed for charging Lithium cells. Never use chargers designed for Nickel Cadmium batteries.
Use charging systems that monitor and control the charge state of each cell in the pack. Unbalanced cells can lead to disaster if it permits overcharge of a single cell in the pack. If the batteries show any sign of swelling, discontinue charging, and remove them to a safe place outside as they could erupt into flames.
Most important: NEVER PLUG IN A BATTERY AND LEAVE IT TO CHARGE UNATTENDED OVERNIGHT. Serious fires have resulted from this practice.
Do not attempt to make your own battery packs from individual cells.
These batteries CANNOT be handled and charged casually such as has been the practice for years with other types of batteries. The consequence of this practice can be very serious resulting in major property damage and/or personal harm.
Sincerely,
Safety
Committee
Academy of Model Aeronautics
5161 E Memorial Drive
Muncie, IN 47302
For more go to: www.modelaircraft.org
Battery Basics
NFO by Bob Aberle
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