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The Handbook of FMA, Inc. Section 3 – Safety, July 2005
This document is a work in progress. Latest revision:
Distributor for Kokam Lithium Polymer Cells 5716A
Charging through a protective circuit Charging in series or in parallel How to use Li Po batteries effectively Gas Gauging using the discharge curve data “So, what do I need for my bird, since it isn’t a Tu-4?” LiPo power suggestions for various electric aircraft types and sizes
FMA, Inc. battery packs are designed to protect against in-the-field risk of unwanted chemical reactions and other electrical and mechanical malfunctions for rechargeable sources that power mission-critical portable and handheld equipment as well as remote control electric aircraft, cars, boats, and robots. The FMA Li Po systems comply with all safety requirements and test guidelines of the IEEE-1625-2004 standard. EP Kokam / FMA provides statistical analysis and sample testing of cells for each production lot to confirm that cells perform to manufacturers' specifications. Tests verify that Li Po packs operate properly for their respective 0 to 45 degree C charge range and 20 to 60 degree C discharge range.
The safety record for Lithium cells in radio control applications is excellent, particularly considering that: n Both manufacturers and modelers were learning how to use and handle the cells. n Many modelers use the cells with no protective circuit during charge or discharge. n Many chargers and charging methods are used.
There have been reports of cells damaged by overcharge. We call them silver sausages because of their shape. In a very few instances, there has been venting with flames. In almost all cases, the cause has been known or was deduced from analysis of the situation. Nearly all these problems were related to charging. One non-charging incident involving a Lithium pack of unknown manufacture occurred because the modeler crashed an airplane and placed it in his car without removing the pack.
Potential cell damage causes include: n Overvoltage during charge. n Use of chargers not designed for Li Po chemistry. n Unbalance of a cell(s) in a pack. Unbalance will be defined later. n Sudden peak surge voltage from the charger when disconnecting. n Charge current set too high for the cell. n Incorrect selection of charge voltage. n Excessive discharge rate. n Use of aluminum soldering paste that deteriorates the tabs and causes a short circuit. n Unreliable chargers. In some chargers with FET switching, the FET shorts when it fails and full supply voltage is applied to the cell. This can be avoided if appropriate crowbar protection[1] or foldback[2] is designed into the charger. n Cell failure that creates an instant unbalance in the pack. n Fundamental risk of lithium ignition with lithium is exposed to air entering through a damaged cell envelope. n Faulty pack assembly. n Physical damage and abuse.
All high energy density cells, including Li Po cells, are safe when handled, interconnected, charged and discharged according to manufacturers’ recommendations, accepted industry practices and common sense. FMA provides a set of precautions in the package for use of the cells and packs it sells. The precautions are listed below, and are also provided on the FMA Direct Web site.
Useful definitions
1. Lithium metal oxide is the base material (cathode) 2. Carbon is the anode in Kokam cells 3. Both are deposited on high quality copper 4. Li-ion is the process 5. Lithium polymer is a subset of li-ion 6. The difference is in the form and configuration 7. An explosion produces a high pressure wave 8. A conflagration produces large amount of flames 9. An open gasoline fire = conflagration 10. A sealed can of gasoline ignited = explosion 11. Li-ion is sealed in a metal can 12. Li-Po is sealed in a plastic envelope 13. Li-ions are vented 14. Li-Po needs no vent 15. Over charge = pressure that must relieve Some of these definitions are self-explanatory. Salient observations are: •Yes, your Li Po cell has good value when dead because there is a nice strip of copper in each plate! •A bit later, we will show you the relative power of Li Ion and Li Po that are deliberately destroyed. •Let’s make it very clear: Li Po cells do not self ignite or vent unless you are doing something to them. •They do not self ignite just from lying on a shelf. So you can discount that if you hear someone say that. •The difference in form between Li Po and most Li Ion is that the Li Ion is in a metal enclosure. •Some Li Ions are cylindrical but there is also a prismatic configuration that is flat quite like a Li Po.
When a Li Po cell is over charged
All high energy density cells used for RC, including NiCd, NiMH and lead acid, as well as Li Po, pose an electrical hazard. If wiring or interconnects are poor or become shorted, these cells are capable of delivering such high currents that the wiring can burn like a filament. Should the pack or wiring be in contact with flammable material, a fire will result.
Under conditions of abuse or error, Lithium cells can vent with flames. During charge, applying a charge voltage of more than six volts per cell for a period of 20 minutes can potentially cause venting and might cause flames depending on current setting.
Li Po is much more tolerant of abuse than has been claimed in a model publication. This chart shows that. Note that the cell under test was charged at 6.75 V right from the start and charge current quickly rose to 18C or 18 times the allowable. The cell took this abuse for over ten minutes before it even started to bulge and withstood it for over 45 minutes before it finally vented. That silver sausage shape holds a lot of gas! Of course, over voltage during charge is the main cause of cell failure, Cells are also ruined by short circuit or heavy overload. When we see those in our service area, they show up as “silver prunes” rather than silver sausages. A puncture of the envelope with penetration of the plates of a fully charged cell means instant ignition and vaporization of the offending object by the extreme high discharge. Finally, running cells so hard that the combination of discharge and ambient temp puts them over 85 degrees C will ruin them for sure.
What Can Burn in a Battery?
Yet, with all the above, you must be aware that Li batteries can catch fire under extenuating circumstances. Metals will burn if subjected to intense heat that ignites them. Li is a metal. You have only to see and hear the roar of the solid rocket boosters in a shuttle launch to see what a half million or so pounds of finely powdered aluminum does when you ignite it with a 250 pound pack of igniter material that is like a bomb going off. It is the finely powdered Li in the cell that creates the real heat. Yes, the electrolyte that is an organic material burns about like paper but does not contribute much. The Kokam HDR cells can sustain up to 18.5 V over charge without igniting (Vs 6.5V) . This feature, in conjunction with the FMA cell balancing chargers, means that Li Po batteries are now very much safer. All that is tucked in the new cells with a miniscule increase in weight. That feature is well worth the tiny increase in weight because the cells are still about 1/5 the weight of an equivalent Ni Cd. As seen earlier, the performance of Kokam HDR cells is superior to all others.
Overcharging can be hazardous My colleagues preceded this test first by putting a Li Ion cell down a ground hog hole. Punxatawny Phil did not come out on ground hog day! In order to shorten the time to vent, we put it on charge via the light at 5 amps for about one hour. Then it was hooked straight across the 12V battery. Note the blast of flame that is released when the cell explodes as the pressure blows out the end cap of this 700 mAh Li Ion cell.
Now, contrast the above with the relatively mild behavior of a KOK 2100 HDR cell under the same circumstances. Once the cell began to vent in the third frame, the entire sequence shown is about one minute. Note that the cell does not explode and does not emit flames. It does vent, it does emit a blast of hot gas, and it does create smoke that would set off a fire alarm. With these HDR cells, there is no conflagration.
Now that the behavior of Li cells has been illustrated and we know the importance of proper handling, let us review proper charging. Li Po cells are charged differently than NiCd/NiMH and all other chemistries except for Li Ion. The Li Po charge schedule is different and you must use a charger designed for either Li Ion or Lithium Polymer cells.
The charge schedule is easily controlled. The proper charger limits current to 1C, where C=cell capacity, e.g. 145 mAh. As cell voltage increases, so must charge voltage increase to force current through the cell until the voltage applied to the cell reaches a maximum of 4.235 V. As cell voltage rises to 4.235V, current approaches zero. When charge current falls to 0.1 C, the cell is full.
The preferred charge rate is 1C such that the cell can be charged to 90% capacity in one hour if the charger is designed to hold charge current at 1C without exceeding 4.235V/cell maximum charge voltage. Lower charge rates are acceptable if longer charge time is tolerable. Li Po cells cannot be charged at high rates such as 4C. The charge algorithm below shows that almost nothing is gained by charging at a rate higher than 2C.
The above chart is very important, so study it carefully. The following charging guidelines emanate from the chart:
n Generally, charging at a 1C charge rate will yield the maximum cycle life n If the FMA Direct Cell Balancing Chargers are used, follow the instructions for “fast Charge” in as little as 20 minutes, but and accept that cycle life will be shortened somewhat by fast charging. n Never exceed a maximum charge voltage of 4.235 VDC.
Charging must be done such that no damage to life or property can occur if (a) a short in wiring or cells, or (b) venting with flames, occurs. Lithium ignites in the presence of oxygen. If the cell envelope ruptures, oxygen enters and combines with the lithium, causing ignition. If the cell envelope does not rupture, then ignition will not occur.
The safety precautions listed earlier include the warning not to charge batteries unattended. The cell under test held out for almost an hour. Leaving any battery on fast charge for an hour without checking it is irresponsible. As stated in the precautions, the cell must be in a safe charging station. If a cell is found to be swelling during charge, remove the charge current immediately. Then allow the cell to cool before taking any other action. You can imagine that rupture of the cell could allow hot gasses and electrolyte to spew out.
Once the cell has cooled, handle it as a fully charged cell with full energy available. This means you do not “poke a hole in it” in preparation for disposal. First, discharge the cell at a reasonable rate. This can be done by using clip leads to attach it to an electric motor, a resistor or some other electrical load. Do not hurry this—a slow, complete discharge to zero volts while still under load is a safe way to do the job. Once the cell is depleted, carefully slice a small slit in the envelope, then immerse the cell in salt water for a few hours. After that, the cell may be disposed of in the trash.
Additional precautions are in order. If shorted, all high energy density cells (including Li Po cells) can heat rapidly, rupturing the envelope or case. A wiring harness connected to a cell or battery, if shorted, can glow like a filament and cause ignition of flammable materials.
Appropriate chargers are available from FMA Direct. Charger specifications may be viewed at the Kokam/FMA web site. .
Use caution with chargers that automatically determine cell count. Keep a close eye on charging until absolutely certain that the charge has selected the proper cell count. Li Po cells are nominally 3.7V under load. Cell voltage at full charge is 4.235V. Two fully charged cells, for example, output 8.4V. Three partially discharged cells may be 2.8V/cell x 3 = 8.4V. If two nearly charged cells are put on charge to “top them off”, some auto-counting charger may incorrectly sense them as three discharged cells and set the charge voltage for three cells. The two cells will receive too much voltage and will definitely be damaged. It is the user’s responsibility to assure a charger’s voltage is properly set.
Charging through a protective circuit
On October 1, 2003, Kokam Engineering Co., Ltd. introduced Safety Guard, a protective circuit that regulates the voltage to a pack under charge to a preset and fixed value no matter what voltage the charger supplies. Its basic function is that of a Protective Circuit Module (PCM) normally used on an OEM pack. However, in RC use, it goes in line for charging a pack and need not be carried with the pack. This is a single-purpose unit that is set to a 2s, 3s or 4s pack voltage by fixed internal circuitry. A 2s pack must be charged via a 2s Safety Guard, 3s pack via a 3s Safety Guard, etc.
Safety Guard specifications state that the unit can protect a pack even with a 30V input. It would still be possible to plug a 2s pack into a 3s Safety Guard. However, that is far more unlikely than remembering to set cell voltage or count by any method and more reliable than an automatic cell count.
In effect, Safety Guard is intended to avoid application of an incorrect charge voltage to a pack. That is its primary use. A secondary use is as a PCM during discharge for systems
needing less than 20 amps current. This means you could use it in line for motors demanding up to 20 amps. It will cut out if current exceeds 20 amps, a good protection from locked rotors in an aborted takeoff if you tip the plane on its nose and forget to throttle back. Theoretically, it could also save an ESC in that circumstance.
Safety Guard can also protect against undervoltage if your ESC does not have cut - off matched to Li Po characteristics. Most ESCs have built-in voltage cut - off, but voltages are set for NiCd and NiMH packs. Note that Safety Guard will produce a slight voltage drop from the FET in line with the current to the ESC. In addition, when battery voltage is cut-off, the BEC is also cut - off, so you would need a separate battery for the receiver and servos.
A Typical PCM
Some suppliers are marketing packs with a PCM built - in. You can recognize them because they have a second charge pigtail. This is an OEM pack we built, so the charge and discharge use the same connector. Such PCMs are used on all OEM packs. PCMs do not balance the cells in a pack. The PCM merely prevents any cell from exceeding 4.2 V by cutting off charge or discharge when one cell in the pack exceeds limits. Thus, the capacity of the pack is determined by the weakest cell in the pack.
How FMA Direct true cell balancing works
Please review the section presented earlier about the FMA SKYVOLT cell balancing system to understand its application. When you recognize that you could do cell balancing manually, you also realize that the key is a super good multi-pin connector with a sense lead to each series node in the pack. The connector used is an FMA proprietary item that is based on a MIL SPEC connector used on packs for the US Navy. The large, gold plated floating pins can carry 65 amps continuous and burst perhaps twice that.
The heart of the unit is the MCU and the software that lets the charger monitor each cell individually plus protect against any failure imaginable even to checking the resistance of each pin on the connector on each frame. It has taken a full year to perfect this charger. We are very proud of it. The SKYVOLT 6S can charge from one to six cells at up to ten amps. Operation is as simple as it can get. Just set the desired charge current at up to 3S and plug the pack in. The charger provides a robust set of protection and diagnostic information and informs you loudly when the pack is ready.
FMA Cell balance charging very simply measures the voltage of each individual cell in the pack as it charges and applies a sophisticated charge control algorithm that: · Does what you could do manually using a DVM with access to each cell · Does it automatically · A microprocessor is the heart · Balances cells to within .010 volts of each other · Allows charge in 20 minutes · Parallel sets are treated as one cell · If used with companion Cell Pro discharge control, no cell should ever be unbalanced again · Extends cell life · Checks every pin each frame to be sure there are no anomalies · Verifies that the cell is behaving properly; e.g. if a cell fails to rise above 2.7V after a preliminary low rate charge, a warning is issued. There are some 14 such checks. Won’t allow operation with a bad cell. · Will not allow any cell to exceed 4,2V · Allows an optional fast charge in approximately 20 minutes
Charging in series or in parallel
Li Po packs may be charged in series or in parallel. Each approach has pros and cons. The Kokam USA Connector Modules permit either method. The FMA cell balancing systems make it simple and easy.. Balance charging results in the longest life and best balance.
When Li Po cells are connected in parallel packs, it is the same as having a cell with greater capacity. There is one important warning: the interconnections and solder joints must have absolute integrity or a cell can be over or undercharged in a parallel string. The factory-made inner strap welds have excellent integrity. Inter-cell connections made, for example, using aluminum paste solder may not have.
A parallel string can be charged with one 4.2V charger, and every cell in the string will be perfectly balanced by being forced to that exact 4.2V. Why not use parallel charging for all packs? Some modelers do. It requires effort to separate a pack into strictly parallel sets for charging, although the Kokam USA Connector Modules do make that easy. With the Modules, a parallel pack could be made up of 1s (single) cells with each cell on an individual connector. On the other hand, if you need to charge 20 KOK 1500 cells in parallel, you’ll need a 4.2V charger that puts out a whopping 30 amps to charge the pack in one hour.
Series charging permits packs be charged without disconnecting and reconnecting. If, say, a 4s1p pack is charged in series, the charge current for the KOK 1500 would be 1.5 amps, not 30 amps. The shortcoming of series charging is that it does not force the cells to stay balanced. Many argue that their Li Po cells do not drift into unbalance at the rate that NiMH or NiCd cells do. This will be true as long as you observe the guidelines in the next section.
If you charge cells or sub-packs in parallel, the Parallel Connector Module connects to the end of the charger leads. If series charging is done, then the Series Module connects to the end of the charger leads.
Packs may be charged in series-parallel by use of the appropriate set of Connector Modules. Suppose we want to charge a 4s3p pack used on the Tu-4. We could simply connect the pack’s power connector to the charger’s connector, then set the charger for 14.8V (for the 4s configuration). It takes the LI PO 402 charger about 3˝ hours to charge the pack (the maximum 1.5 amp current is divided between three parallel packs, with each getting 500 ma.). If series-parallel charging is done, it is wise to check the individual cells periodically with a digital voltmeter make sure that the cells stay in balance.
When in doubt!!
No matter what you do, no matter how safely you operate, no matter how cautious you are, a can of gasoline may spill or be dropped or get hit by flying debris from a rotary lawn mower. So it is with Li Po safety. Put a baffle in a tool - box or ammo box and drill a few very small pressure relief holes. Better yet, if you are a craftsman, put a small muffler in the end that relieves pressure and snuffs any flame.
Li Po packs actually incur damage during discharge that leads to subsequent failure and potential for venting with flames on subsequent charges. Thus, it is vitally important to understand the effects of discharging at too high a rate; the effects of resultant temperature rise, running cells to too low a voltage, and the mechanisms set off by cell unbalance.
What is cell unbalance?
Cell unbalance ruins packs and leads to venting. This is the devil we know! For some reason, users seem to think that Li Pos are more affected than other chemistries. Not so! Li Pos actually are better at retaining balance. It’s just that the effect is much more dramatic when you see a silver sausage vs. a Ni MH that has vented and turns white on the end after a while as it loses capacity and electrolyte. We are not at all concerned about operation in the green box shown below. If all cells were operated there, cell unbalance would not be so much a problem. It is the red box to the right that is the “zone of temptation”. Some seem determined to operate there and then complain when they have problems.
Thermal runaway is what we are really trying to avoid. It is important to understand the phenomenon of thermal runaway that destroys Li Ion/Li Po cells. All the things we can do, i.e., cell balancing, proper cut - off, and better resistance to over voltage do help avoid thermal runaway. However, if anything ever happens to initiate thermal runaway, no external device, PCM, polyswitch, internal switch, or the like can stop it. The most dramatic causes is puncture that shorts the plates or a massive, internal short. Once the cell starts to break down and ignition starts, nothing can stop it except to starve the cell of oxygen. This is why the ultimate safety is to charge in a fire - safe box. If we keep the cells in a pack perfectly balanced throughout the life of the pack, assure cut-off in the face of massive overload, and use proper maintenance to avoid a dead cell in a pack, the possibility of thermal runaway is almost zero from normal operation. The potential for cell damage that could inflict puncture and plate shorting can be ameliorated by use of IMPAD to surround the pack.
Yes, you have to worry about Li Pos becoming unbalanced with age. It is guaranteed that Ni Cds and Ni MH become unbalanced within as short as ten cycles even when new if discharged at high rates. Active cell balancing can prevent all of this for either chemistry and nothing else will. Cell balance charge is, in effect, charging each cell in a pack individually to the precise end voltage or capacity at which it is rated. The difference is that Li Pos are arbitrarily 100% full when they reach 4.2V while Ni Cd and Ni MH are full when they reach a nominal 1.6V/cell but measurable only by detecting the peak during overcharge at which voltage/time slope goes negative.
It is best and safest if all charging is automatic and does not demand of the user that he remember to do anything except plug the pack in for charge and take it off when the charger says it is full. Only a cell balancing charger or very tedious and smart manual checks can provide cell balancing protection. A manual arrangement can prevent cell unbalance so long as you remember to use it and remember to check cell balance as charge progresses.! You can have a manual balancer from FMA as well through a simple, inexpensive test fixture that plugs into the balancing charge connector and uses a rotary switch to sequence each cell to a pair of pin jacks for check with a DVM.
Packs that have a PCM built in do not help balance a low cell in a pack. The Kokam Safety Guard falls into this category. Be careful when you buy a pack to find out if the supplier is using such a PCM. You can usually tell by the fact that a second connector and cable are provided for charging. Safety Guard checks pack voltage end to end and definitely prevents pack fires but does not ensure that you never get a swelled cell. PCMs only cut off charge when the first cell in the pack reaches 4.2V. That may leave other cells in the pack at lower end voltage and low on capacity.
The most frequently - asked question for all cell chemistries is: “What capacity and voltage should my pack be to fly my particular model for XX minutes?” At FMA Direct, this question gets answered as best we can many times every day.
FMA Direct provides an online tool, LiPo Calc, for estimating the LiPo pack needed when various parameters are known.
LiPo Calc and its help - page are reproduced below. LiPo Calc is driven by the wattage required to fly the model and the watt-hours needed to fly for an expected period. Be aware that LiPo Calc does not take into account the fact that cells lose capacity, therefore run time, and with increasing current draw.
ElectriCalc and MotoCalc programs are also helpful in calculating theoretical model performance.
Use FMA’s LiPo Calc II to preview LiPo pack configurations based on the operating and flight parameters for the motor and ESC selected. LiPo Calc II is driven by the wattage required to fly the model and the watt-hours needed to fly for an expected period. Be aware that LiPo Calc II does not take into account the fact that cells lose capacity, therefore run time, with increasing current draw.
To use LiPo Calc II, fill in the fields at the top, then click the Update Values button. LiPo Calc II fills in the table, providing a lot of data to help you choose an appropriate pack configuration. Click a column heading for an explanation of the data in that column. Complete instructions are in the LiPo Calc help file (“Click here for help”).
Voltage and current required by the propulsion system
Configuration
(as a multiple of cell capacity)
As a general rule, Li Po cells should not be forced to deliver continuous current above the multiple of C where the curve depresses. However, please recognize that run time is going to be very short at a 20C discharge rate. As a “rule of thumb,” heating and deterioration of cell capacity will result when cell capacity is depressed below about 70%. Kokam USA specifies cell performance as times C for continuous operation with bursts to a higher multiple, say 10C.
Earlier, the curve above was used to illustrate how cells run out in discharge and the voltage diverges. That divergence can lead to cell damage if one cell goes below 2.5V and is driven there for more than a moment. As cells age, the divergence may become more pronounced, particularly near the end of cycle life. Grading and selecting cells adds cost to a pack, and car racers are willing to pay extra to get that competitive edge. They drive packs right down to the last mA and the traces above result. NiCd and NiMH cells are more tolerant of being driven down almost to zero.
The minimum voltage for Li Po is conservatively set for protection of the cells. There is really very little to be gained by driving the cells to the bitter end. The above curves are exaggerated to show that there is a divergence. In fact, the difference between the break -point on the curve and the cut-off is only about 1% of total capacity.
How to use Li Po batteries effectively
Operating Li Po batteries. Motors, and ESCs is not unlike operating a model engine. The cost of the propulsion system is directly proportional to the amount of power required for your model. Big airplanes require big engines and large electric power systems. The bigger they are, the more they cost. Electric power, once you know how to use it, is much less costly to operate over the life cycle than an engine. Engines are fussy, can be hard to start, and require care and knowledge every time you start one. When you finish for the day, the model has to have all the oil cleaned off it and the engine requires careful maintenance if long life is desired. Engines that are not carefully prepared for storage can be ruined completely over one winter of storage. One bad setting of the needle valve can ruin the engine. The engine overheats due to lack of proper lubrication and is ruined. The uninitiated may put too large a propeller on the engine to over load it and, again, cause it to overheat, seize and be ruined.
Think of the curve above as you would a power curve for an engine. In full - scale aviation, engine gauges have zones for proper operation marked on them. In particular, rpm and temperature have arcs of yellow where operation for specified short terms such as take - off are marked in yellow. Normal cruise settings are marked with a narrow white arc. The absolute maximum that must never be exceeded has a red line. Thus, most are familiar with the popular term about something being “red lined”.
Confusion has come about because too many do not understand or misinterpret the rating criteria for Li Po cells. Here are the facts:
1. Cell capacity and discharge rates are specified at 0.5 times the rated capacity for the cell (0.5C) as the discharge rate. That is, if a cell has a nominal capacity of 1000 mAh, that capacity is measured at a discharge rate of 0.2X1000 = 200 milliamperes. This is by international standard. The specimen data for the cell above is for a nominal 2000 mAh cell. The scheme is not too useful for high current drain unless the curves are available for the expected operating range.
2. Li Po cells deliver a nominal 3.7V when discharged at 0.2C. Note that this is a nominal level, The average voltage for the particular cell above is almost 3.8V. This will vary somewhat from cell to cell but is the statistical average of many, many cells.
3. The discharge curve shows lower voltage and lower capacity as discharge rate is increased. The industry has drifted into the habit of specifying discharge rate as a multiple of the nominal C for a cell even though the international standard warns against that. This is because C is in mAh or Ah, not amperes. This misclassification has created confusion.
4. That confusion created the “discharge rate race” in which manufacturers have begun to claim ever-higher “Times C” rates. The maximum discharge rate is NOT the rate at which cell capacity is specified. Rather, it is the maximum, or “REDLINE” discharge that must not be exceeded and above which cell damage will almost certainly occur.
5. By international standard, all cells are rated for capacity at 0.2C, where C = nominal capacity of the cell when discharged at a rate equal to 0.2 times the predicted capacity for the cell. The rated cell capacity is actually statistically established since the actual is not known until tested. The formal discharge curves for the KOK 2000/15C above is marked to show the cautionary “red zone”. In general no cell should be operated for longer than a duty cycle of 6 sec out of 60 in the red zone. This zone is also referred to as “burst capability”. 6. We accept some responsibility for confusion caused by ignoring the international standard warning that multiples of C should not be used to designate discharge rate. Further confusion was introduced when we introduced the term X C to designate the MAXIMUM discharge rate for increasing discharge rate cells (HDR). In fact, Kokam rated the KOK 2100/20C cell at 20 C as the first time that was done. In retrospect, that was a mistake that has added to confusion. The proper way as defined by international standard is to rate the cells at 0.2C, then provide discharge curves such as that above that let you see what will be delivered as discharge current increases. 7. The discharge curves for all Kokam RC cells are/will be available in the Li Po Compendium that is on the FMA Direct web site. The proper way to use the cells is to follow the discipline indicated just as if you were flying full - scale. If you ever exceed the red line on an airplane engine, FAA requires the engine to be torn down and rebuilt with magnafluxing of the parts. When you shell out $25K for that even for a four cylinder engine, you think twice! 8. How to check? A Whatmeter or an ammeter is required to know what the current drain is. The worst-case condition is “static”: i.e. on the ground with the airplane stationary and the prop turning. It is reassuring to know that current is always lower in flight due to the phenomena called “unloading”. The prop has maximum inefficiency when held static and reaches maximum efficiency at the airspeed for which it is designed. The maximum static current must stay within the guidelines shown in the typical curve. 9. When using Li Po packs, we recommend that you: n Set the cut-off above 2.5V, or nearer 3V in most situations. n Avoid designing your pack so that it has to operate at the maximum allowable discharge rate. n Stay out of the “zone of temptation.”
For aircraft, the “zone of temptation” is usually entered by repeatedly restarting after the first cut-off occurs. As with many rules, there is an exception. Some Kokam HC cells, when operated right up against the maximum allowable discharge are a little above the 3V line. A sudden application of throttle in that situation could cause a motor cut-off. Thus, the cut-off for the KOK 700, 1500 and 2AH should be set for 2.8V. In general, experience is showing that most airplanes have stopped flying before cut-off occurs, so this may be a moot point.
Look for the
discharge curves for all FMA Li Po cells here in the Li Po Compendium under
Data Sheets. Insist on having that data for any Li Po batteries you buy or be
at risk. Every FMA Li Po pack has the discharge curve in the package. Not
having the discharge data is like flying full scale with no gauges! Gas Gauging using the discharge curve data
Please refer to the discharge curves above for the discussion of gas gauging.
1. Most OEM applications of Li Pos such as cell phones use a “gas gauge” to inform the user of the status of the battery. 2. RC applications are so price competitive that the added cost of individual pack gauging is not practical. 3. However, if the user has a digital voltmeter (DVM) a good emulation can be had. 4. The receiver and servos and ESC current without the motor running approximate the 0.2C load at which the cell is rated as shown in the 0.2C curve above. The discharge curve for the pack is nominally 3.7V/cell X # of cells at 0.2C. Recognize that this is a nominal value and will vary with the specific pack, how close the current is to 0.2C and how old the pack is. 5. The Kokam cells are shipped at half - charge and hundreds checked prior to in-house pack assembly measured exactly 3.81 at 50% capacity before charging. It is well to check this voltage before charging in order to ”calibrate” your set - up. 6. For the example curve: if your DVM says 3.5V under the light load, the cell has been depleted to 3.68V = 1625 mAh. That is about 80% of capacity and time to stop flying if you wish to maximize cycle life or minimize risk of a cut – off and a short flight.. 7. Keep track of pack voltage after each flight, track it against the discharge curve, then calibrate when the pack is recharged. Also, keep a record of flight time. Record the capacity it takes to charge the pack. Divide the capacity needed for charge by flight time in minutes to get the average capacity consumed per minute. Now you have it gas - gauged and can use the DVM to see what is left after each flight or use the average consumption per minute to estimate capacity used. 8. Always remember that almost all crashes occur on “that one last flight”! Leave a margin.
“So, what do I need for my bird, since it isn’t a Tu-4?”
Li PoCalc will do a very good job of sizing a pack if you know the input parameters. However, as stated earlier, that question is the one we hear most often. The tables below are provided to get you into the ball - park. It is virtually impossible to cover every combination of propulsion package in use or coming on the scene. However, we will attempt to provide as much information as can be assembled right now.
First, standard inventory packs can be assembled in series or parallel using the Connector Modules as shown in the next diagram. Need higher voltage? Plug packs into a Series Connector Module. Example: plug a 2s and a 3s pack into a Series Connector Module to get 18.5 Volts output (all cells must have the same capacity).
*Peak burst current is a cycle consisting of less than 12 seconds of peak current followed by 50 seconds at 50-60% of peak current.
Need higher current or longer flights? Plug packs into a Parallel Connector Module. Example: plug four 1s packs into a Parallel Connector Module to get 4 times the output of a single cell (all cells must have the same capacity).
The table above is for the peak current expected. The table below is for continuous operation. Remember that pack size is set by the peak current while duration is set by the average duty cycle.
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