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Contents:


  • Introduction
  • Wing Geometry
  • Forces in Flight
  • Stability Concepts
  • Airfoil Simulator
  • Stall and Spin
  • Beginners' Guide
  • Trainer Design

  • Main Page



    • Beginners' Guide

      | First Model | Radio Control | Servos | Glow Engines | Pulsejet | Turbines | Electrics |



    • Batteries

      Batteries are available in different sizes, weights, voltages and capacities C, which
      refer to their stored energy expressed either in amps-hour Ah or milliamps-hour mAh.
      For example, a battery with a capacity of 500mAh should deliver 500mA during
      one hour before it gets totaly discharged (flat).

      Radio control sytems are usually
      powered by rechargeable batteries.
      Rechargeable battery types available
      on the market today are:
      Nickel-Cadmium (NiCads)
      Nickel-Metal hydride (NiMH) and
      Lithium-Polymer (Lipo) batteries.
      Even Lead-Acid batteries are also
      used as ground power source.

      Normally the NiCads stand more "abuse" which means that they may be charged
      at higher rate (normally 2 - 4C) and have the ability to deliver higher current, i.e.
      discharge rates up to 2C continuous or 8 to 10C during 4 - 5 minutes and even up
      to 100C during very short time.
      They have some designations such as the Sanyo AE for high capacity and AR or
      SCR for quick charge/discharge.

      A NiCad cell consists basically in a positive plate foil of nickel metal with nickel
      oxide/hydroxide, a negative plate foil of cadmium metal with cadmium hydroxide
      and an isolating porous separator film moistened with an electrolyte of potassium
      hydroxide (caustic potash).
      The two plates are sandwiched between the isolating porous separator films,
      rolled up and enclosed in a nickel-plated steel can.
      A spring-loaded vent is fitted at the positive terminal end in order to release the
      electrolyte and/or gasses, in case overpressure occurs due to overcharge.
      See picture below.



      The NiMH have higher capacity/weight compared with the NiCads but are more
      sensitive to high charge rates (max recommended 1C) and normally it is not
      recommended to discharge the NiMH batteries at higher rates than 3 - 5C.

      The NiMH self-discharge rate is also about 50% higher than the NiCads.
      However, the NiMH are more environment-friendly.

         A new type of NiMH battery known as HeCell has recently been 
         developed, which is claimed to allow higher discharge rates than 
         the conventional ones (about 12 - 16C). 
      

      Both battery types lose their stored charge due to internal chemical action, even
      when not in use.
      Normally the NiCads lose around 10% of its charge in the first 24 hours after
      been charged and keep losing it by 10% per month.
      The rate of self-discharge doubles for a rise in temperature of 10 degrees C.
      Some NiCads can discharge themselves completely in a period of six months.

      The best way to keep batteries which are not in use for a long time, is by having
      them stored in the refrigerator (not in the freezer).
      Just allow the battery to reach the ambient temperature before using/recharging.

      Some manufacturers claim that these battery types are able to stand at least
      1000 charges/discharges during their lifetime, assuming they have been subject
      to the ideal charging and handling methods.
      In practice however, we may expect about 600 - 800 charges/discharges.

      A safe method to charge both the NiCads and the NiMHs is by using a constant
      charge current (CC) at 1/10 of their capacity (0.1C) during 14 hours.
      For other charge current values one may use the following formula:
      Charge Time (Hours) = 1.4 x Battery Capacity / Charge Current (assuming that
      a constant charge current is used).

      However, low cost CC chargers provide no way of detecting when the battery is
      fully charged.
      The user is then expected to estimate the charging time based on the constant
      charging current value and the battery capacity, according to the formula above.
      And providing the NiCads' are discharged to about 1.1V p/cell each time before
      recharging, this charging method can be used to achieve a reasonably long
      battery life. Since repeatedly recharging an already fully charged NiCad or one
      with a large part of its charge remaining will degrade its performance.

      Some chargers provide the option to discharge the batteries down to about
      1.1V per cell before starting the charging process.
      There are also fast battery chargers on the market charging from 1C up to 4C.
      But due to the high charging current level, it is required a reliable method of
      stopping the charge once the battery is fully charged, otherwise overheating and
      battery damage may occur.

      Since the NiMHs' and NiCads' voltage actually starts dropping after they have
      reached the fully charged state, the fast chargers use the so-called Delta Peak
      detecting method.
      There are "negative delta V (-DV)" and "zero delta V (0D)" detectors.
      Also "change of temperature (dT/dt)" detectors are commonly used.
      Some manufacturers use negative or zero delta V together with change of temp.
      detection, in case of one method fails to detect.
      Since NiMHs' voltage drop (delta V) after the fully charged state is lower than
      the NiCads, a more sensitive delta V charger is required for the NiMH batteries.
      Some chargers allow the user to set the value of the delta peak detection, which
      may be between 10 - 20mV per cell for NiCads and 5 - 10mV for NiMHs.
      A too low value may cause false peak detection due to electric noise, preventing
      the batteries from getting fully charged, whereas a too large value may result in
      overcharge, which reduces the batteries' life.

      Some fast chargers offer the possibility to automatically change over to slow
      charge (trickle-charge, for ex. at 0.05C) when the fully charge status is detected.

      The graph on the right
      shows the voltage and
      temperature variation
      of a four cell NiCad
      during charging at 1C
      constant charge current.

      Notice how the voltage
      drops after it has reached
      a top value, whereas the
      temperature keeps rising.

      The battery is considered
      fully charged when the
      temp. rises about 10C
      above the ambient temp.
      (e.g. 24 + 10 = 34C )
      The NiMH batteries tend to dissipate heat during all the charging process, while
      the NiCads get warm only when they reach the full charge point.
      The nominal voltage is 1.2V per cell for both battery types and a charged cell
      may have about 1.45 - 1.50V.

      It's not possible to know exactly the NiCad's or NiMH's cell charge status by only
      measuring it's terminal voltage, as the cell's charge status is not a linear function
      of the cell's voltage.
      A reliable method to know how much charge is left or whether a cell still has its
      nominal capacity, is by discharging it with a known constant current and measure
      the time until the cell voltage reaches about 1.1V.
      For example, it should take about two hours to discharge a fully charged 500mAh
      cell by using a constant discharging current of 250mAh.

      Battery researchers have in the recent years come to conclusion that NiCads
      respond better to a pulsed charging waveform than to a steady DC current.
      By applying the charge current in one-second pulses with brief "rest" periods
      between them, ions are able to diffuse over the plate area and the cells are
      better able to absorb the charge.

      This is particularly true at the higher charge rates used by fast chargers.
      These chargers have a microprocessor that samples the "rest" periods between
      the charging pulses to read the battery terminal voltage.
      Another interesting discovery is that the charging process actually improves even
      further if during the "rest period" between charging pulses, the cells are subject
      to very brief discharging pulses with an amplitude of about 2.5 times the charging
      current, but lasting only about 5mS.


      It is claimed that these short discharge pulses actually dislodge oxygen bubbles
      from the plates and help them diffuse during the "rest period". The use of these
      brief discharge pulses is known as "burp charging".
      Tests done by both US military and NASA have shown that NiCads charged by
      using fast chargers employing the burped pulse system tend to last up to Twice
      as long as those charged by traditional CC chargers.
      Many of the high-end fast pulse chargers for NiCads use a charging method
      according to those findings.

      A battery pack consists of several cells connected in series, which inevitably age
      at different rates and gradually develop individual different charge status, and
      since the battery pack as a whole is charged and discharged repeatedly, these
      differences may become accentuated.
      The result is that some weaker cells can eventually be discharged well below
      1.0 V and even driven into reverse polarity before the others reach the fully
      discharged state.
      During the recharging process, the weaker cells will be improperly recharged
      and tend to suffer increased crystal growth, while the others will absorb most
      of the charge and overheat, which dramatically degrades the whole battery pack
      performance.
      It's therefore advisable checking if the battery cells get different temperatures
      during the charging process, specially when high charge current rates are used.

      It's claimed that individual cell differences may level out by slow charging the
      battery pack from time to time at 0.1C during 14h or so.

      Triton Computerized Charger, Discharger
        TRITON Charger, Discharger
        Handles 1-24 NiCd or NiMH cells, 1-4 Li-Ion cells
        or 6,12, and 24V Lead Acid batteries.
        Se product review here


      For those who like to tinker with electronics and can't afford an expensive and
      sophisticated charger, there's a cheap alternative based on the National
      Semiconductorâ LM317 low cost regulator.
      The circuit diagram below shows a constant current charger using the LM317.



      The constant current may be set anywhere between 10mA and 1.5A by choosing
      the appropriate resistor R.
      R = 1.25 / I
      Where R is the resistor value in ohms, 1.25 is a reference drop voltage in Volts
      and I is the constant current in Amps.

      For example, to charge a 500mAH battery at 0.1C, (50mA) the R value will be:
      1.25 / 0.05 = 25ohm.
      The dissipated power on the resistor R in this example is:
      P = V x I = 1.25 x 0.05 = 0.0625W or 62.5mW.

      The dissipated power on the LM317 IC is:
      (Vin - Vout) x Charging Current.
      It's advisable to use a heatsink to prevent the IC from getting too hot.
      Notice that the IC's metal package or tab also carries the Vout, so it's necessary
      to use isolating washers in case you attach the heatsink to a metal case.

      NiCads and NiMHs may be on charge during relatively long time without the risk
      of overcharging damage when using a constant current equal or less than 0.1C.
      However, it is not advisable to have the batteries continuously on charge longer
      than 24h, so one may connect the charger to a timer in order to cut the charging
      after about 14 -18h.

      For those who prefer a more sophisticated D.I.Y. NiCad charger based on
      delta peak method, as well as other interesting circuits, check here

      New rechargeable battery types, such as the Li-Ion (liquid electrolyte), the
      Lithium-Ion-Polymer (gel flat electrolyte) and especially the Lithium-Polymer
      (solid polymer electrolyte) are now often used with slow-flyers, indoors and
      even in much bigger models.
      A Lithium-Polymer cell (Li-poly or Lipo) has 3.7V nominal voltage, 4.2V max
      and 3.0V minimum.
      Other types may have different nominal voltages.
      These battery types have much higher energy density than NiCads and NiMHs.

      1300mah/11.1V 20C Li-poly Battery Pack W/ Balancer

      The max charge rate recommended is 1C, while the discharge rate should not be
      higher than 3 - 4C continuous or 5 - 6C during short time for the earlier types.
      Nowadays however some manufacturers offer discharge rates up to above 20C.

      For the same capacity, the battery with higher recommended max discharge rate
      has lower internal resistance, which provides better ability to deliver power.
      The self-discharge rate is claimed to be very low, typically 5% per year.

      These batteries cannot be charged with the same chargers that are designed for
      only NiCads or NiMH.

      Smart Plus 2 Cell - 5 Cell Li-poly Balance Charger

      In order to correctly charge the Li-ion/Lithium-polymer batteries, it must be taken
      into account the number of cells in the actual battery pack, since both the max
      charging current and voltage have to be set according to the cells' specifications.

      Charging these batteries with a wrong charger may cause them to explode!
      Also a short circuited pack may easily catch fire.
      According to Kokam, the Lithium-polymer batteries should not be discharged
      below 2.5V per cell, otherwise a rapid deterioration will occur.

      The basic charging procedure is by limiting the current (from 0.2 C to max 1C
      depending on manufacturer) until the battery reaches 4.2 V/cell and keeping this
      voltage until the charge current has dropped to 10% of the capacity C.
      Since the batteries only have 40 to 70% of full capacity when 4.2V/cell is reached,
      it's necessary to continue charging them until the current drops as described above.
      A charge timer should be used to terminate the charge in case the top voltage
      and/or termination current never reach their values within a certain time, which
      depends on the initial charging current, (e.g. 2 hours at 1C or 10 hours at 0.2C).
      Trickle charging is not good for Lithium batteries, as the chemistry cannot accept
      an overcharge without causing damage to the cells.

      Panasonic's charge curve for their 830mAh cells is shown below:


      The circuit diagram below shows a simple Li-ion/Lithium-polymer charger based
      on National Semiconductor LM317 low cost regulator.



      Before connecting the cells to the charger the max charging voltage has to be set
      by adjusting P1 (2k potentiometer).
      The max charging voltage must not exceed 4.2V per cell (Kokam), e.g. 8.4V for
      two serial connected cells. It is recommended using a digital voltmeter.
      The max charging current is set by choosing the value of Rx.
      Rx = 0.6 / max charging current

      For example, for a max charging current of 600mA, Rx should be 0.6 / 0.6 = 1ohm,
      while for a max charging current of 1.2A it should be 0.6 / 1.2 = 0.5ohm.
      The dissipated power on Rx at a charging current of 1.2A is:
      P = V x I = 0.6 x 1.2 = 0.72W

      The dissipated power on the LM317 IC is:
      (Vin - Vout) x Charging Current.
      It's advisable to use a heatsink to prevent the IC from getting too hot.
      Notice that the IC's metal package or tab also carries the Vout, so it's necessary
      to use isolating washers in case you attach the heatsink to a metal case.

      The LM317's max output current is 1.5A. For higher charging currents one may
      use the LM350 rated at 3A or the LM1084 rated at 5A.

      Note: if a Li-ion battery gets discharged below 2.9V/cell, it needs to be slow charged at 0.1C until
      3.0V/cell is reached before a higher charging current rate may be used.
      Also discharging below 2.3V/cell will damage the battery.

      According to the manufacturers the Li-ion batteries should be stored charged to about 30 - 50% of
      capacity at room temperature.
      For prolonged storage periods, store discharged (i.e. 2.5 to 3.0V/cell) at -20 to 25 C.


      Important!
      Make sure to set your charger to the correct voltage according to the number of cells.
      Failure to do this may result in battery fire!

      Before you charge a new Lithium pack, check the voltage of each cell individually.
      This is absolutely critical as an unbalanced pack may explode while charging even if the correct
      cell count was chosen.
      If the voltage difference between cells is greater than 0.1V, charge each cell individually to 4.2V
      so that they are all equal.
      If after discharge, the pack still is unbalanced you have a faulty cell that must be replaced.

      Do not charge at more than 1C.
      NEVER charge the batteries unattended!

      Caution:
      If you crash with Lithium cells there is a risk that they get a latent internal short-circuit.
      The cells may still look just fine but, if you crash in any way remove the battery pack carefully
      from the model and place it on a non-flammable place, as these cells may catch fire later on.
      (A box with sand is a cheap fire extinguisher).
      Don't use Lithium batteries when flying in areas with large amounts of dry vegetation, as a crash
      may result in a serious forest fire.

      A new sort of Lithium (Saphion) cells has now been introduced into the market.
      These cells are claimed safe since they don't burst into flames when abused like
      the traditional Li-Ion-Polymer do.
      Their safety aspects result from the incorporation of phosphates as the cathode
      material, which are stable in overcharge or short circuit conditions and also have
      the ability to withstand high temperatures without decomposing.
      When abuse occurs, phosphates are not prone to thermal runaway and don't burn.

      These cells have a nominal voltage of 3.2V, can be discharged down to 2V and
      charged to 4.2V.
      The recommended discharge rate is 5 to 6C continuous for a long life or higher
      discharge rates for a shorter life.




      The lead acid batteries have much lower
      energy/weight ratio than all those previously
      mentioned. Which means that the lead acid
      batteries are heavier for the same capacity.

      They are not suitable to be used airborne, but
      since they are rather cheap, they are often used
      on the flying fields as ground power supply for
      engine starters and/or to charge the smaller ones.
      There are various versions of lead acid batteries:
      The Gel-Cell, the Absorbed Glass Mat (AGM) and the Wet Cell.
      The Gel-Cell and the AGM batteries cost about twice as much as the Wet Cell.
      However, they store very well and do not tend to sulfate or degrade as easily as
      the Wet Cell.
      Lead acid batteries get "sulfated" when the soft lead sulfate normally formed on the
      positive and negative plates' surfaces re-crystallises into hard lead sulfate when
      the batteries are left uncharged during long time. This reduces the battery's
      capacity and ability to be recharged.
      Adding Silica Gel to the sulphuric acid turns the electrolyte into a solid mass that
      looks like jelly, hence the name Gel-Cell. This prevents acid spillage even when
      the battery is broken.
      The sulphuric acid in AGM batteries is absorbed into fine fibreglass mats, they have
      the same advantage of the gelled batteries but can take more abuse.
      Both the Gel-Cell and AGM are the safest lead acid batteries one can use.
      However, Gel-Cell and some AGM batteries require a slower charging rate. These
      batteries may be damaged if fast charged on a conventional car charger.

      There are sealed (maintenance free) and serviceable non-sealed Wet Cell
      batteries. Non-sealed batteries are recommended in hot climates since distilled
      water can be added through the filler caps when the electrolyte evaporates due
      to the high environment temperature.

      The lead acid batteries have a self - discharge rate of about 1% to 25% a month.
      They will discharge faster at higher temperature. For example, a battery stored at
      35C (95F) will self-discharge twice as fast than one stored at 24C (75F).

      Lead acid batteries left uncharged during long time will become fully discharged
      and sulfated. The best way to prevent sulfation is by periodically recharging the
      battery when it drops below 80% of its charge.
      It is possible to determine a non-sealed battery's charge status by measuring the
      concentration of the sulfuric acid of the battery electrolyte ("battery acid") with a
      hydrometer.

      These batteries are built with different characteristics depending on application.
      For example, starting batteries (also called SLI - Starting, Lightning, Ignition) have
      the ability to deliver large starting current during very short time (cranking amps).
      They have many thin plates of Lead "sponge", which gives a large surface area.
      Starting batteries are mainly intended to start engines when the batteries seldom
      get deep discharged, because if they often get deep discharged the Lead sponge
      falls faster to the bottom of the cells, significantly reducing their lifespan.

      Another type are the deep cycle batteries, which may be discharged down to 20%
      of the full charge, time after time, without reducing their lifespan as their plates are
      much thicker, however, these batteries lack the ability to deliver large current during
      short time compared with the starting batteries.
      Deep cycle batteries are therefore used where current is needed during long time,
      such as in forklifts, golf carts or solar electric backup power.
      If a deep cycle battery is also going to be used as a starting battery, it should be
      oversized about 20% in relation to the recommended starting battery's size in order
      to provide the same cranking amps.

      The lead acid batteries have normally 3 or 6 cells connected in series.
      Each cell has a nominal voltage of 2V resulting in a nominal pack voltage of 6V
      and 12V respectively.
      They are usually charged with a constant voltage of 2.4 - 2.5V per cell having the
      charging current limited to 1/10C. It is not recommended charging these batteries
      with a charging current exceeding 1/3C.
      A lead acid battery pack is considered fully charged when the charging current
      falls below 10mA and/or the cell voltage reaches 2.4 - 2.5V.

      Should a lead acid battery be continuously left on charge (when used as power
      backup); the charging voltage should not exceed 2.25 - 2.30V per cell.
      It is also advisable to charge these batteries in a well-ventilated area/room, since
      it produces hydrogen-oxygen gases that can be explosive and also the electrolyte
      contains sulfuric acid that can cause severe burns.
      Lead acid batteries' lifespan is about 4 to 8 years depending on the treatment.



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