RCU Review: Greg Covey's Amp'd Issue 14: Storing Lithium for Longevity!


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    Contributed by: Greg Covey | Published: November 2009 | Views: 35681 | email icon Email this Article | PDFpdf icon

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    Issue 14
    Article By Greg Covey

    Print Issue 14 "Storing Lithium for Longevity"


    Although new market trends are lowering prices, most of us consider the cost of Lithium batteries to be a significant part of our electric flight setup. If we had been previously flying with glow or gas engines, suddenly paying for fuel "up front" can be a real paradigm shift. When we buy a battery pack, we are essentially paying for hundreds of flights all at once.

    Batteries are perishable products that start deteriorating right from the moment they leave the factory. Much like cars and computers, our flight packs loose value after they are purchased, even if we don't use them. As our batteries loose capacity over time, or from abuse, we start to realize that it is important to maximize our investment.

    As I prepare for a long cold winter here in upstate, NY, my focus in this month's issue of AMP'D is to show some simple preventive measures that anyone can apply to slow the aging process of Lithium packs, especially during the off-season months when they may not even be used.


    Storing Lipo Packs

    The Battery University states that the recommended storage temperature for most batteries is 15°C (59°F). While lead-acid batteries must always be kept at full charge, nickel and lithium-based chemistries should be stored at 40% state of charge. This level minimizes age-related capacity loss, yet keeps the battery in operating condition even with some self-discharge.

    While capacity loss during a battery's life cannot be eliminated, following a few simple guidelines will minimize the loss of capacity:

    • Keep batteries in a cool and dry storage area. Refrigeration is recommended but freezers should be avoided. When refrigerated, the battery should be placed in a plastic bag to protect against condensation


    • Do not fully charge lithium and nickel-based batteries before storage. Keep them partially charged and apply a full charge before use. Store lithium-ion at about 40% state-of-charge (3.75-3.80V/cell open terminal). Lead-acid batteries must be stored fully charged.


    • Do not store lithium-ion fully depleted. If empty, charge for about 30 minutes (at a 1 x Capacity rate) before storage. Self-discharge on a depleted battery may cause damage and prevent a recharge.


    • Do not stockpile lithium-ion batteries; avoid buying dated stock, even if offered at a reduced price. Observe the manufacturing date, if available.

    The best possible capacity retention for LiPos is obtained when you store them about half-charged and keep them cold around 0° (C) or 32° (F). This results in approximately 2% loss of capacity per year. The next best is a 4% loss per year when stored half-charged at 25° (C) or 75° (F).

    For winter storage of LiPos in upstate NY, my garage stays around the freezing temperatures for most of the "building" season. The remaining task is to get my packs at around the half-charged voltage and I accomplish this in several ways. Once the pack is around half charge, I usually place a 50% circle sticker on it so that I remember which packs were done. The packs are then sealed in plastic bags, with as much air squeezed out as possible to reduce condensation, and placed in a box to be stored in the garage.



    Discharging Packs for Storage

    When discharging my packs for off-season storage, I first organize the packs in various cell counts, capacities, and discharge rating. My larger more expensive packs get first attention and I seldom discharge packs that are smaller than 1500mAh. Once the packs are sorted, there are a number of ways to discharge them to 50%. Although discharging packs does not have the potential for hazard associated with over-charging, it is important to stay nearby and not let your packs get discharged below 3v per cell for an extended period.

    One way to get my packs at the half-charge voltage of 3.8v/cell is to charge them after my last flight to only half way. Some chargers currently on the market display a fuel gauge to help make this process easier. Once the pack is at 50% fuel level, you stop the charge. The FMA Direct Cellpro Product Line of chargers, like the 4s shown on the left, have a built-in STORE feature that automatically stops the charge at 50%. This is a great feature to help automate your battery storage process.

    Another way I used to get my packs at the half-charge voltage was to use a series of light bulbs like the BC015 8-bulb Battery Dumper that Hobby Lobby used to sell.

    I used a wattmeter in series with the BC015 Battery Dumper to monitor the voltage for a half-charged state and maintain a reasonable current draw for the pack. The number of bulbs can be varied to suit the pack voltage and battery discharge rating. This process typically took about 10-15 minutes per pack so you had to keep a close eye on it.

    The bulbs produced plenty of heat so I would typically do this process on the cement floor in the garage. After letting the pack sit unloaded for a few minutes, I would then verify the pack voltage using a simple voltmeter. This inexpensive approach to discharging batteries required skill and attention.

    My current procedure to discharge packs to 50% uses several of my old Astro Flight Lithium 109 Charger/Dischargers. It discharges at a much slower rate than the bulb technique so a fully charged 5AH pack takes a few hours to reach half charge. The procedure is as follows:
    1. Plug pack to be discharged into a powered-down Astro Lithium 109.
    2. Apply power to the charger and it should go into a slow discharge mode.
    3. The display will read pack voltage and capacity removed in mAh. Let the pack discharge until you approximately see the appropriate value below. An exact 50% discharge is not critical but it is still a manual process.

      ½ Voltage Value for Various Packs
      - 3s = 11.4v
      - 4s = 15.2v
      - 5s = 19v
      - 6s = 22.9v

    After disconnecting the pack from the charger, you can let it sit for a few minutes or so and confirm the pack voltage using a multi-meter.

    Serious battery users may consider using the Computerized Battery Analyzer (or CBA) from West Mountain Radio. In addition to performing professional quality battery tests and analysis, the CBA has a user selectable discharge test current setting from 0 to 40 Amps. It automatically shuts off the discharge at a safe pre-determined minimum battery voltage.

    The CBA is much more than a simple battery voltage tester or a battery load tester. Unlike a simple load tester the CBA will test virtually any type or size of battery, any chemistry, any number of cells up to 55 volts. The software supplied with the CBA is easy and intuitive to use. It provides automatic sensing of the battery cell count and recommends a safe maximum discharge current and minimum safe cutoff voltage for your batteries.

    Gas Gauging LiPo Packs

    Many OEM applications of LiPos such as cell phones use a "gas gauge" to inform the user of the status of the battery. Since most R/C applications are price sensitive, the added cost of individual pack gauging is not practical. However, if the user has a simple digital voltmeter, a good gas-gauging emulation can be had by using a gas gauge load or using the RC system as the load.

    The receiver, servos, and ESC combined current without the motor running approximate the 0.5C load at which the cell is rated as shown in the 0.5C curve (right). The discharge curve for the pack is nominally 3.7v/cell x (# of cells at 0.5C). Recognize that this is a nominal value and will vary with the specific pack, how close the current is to 0.5C and how old the pack is.

    Most LiPo packs are shipped from the manufacturer at half-charge and checked prior to assembly to measure exactly 3.81v or 50% capacity.

    It is well to check this voltage before storing the pack or charging it in order to "calibrate" your setup. For the example curve: if your DVM says 3.5V under the light load, the 2150mAh cell has been depleted to 3.68V which equals ¼ capacity remaining. That is about 80% of capacity and time to stop flying if you wish to maximize cycle life or minimize risk of an ESC cut-off. The last ¼ pack capacity is sometimes referred to as the "zone of temptation" as it can be used to maximize flight time but often leads to premature loss in pack capacity.

    For the most accurate results, keep track of pack voltage after each flight, track it against the discharge curve, and 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.

    In most cases, I use a subset of the gas-gauging technique just described as a simple field tool. I print out a copy of the example voltages below that I extracted from the chart and keep it in my field box. I then measure the pack voltage after flight using a multi-meter. For best accuracy, this can be done with the motor stopped and the receiver and servos still energized, but, I often do it on an unloaded pack as well. After I roughly determine the remaining capacity of the pack, I may decide to adjust the flight timer on my transmitter. Eventually, this iterative process allows me to hone into each model's flight duration so it stays in memory with the rest of my programmable settings. In this manner, I can successfully "gauge" dozens of setups and keep the results in my transmitter. Some of my planes are not fun to do a "dead stick" landing.

    Example Voltages: Using the chart below and a simple voltmeter, you can determine the capacity left in a LiPo pack.

    Half Pack Voltage
    6s pack = 22.86v
    5s pack = 19.05v
    4s pack = 15.24v
    3s pack = 11.43v
    2s pack = 7.62v
    1s pack = 3.81v

    Quarter Pack Voltage
    6s pack = 22.2v
    5s pack = 18.5v
    4s pack = 14.8v
    3s pack = 11.1v
    2s pack = 7.4v
    1s pack = 3.7v


    Summary

    Unlike NiCad and NiMH packs, LiPo batteries can be stored for several months without significantly losing charge. However, if storing for long periods, you can minimize the loss of capacity by discharging the battery to 40% or 50% and keep it refrigerated but not frozen. By sealing the packs in plastic bags during cold storage, you can eliminate condensation from forming that could damage the pack.

    This month's issue of AMP'D we learned that the best possible capacity retention for LiPos is obtained when you store them about half-charged and keep them cold but not frozen. This simple process is not hazardous and results in only about 2% loss of pack capacity per year and is a great way to maximize your investment in electric flight.

    Remember to keep a few packs handy for some fun in the snow! Let the packs warm to room temperature overnight and then charge them up. Keep them warm until you are ready to fly for best performance.

    When you fly electric, fly clean, fly quiet, and fly safe!

    Special thanks for contributions by:
    "Papa Jeff" Ring and Lynn Bowerman

    This section of AMP'D cover some of the questions that our readers have sent in and I thought would be interesting for others.

    Richard D. asks:

    Hi Greg,

    I am interested in your Fly Fly BAe Hawk on RCU. Did you run the Fly Fly 90mm fan with an Ammo-36-56-1800 motor and 6-cell Lipo 22.2V? How good is it with this set up? Have you tried with Midi 90mm rotor?

    Thanks in advance for your response

    FlyFly Hawk Video with AMMO Motor and Midi Rotor

    Download and Watch in Windows Media Player here!

    Hi Richard,

    The best combination for me has been using the Midi 90mm rotor, FlyFly DF, Ammo 36-56-1800 motor, 60-80amp ESC, and 6s 25C LiPo pack.

    You can see this combination fly in my BAe Hawk in the link below. Although the FlyFly rotor worked fine, the Midi rotor had less vibration and gave similar thrust for less current draw. It was more efficient than the FlyFly rotor.

    Good luck!

    Ask questions by e-mailing me at greg@rcuniverse.com

    The Bind-N-Fly? Revolution Continues with the ParkZone Habu EDF BNF Sport Jet


    SKS Video Productions - Neat Fair 2009



    FMA Direct - RTF Airplanes with On-Board Flight Stabilization


    Castle Creations CC BEC Pro for up to 20-amps Peak on 10s LiPo

    Print Issue 14 "Storing Lithium for Longevity"

    Comments on RCU Review: Greg Covey's Amp'd Issue 14: Storing Lithium for Longevity!

    Posted by: byjoe on 12/07/2009
    Good info.Great
    Page: 1

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