Which Type Of Battery Is Best For HAB Use?

April 2011

I'm often asked the above question and the answer depends on many things. Those with the knowledge and experience to select the best battery for a particular application, piece of equipment or HAB flight may choose something different for a specific reason, but Energizer Lithium Iron Disulfide cells are hard to beat for HAB use and the #1 choice by most, mainly because of their −40C temperature rating and light weight.

Following is some other information from the Energizer Lithium Iron Disulfide, or LiFeS2, Cell Application Manual that I found interesting or good to keep in mind when using these cells.

  • Energy density is twice that of an alkaline cell.
  • mAh capacity is reduced very little by higher discharge current rates, unlike alkaline cells.
  • mAh capacity is reduced little at −40C with a 25ma load, but with a 200ma load the capacity starts to drop off at −20C and is ≈ 1/2 at −40C. Note: Cells still work fine at −40C and capacity is not lost, it's simply unavailable due to the reduced chemical activity and becomes available again at warmer temperatures.
  • Rated voltage is 1.5 volts, but Open Circuit Voltage (OCV) of a fresh cell can be 1.79 to 1.83 volts.
  • OCV  drops dramatically when a load is applied, but recovers slowly over time when a load is removed.
  • Any cell with an OCV of <1.70 volts (after it has been given time to recover) is completely discharged.
  • Cells utilize a PTC (positive temperature coefficient) over current safety device that's designed to reversibly shut down and protect the battery from high currents and temperatures by limiting the current when the PTC temperature exceeds 85C. The PTC will rarely activate during non-abuse conditions, but keep in mind that a combination of high discharge current and high ambient temperature could cause the PTC to limit current.

I did some quick tests and calculations to determine how long Energizer Lithium L91 & L92 cells would power a RTrak-HAB V1.10 tracker with an active antenna which requires a minimum input voltage of 5.2V and  ≈ 88 ma when idle and ≈ 310 ma transmitting. The 5.2V min. input voltage means only 4 cells are required and for a 30 sec. beacon rate with 1 sec. transmissions the average current works out to ≈ 95 ma which means:

  • 4 Lithium L91 AA Cells that weigh 58g and provide ≈ 3000 mAh
    would last ≈ 31.5 hours or ≈ 36.1 hours with the power on LED disabled to save 12 ma.
  • 4 Lithium L92 AAA Cells that weigh 30.4g and provide ≈ 1200 mAh would save 27.6g and
    would last ≈ 12.6 hours or ≈ 14.4 hours with the power on LED disabled to save 12 ma.

I'm always looking for ways to reduce weight in order to reach the highest possible altitude so also did some quick tests with a SparkFun LiPower Boost Converter providing the tracker with 5.0V and with an input voltage of

  • 3.0V the converter input current with the tracker idle = 145 ma and while transmitting =   550 ma and with
  • 2.0V the converter input current with the tracker idle = 230 ma and while transmitting = 1050 ma.

Without knowing what the discharge voltage curve of a battery powering a RTrak using a converter would be it's not possible to accurately calculate how long it would last, but using the same formulas as used above for 4 cells:

  • 2 Lithium L91 AA Cells weighing 29g would last ≈ 18.9 hours at 3.0V and 11.7 hours at 2.0V
    or, with the power LED disabled, 21.7 hours at 3.0V and 13.3 hours at 2.0V
  • 2 Lithium L92 AAA Cells weighing 15.2g would last ≈ 7.6 hours at 3.0V and 4.7 hours at 2.0V
    or, with the power LED disabled, 8.7 hours at 3.0V and 5.3 hours at 2.0V

A few grams have to be added for the weight of the converter of course. The converter output would also have to be made >5.2V to allow use of the tracker A/D converter which would reduce the times a bit, unless one bypassed the 5V regulator. And tests using L91 & L92 cells are needed to accurately know what operating times would be, but the calculated times at least help choose which power sources are most likely the best to test further which are:

  • 4 Lithium L91 AA Cells weighing 58g for the longest operating time,
  • 2 Lithium L91 AA Cells with a converter weighing ≈30g for ≈50% less weight and
  • 2 Lithium L92 AAA Cells with a converter weighing ≈16g for the lightest weight.

Energizer Lithium cells aren't cheap so I don't plan on doing any actual battery life tests until one of the sources is used for a flight to allow simply leaving the tracker turned on and using the battery already used for the flight which would likely be discarded anyway with not knowing for sure how much longer it would last.

I also plan to check out using rechargeable Lithium Polymer Cells with a Sparkfun LiPo Charger/Booster which is simply a LiPower Boost Converter with a LiPo Charger to save having to remove a battery to charge it. Being rechargeable, this type of battery will allow some actual battery life tests to be done and having a number of cells of different capacity should provide a much better idea of how long L91 & L92 cells with a converter will last.

More to follow ........

 


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