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L91 Overview - No, the Energizer bunny is not going to pop out of the batteries. The AA size, L91 Energizer battery is the latest in a family of Lithium based primary (non-rechargeable) cells. The L91 is composed of Lithium/Iron Disulfide (Li/Fe S2). The cells specified operating range is -40º C to 60º C, which makes them ideal for balloon based payloads. However, they do not perform well at high temperatures. So heat soaking a payload in a car on a hot summer day is not recommended. The cells claim 2.9 AH (Amp Hours) of service. Testing with the Pico Beacon has proven they meet this claim. With the payload cold soaked in a home freezer, the cells met their capacity claim with the GPS in continue operation (100mA at 3VDC) and transmitting every twenty seconds for one second (900mA at 3VDC). Battery capacity is effected by current draw. Availability - The cells are widely available at national retailers and easily disposed of. Up until now, we've been flying Li SO2 cells that were part of a BA-5590 battery pack. The batteries are difficult to extract and properly dispose of. A combination of the L91 cells and rechargeable Lithium Ion battery technology will replace the BA-5590 batteries. Other Technology - The L91 does perform better than other common Lithium photo cells (such as the CR123A). These batteries are rated to only -20º C. The battery chemistry is different and is typically Lithium/Manganese Dioxide (Li/Mn O2). Other secondary (rechargeable) cells such a NiCd (Nickel Cadmium), NiMH (Nickel Metal Hydride), and (Li-Ion) Lithium Ion generally are usable down to only -15º C before the internal electrolyte starts to freeze and the impedance goes up. With an increase in impedance, the cells won't produce full current, such as required during a transmit operation. Fresh Cells - Although the nominal voltage of the AA cell is 1.5VDC, new cells provide 1.7VDC during the first 3-5% of their operational life. If a number of cells are used in series, the additional voltage needs to be handled by the system's voltage regulator.  Flight Batteries. (Two required for Pico Beacon). Shown below is the performance of the two cells wire in series under load (950mA transmit) in extreme cold. Although the battery voltage drops 600mV through out the temperature range, the cells are able to provide 1 amp of current required for the transmit operation. The cells are usable down to 1V each, so there is a 20-25% margin available even at -55° F.  2 cells - Battery Voltage / Temperature vs. Altitude. |
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