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Li-Ion The video transmitter was the first payload flown to utilize a non-primary (rechargeable) batteries. Li-Ion cells have been available for some time and are used extensively in cell phones and laptop computers. The biggest drawback to these types of cells for a high altitude flight is their reduced output power at colder temperatures. Primary cells typically used in balloon flights are rated to -40° C, while non-primary cells are typically rated 0° C or sometimes -20° C. In the video transmitter application, the payload generates nearly 1.5 watts of heat. This is enough to keep the payload compartment and the battery pack warm. In addition, the payload draws almost 900mA during operation. This reasonably high current draw warms the battery internally. The batteries used in this application are Panasonic CGR18650A cells. The cells provide a nominal voltage of 3.7 VDC at 2000mAh. The cells are arranged in a 2S2P (2 series / 2 parallel) configuration to provide a 7.4 VDC / 4000mAh pack. In addition, a built in protection circuits provides under voltage and short circuit protection. The pack and charger used in this application were purchased from Electritek-AVT, Inc. for less than $100. Additional battery packs may be purchased for around $35 and include the newer CGR18650C cells that provide 3.7 VDC / 2150mAh. Although lower cost, first generation Li-Ion cells may be obtained on the surplus market, they are not recommended for high reliability applications such as balloon payloads. The cells typically have 20-35% less capacity than the latest generation of cells, are prone to high impedance failure at cold, and carry a greater risk of thermal runaway (fire/explosion) during recharge. Factory fresh cells may be obtained from a variety of reputable vendors at only a slightly higher cost. Although it is tempting to use a constant current/constant voltage laboratory supply or home brew circuit to charge the cells, a smart charger should always be used. The smart charger provides the proper voltage/current for over discharged cells, special current curves to prevent thermal runaway, and charge timers. From the Panasonic web site, "...lithium ion in most cases uses Lithium Cobalt Nickel Dioxide which has a tendency to undergo thermal runaway...When you heat this material up, it [can] reach an onset temperature that begins to self-heat and progresses into fire and explosion..." Panasonic offers an excellent safety video that demonstrates how single AA cell exploding can do a lot of damage.  Typical Li-Ion battery Pack. |
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