====== Power Supply ======

For balloon payload power supplies think - compact, lightweight and able to withstand cold – the obvious choice is some form of lithium battery pack – but which?

  * **Lithium Thionyl Chloride**  (Li-SOCl2) - these typically have a voltage of 3.6V per cell – very good operating temperature range (some to –60degC)  however they a poor current capability (with an  AA sized battery typically not able to sustain more than a few 10s of milliamps)

  * **Photo Lithium** (Lithium Manganese dioxide Li-MnO2) – normally 3.0V per cell – good operating temperature (down to –40deg C) – more able to stand sustained loads – with AA batteries offering sustained loads of 100mA or so and occasional peaks upto an amp (enough to operate motor wind on mechanical cameras). These have a very similar energy density per unit weight to Lithium Thionyl chloride.

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  * **Lithium Iron DiSulfide** (Li/FeS2) - e.g. Energizer E2 L91 or L92 - These are 1.5V batteries which can be used as a direct replacement for ordinary Alkalines. They have Good temperature characteristics  (down to -40 deg C).  As good if not better current handling characteristics to the Photo Lithiums. Similar (possibly better) energy density to Lithium Thionyl Chloride/Photo Lithiums. Energizer publish a comprehensive datasheet including temperature performance profiles for their cells [[http://data.energizer.com/PDFs/lithiuml91l92_appman.pdf|here]].

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  * **Lithium Polymer** (Li-Poly) – normal voltage 3.7V – fair operating temperature range (down to –20 deg C see [[http://www.ibt-power.com/Battery_packs/Li_Polymer/Lithium_polymer_tech.html]]) – excellent current handling characteristics. The big advantage of these cells is that they are re-chargeable (with the right charger). These have about half the energy density per unit weight of Lithium Thionyl chloride/Photo Lithium/Lithium Iron DiSulfide. Li-pos are well known for how easily they combust. Charging and discharging too fast, discharging too much and puncturing can all cause fire or explosions.

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  * **Lithium Ion** (Li-ion) - similar in most respects to Lithium polymer, see [[http://www.ibt-power.com/Battery_packs/Li_Ion/Lithium_ion_tech.html]]


**Solar Cells** are a possible alternative to batteries – there is plenty of sunlight above the clouds – or what about a combination of Li-Poly/Li-Ion with a solar cell charger (but be aware that incorrect charging of these batteries can cause a fire). These solar cells are on sale at Maplins [[http://www.maplin.co.uk/Module.aspx?ModuleNo=2310&criteria=solar%20cell&doy=7m7|BZ48C]] - they put out 0.45V at 400mA in bright sunlight - strip them out of their plastic case and they weigh just 8g each - size 57mm x 88mm x 2mm. (As of 14/11/09, BZ48C has been discontinued by Maplin, [[http://www.maplin.co.uk/Module.aspx?ModuleNo=97350|Order Code: N80CH]] seems to be the nearest replacement.)

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You can also find small-size solar panels suitable for airborne purposes on: [[http://www.solarbotics.com/]]