Although lipo batteries have only recently become commercially available (compared to the earlier Nickel Cadmium/Nickel Metal Hydride types), their performance characteristics have quickly taken them to the top of the modellers wish list. The energy density of these cells (the watt minutes/gram) is way above the other cell types we have used, and this, together with their ability to deliver high levels of power, is the reason why they are so attractive to the modeller. Their effect on electric powered model flight has been little short of amazing, and although there is little data to support the statistics, it seems likely that the proportion of powered sports flying using electric power now exceeds 50%. This rate of advance has certain disadvantages, and in this case the main one seems to be a lack of technical information relating to lipos. Whilst some technology is common to all batteries, each particular type has a different chemistry, often a different physical form, and usually quite different procedures in use. Since lipo cells and batteries are the latest to be developed, it is logical to assume that users are less familiar with them than with older types. This text will attempt to remedy that shortfall, at least to some degree.
Electric cells and batteries fall into two broad categories. They are either primary or secondary, dependent upon whether or not they are rechargeable. Primary cells are single use, non rechargeable units, whereas secondary cells are repeated use, rechargeable ones. In modelling, we use both types e.g. the carbonzinc primary cell to power a tachometer, and the nickel metal hydride secondary cell in a glow driver. Lipo cells and batteries are therefore clearly secondary units.
Capacity. If we consider the capacity of a lipo cell/battery then we need to adopt a slightly different system to that used with previous cell types. Whilst the capacity itself is measured in ampere hours (Ah) or milliampere hours (mAh) for smaller packs, we also link this to a C rating for the pack which is actually a measure of rate of discharge (or charge). A 1C rate is equivalent to a complete discharge in one hour so that the current drawn will be the Ah capacity numerically expressed in Amps. Multiples of C (2C, 5C, 20C etc.) would involve a current draw increased by the same multiple with the time period decreased in the same ratio. A theoretical example would be a 3500 mAh lipo battery discharged at 2C when 7000 mA (7.0 Amps) drawn from the pack would last for 30 minutes, or the same battery recharged at 0.5C when the charging current would be 1750 mAh (1.75 Amps) and the pack would take 2 hours to fully recharge from empty. These values are purely theoretical since they take no account of losses during the process.
One additional application of C ratings is in terms of maximum discharge rates. The maximum current which can be safely drawn from a battery is one way of measuring the quality of a pack, so identical capacity packs can be rated differently. A 2200 mAh lipo battery pack rated at 20C should be limited to a maximum discharge current of "20x2 = 40 Amps", whereas a 3s 2200mah battery pack rated at 50C can theoretically be loaded at "50x2 = 100 Amps". These C ratings are established by the manufacturers and there is some variation in the interpretation of this assessment. Modellers are therefore recommended to approach such maximum currents with caution.
Buying used batteries
Extreme caution should be exercised when considering buying used batteries, as you will usually have no real idea of the history of the battery or its 3 + 3 can provide more than 6 If may be worth buying two smaller batteries instead of one larger one. For example, if you needed a 6S 3,700mAh lipo, consider purchasing two 3S 3,700mAh batteries instead, which would be used in series and dedicated to that model. In this way, if damage occurred to one of the batteries, at least one would still be able to use the other one in a smaller model, thus retaining some of the value of your investment.
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