Lithium Polymer Batteries December 5, 2006
Lithium Batteries are found in many modern electronic devices, such as laptops and cell phones. They are significantly lighter than NiMH or NiCD batteries, and have higher capacity for the same size. Over the last five years they have worked their way into the hobby market, for use in electric RC helicopters, boats, cars, and particularly rc model airplanes. Their light weight and high capacity make them ideal for long flight times, while also providing more power.
Voltage, Cell Count And C Rating:
The nominal voltage of each cell is 3.7V, but can go down to 3.3V during discharge, and up to 4.2V when fully charged. A battery pack is usually composed of two or more cells put together in series for increased voltage, or in parallel for increased capacity. The C rating denotes how quickly a battery can be discharged (a rating of 1C continuous would mean that a 2000mAh battery should not be discharged any faster than 2000mA or 2A, which would take one hour). A 2000mAh pack rated at 12C continuous would be able to discharge at 12 times its capacity (12 x 2000mA = 24000mA or 24A) at which rate it would discharge in 1/12th of an hour. If you know how much continuous current you will be drawing and the capacity of the pack you want to use, you can easily determince what C rating you require. If you are drawing 5A from a 1320mAh pack, just take the current and divide by the capacity: 5A = 5000mA, 5000mA / 1320mAh = 3.8C. Using a pack with a higher C rating than you require will leave some headroom, and extend the life of your battery. Batteries are also given a C rating in terms of burst, which is how quickly the battery is able to discharge for a short period. A burst rating of 20C would mean a 2000mAh battery could supply 20 x 2000mA = 40000mA or 40A for a few seconds at a time.
Lithium Batteries are also know as LiPo, Li-Po, LiPoly, or Li-Poly. The pack configuration is denoted by the number of cells in series and the number of cells in parallel. A 3s2p pack would have three cells in series, and 2 cells in parallel, using a total of 3×2=6 cells. A 4000mAh 3s2p pack would have a capacity of 4000mAh, and a voltage of 11.1V (3 x 3.7V). It would internally consist of six 3.7V 2000mAh cells. The cells would be doubled up (the 2p part of 3s2p) to get 4000mAh, and there would be three in series (the 3s part of 3s2p) to get 3 x 3.7V = 11.1V.
Which Battery Is Best For Me?:
To select a battery, you first need to know what voltage you require and how much current you will be drawing continuously. If you have a motor that works with 11.1V, you would need a 3 cell battery. If you need to draw 20A, and you would like to have a 10 minute (1/6th of an hour = 6C) flight, you would need a battery with 20A / 6 = 3.3A = 3300mAh. This means you would need a 3s 3300mAh battery with a C rating of 6 or higher.
Never charge a lithium battery if it is below 3.0V per cell, puffed up, or damaged. Always place on a fire-proof surface when charging. Only use chargers designed to work with Lithium Polymer batteries. Never leave your battery unattended while charging.
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