Each cell of a Lithium Polymer battery has a nominal voltage of 3.7v, and they are connected in series. This results in a 2-cell Lipo being referred to as a 7.4v Lipo, and a 3-cell Lipo being referred to as an 11.1v Lipo. Lipo cells must maintain a voltage between about 2.7 volts and 4.2 volts, and allowing a cell to go much above or below this voltage range can result in damage to the cell. The only way to assure that each cell stays within this voltage range is to charge the cells individually to a specified voltage with a 'balance' charger, or to bring each cell of a battery pack to the same voltage as the other cell(s) in the pack with a 'balancing' device. A special connector on Lipo battery packs, called a 'balance connector', makes this possible. A battery cell 'balancing' device simply balances the cell voltages - it does not actually charge the cells. We recommend using a Balance Charger that monitors and charges the individual cells of a Lipo.
2. What is a 'C' rating?
The 'C' rating of a Lipo indicates the maximum safe discharge rate. The 'C' refers to battery Capacity measured in milliamp hours (mah). The 'C rating' is a number that is multiplied by the capacity of the battery to give you the discharge rate. (It would be simpler to just indicate the maximum safe discharge rate in amps on the battery pack, but why make it simple?!) The way to determine the maximum discharge rate in AMPS is to take the capacity of the Lipo (mah) and multiply by the 'C rating', then divide the result by 1000. For example, a 1200mah 12C Lipo has a maximum safe discharge rate of 14.4 amps (1200 x 12 = 14400, divided by 1000 = 14.4 amps).
Keep in mind that amp output tells only half the story. You must also know the voltage to determine the total energy output in watts: Amps x Volts = Watts. So if your 1200 12C Lipo has 2 cells (7.4 volts), then the total energy output would be 106.56 watts (14.4 amps x 7.4 volts = 106.56 watts). If the Lipo has 3 cells (11.1 volts), then the total energy output would be 159.84 watts (14.4 amps x 11.1 volts = 159.84 watts). More watts will produce more thrust - all things being equal.
If you want to know the true usable energy in a Lipo, you would hook up a meter between the battery and an electrical load (motor/prop) and run from a fully charged state to motor cut-off, measuring the energy used in watt hours (wh).
3. How fast can I charge a Lipo battery?
Most Lipos are designed to be charged at a 1C maximum rate. This means that the charge rate (milliamps) must not exceed the capacity (mah) of the Lipo. So a 500 mah Lipo should not be charged at a rate greater than 500 milliamps (0.5 amps), and an 1800 mah Lipo should not be charged at a rate greater than 1800 mah (1.8 amps). THIS IS VERY IMPORTANT!!! BE CERTAIN OF YOUR CHARGE RATE!!! You might consider charging at a lower rate than 1C if you're not in a hurry, as this may help extend the life of your Lipos.
4. When should I charge my Lipos?
Lipo batteries are not fully charged when shipped, so be sure to balance charge them before the first use. Lipo batteries will hold a charge for an extremely long time, so there is no need to "top them off" right before you fly. I recommend that you allow them to cool for about 20 minutes after flying, and then go ahead and charge them up so they are ready to use for your next flight. Some chargers will slowly discharge a Lipo battery after charging is completed, so always unhook the Lipo from the charger immediately after charging is finished. If it may be several weeks before you use the battery again, you may want to discharge them to about 3.8 volts per cell, as this may extend the batterie's useful life. Lipo batteries do not develop a 'memory', so partially discharging and then charging them does not effect their ability to take a full charge.
The information above is provided free of charge. If you would like black and white printed copies, you may purchase them for $0.10 each.
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