If your ESC has an automatic lithium mode. Use it, it will correctly sense the number of cells and set the auto cut-off appropriately.
If you have previously been flying with NiCad or NiMH batteries, switching over to lithium polymer will result in a different number of cells being used. If you had 6 to 7 round cells then 2 lithium polymer cells will correctly duplicate the voltage of those cells. If you had 10-11 cells then 3 lithium polymer cells would be right for you. There are a lot of 8 cell flyer's out there that are stuck between 2 and 3 cells. In my experience the best option is to determine how many watts you were using before and duplicate that with your Li-Pos, Motor, and Prop. For example. If you were running 8 cells (9.6volts) at 10 amps on a speed 400 airplane, then you have 9.6 x10, 96 watts. So if you went with 2 lithium polymer cells (7.2 volts nominal) then you'd need to change your prop such that you used 13 amps. If you went to 3 Li-Po cells (10.8 volts nominal) then you'd need to reduce the amperage to 8.9 amps. These estimates are approximate, and some experimentation is required for best results but conserving Watts is a good way to start.
The safe discharge rate of a battery is governed by it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. All RC batteries are rated in milliamp hours. If a battery is rated at 2000mah and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mA x 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x 3).
All batteries have limitations on how fast they can discharge. Because of this many Li-Poly batteries are connected in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have 2 x 2000mAh cells and you wire them in parallel then the result is the same as
1 x 4000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the 2000mAh cells could discharge at a maximum of 5C, or 10 amps then the new 4000mAh cell can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use Li-Poly batteries at higher currents than single cells could produce.
The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 3S means it's a 3 cell pack. The number in front of P means the number of cells in parallel. So a 3S4P pack of 2100mAh cells has a total of 12 cells inside. It will have the voltage of any other 3S pack since the number of cells in series determines the voltage. It will have the current handling of 4 times the maximum C rating of the 12 individual cells. So say our 3S4P pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).
With so many choices out there it is difficult to determine what is marketing hype, what is brand
loyalty, and what is outright lies. Battery manufacturers are constantly trying steal a lead on each other. While capitalism can drive prices down, it can also lead to false claims about products.
One way to find out the best choice of battery is to look at graphs of the battery’s performance. Looking at how low the voltage of the cell drops at various currents will give you a comparison of that battery with similar size/weight batteries.
If graphs aren't your thing then simply look at what other people are using in successful setups that are similar to your application. If a lot of people are reporting long flight times and lots of power from airplane X, with power system Y, and battery Z and you do the same, then if your setup is similar the same battery will probably work well for you.
It pays to learn something about Watts, Volts, and Amps. It will serve you well in not only figuring out what battery is best but also in understanding electric aircraft. A 30C battery is not necessarily any better than a 10 or 20C battery. A higher C rating means it can discharge faster but at the same time a battery discharged at 20C continuously will be empty in 3 minutes. Do you really only want to use the battery for 3 minutes? Having burst power capability can be useful in helicopters, boats and 3D aircraft but in almost all other applications actually running a battery at or above 20C is a waste of power reserve. It is better to run batteries at 8-10 C and have a little headroom if needed.
A final note on choosing a battery, cheap is not necessarily the best option. Confirm that your batteries are capable of running at the current level you require. Running a cell at a higher C rating than the battery can handle can not only damage your batteries, but it can also damage your speed controller. It is advisable to buy a better battery than you need than to destroy your electronics.
Lithium batteries like heat, but not too much. In the winter time, try to protect your batteries from the cold as much as possible. Leave them in the car while your flying, or keep them in your pocket. At the same time don't let them heat up too much. Try to keep your batteries from reaching 160F after use. This will prolong the life of the cells. A good way to measure temperature is a handheld IR meter, available at most hobby shops.
More useful hints and tips to help you enjoy your model flying next month.
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