Lithium polymer batteries are now being widely used in hobby and UAV applications. They work effectively because they can hold a large amount of current and are lighter than nickel metal and ni-cad batteries. But with this improvement in battery life come potential hazards.
Voltage and Cell Count:
Lipo battery act differently than NiCad or NiMH batteries do when charging and discharging. Lithium batteries are fully charged when each cell has a voltage of 4.2 volts. They’re fully discharged when each cell has a voltage of 3.0 volts. It is necessary not to exceed both the high voltage of 4.2 volts and the low voltage of 3.0 resting volts or 2.5 during discharge. Exceeding these limits can harm the battery.
The way to ensure that you do not go below 2.5 volts while flying is to set the low voltage cutoff (LVC) of your electronic speed control (ESC). It important to use a programmable ESC since the correct voltage cutoff is critical to the life of your batteries. Use the ESC’s programming mode to set the LVC to 2 cell lipo battery with a hard cutoff, or 3.0 volts per cell with a soft cutoff. If your ESC does not have hard or soft cutoff, use 3.0 volts per cell. You’ll know when flying that it is time to land when you experience a sudden drop in power caused by the LVC.
If you have previously been flying with NiCad or NiMH batteries, switching over to lithium polymer will lead to a different number of cells being used. If you had 6 to 7 cells of 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 best 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 Lithium Polymers, 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 LiPoly’s (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.
10C from 3S4P?
How fast a battery can discharge is it’s maximum current capacity. Current is generally rated in C’s for the battery. C is how long it takes to discharge the RC airplane batteries in fractions of an hour. For example 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ? or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh 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).
Currently LiPoly technology does not allow currents as high as NiCad or NiMH batteries do. Because of this many LiPoly batteries are put in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to negative and positive to negative they become like one battery with double the capacity. If you have 2 2000mAh cells and you wire them in parallel then the result is the same as 1 4000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the 2000mAh cell lipo battery could discharge at a maximum of 5C, or 10 amps then the new 4000mAh cell lipo battery can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use LiPoly 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 quantity 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 Gens Ace 3S4P lipo battery pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3×3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).