Demystifying Lithium Polymer Batteries
Lithium Polymer Batteries have grown in popularity and now are used by the majority of multirotors. They are often abbreviated as LiPo or LiPoly. Their weight, capacity, and discharge rates make them ideal for use in RC aircraft.
However, LiPo batteries must be treated with care. If they are charged incorrectly, stored in improper temperatures, or are punctured, they may catch on fire and/or explode. See here for more details.
Here is a typical battery listing from HobbyKing:
Turnigy 2200mAh 3S 25C Lipo Pack
A typical lithium polymer battery contains a certain amount of cells stacked on top of each other. They are usually wired in sereis with about 3.7 volts per cell. The number of cells is represented by the letter S. For example, a battery labeled 3S would have 3 LiPo cells in series and a voltage of 3.7*3 = 11.1 volts.
2200 mAh Meaning
mAh stands for milliamp hours, which means the maximum number of milliamps a battery can discharge for an hour. In this case there are one thousand milliamps in an amp, so the battery has 2.2 Amp Hours. This means that it can discharge 2.2 Amps for 1 hour. The larger the number, the longer your multirotor will be able to run.
C stands for how much current the battery can discharge. The higher the number, the more current the battery will be able to discharge. To calculate the maximum current, multiply the number of Amp Hours by the C rating. In this case the battery has 2.2 Amp Hours and a C rating of 25.
2.2*25 = 55 Amps. So this battery can stably deliver 55 Amps of current. There is also a number known as the Peak Discharge Rating, also measured in C. It represents the maximum number amps the battery can deliver for a short period of time (~10 seconds)
The white colored plug that is sticking out of the battery is known as the Balance Plug. It has 4 wires, one for ground, and one for each of the cell. You can use the plug to measure the individual voltages of each cell. When charging, the balance plug is plugged in to the charger so the charger can equalize the voltages of each cell. The voltage of each cell in a lipo battery should be around the same.
The yellow plug coming out of the charger is the discharge/charge plug that allows the battery to power the electronics of the multirotor, or to charge it too. The battery connector shown here is known as the XT60, and can be used to deliver up to 60 amps in an aircraft. The battery connector used is dependent on how much current the battery is designed to be able to discharge. Other popular connectors include XT90, 4MM HXT, and JST.
Charging and Discharging
A lithium polymer battery must be charged using a specialized lithium polymer charger such as an inexpensive one like this:
The charger can be powered by a 12V power supply and comes with an adapter that converts the jack plug on the charger to two convenient alligator clips.
Notice the XT60 Plugs that allow you to charge XT60 Batteries
The number of Watts represents how “powerful” the charger is. Each battery also has a charge rating. Most batteries are charged at 1C, or their rated Amp Hours. For example, the battery above charged at 1C would be charged at 2.2 Amps. Charging at 1C is safe and is less likely to cause a battery fire, but also takes longer. Charging at 1C takes around an hour, while charging at 2C takes around 30 minutes. To calculate if your charger is powerful enough for your battery, multiply the voltage of your battery, 11.1, by the current you plan on charging the battery with, 1C or 2.2 Amps. 11.1*2.2 = 24.42W, which is comfortably below the charger’s maximum 40W.
A fully charged lithium polymer battery has around 4.2V per cell, so the battery above could be charged to around 12.6V. However, LiPo batteries should not be kept charged. If you are not going to use a LiPo battery for several days, you must discharge the battery to around 3.8V per cell, which is about 11.4V for the battery above. A fully discharged lithium polymer battery cell has around 3V, and you should never use your batteries below that voltage.
A common condition known as “puffing” occurs in LiPo batteries when the cells expand and make the battery “fatter.” It often happens because of mistreatment to the batteries. Puffed batteries are extremely dangerous and likely to catch on fire. If your battery puffs, move it to a safe location and watch it to make sure it is stable. You may want to safely dispose of it.
Some people also like to charge their lithium polymer batteries in a LiPo safe bag to prevent a LiPo fire from getting out of control if it happens during charging. Some also put their LiPo batteries in a metal box to prevent a fire from spreading.