LiPo Batteries Explained
By Texas Wings member Jack Kreska - posted 7/16/2019
A Lithium Polymer (LiPo) battery is constructed from one or more cells of LiPo. The cells can vary in length, width, and thickness. The wider, thicker, or longer the cells are, the more current (amps) they can supply.
Each cell has a nominal voltage of 3.7 volts. That means we can have 3.7v, 7.4v, 11.1v, 14.8v, 18.5v, or 22.2v batteries, as determined by the number of cells used. When fully charged a cell has 4.2 volts.
LiPo batteries contain 2, 3, 4, 5, 6, etc. cells connected in series (S). So, a pack with 2 cells is a “2S” battery, and a pack with 3 cells is a “3S” battery. A 2S battery has 7.4 volts, and a 3S battery has 11.1 volts, etc.
LiPo batteries that we use are constructed using two cells (2S), three cells (3S), four cells (4S), five cells(5S), or six cells (6S).
“Series” connection means the cells are connected inside the pack so the first cell’s positive tab is connected to the second cell’s negative tab. This combines their voltages; the two 3.7 volt cells deliver 7.4 volts combined. The other way to connect the cells is in parallel (P), which means the cell’s tabs are connected “positive to positive” and "negative to negative.” This combines their capacities. If you look at the specifications of your LiPo, you may see that your 2S pack is actually 2S2P, meaning it has two pairs of cells, each connected in parallel (2P), and the two pairs of cells are then connected in series (2S).
Are you confused yet? Hang on, it gets worse.
Enough for voltage, let’s examine current (amps).
LiPo batteries are labeled in voltage, number of cells, and milliamp-hours. A milliamp (mA) is 1/1000th of an amp. Milliamp-hour is a measure of current (mA) available for one hour. That means a battery marked as 1800 mAh is able to supply 1800 milliamps (1.8 amps) for one hour. So why can’t we fly for an hour? Let’s see.
If you can find the current used by your motor (good luck), you can calculate how long it will run with a particular battery. The small motors on planes like the foamboard F22 use about 25 amps or so at full throttle. However, the battery is only rated for 2200 mAh or 2.2 amp-hours. We will have to draw significantly more amps from the LiPo in order to run that motor. Conveniently LiPo batteries are capable of suppling many times the number of milliamps (or amps) they’re labeled to supply.
In the case of the F22, we can calculate how long we can run the motor using a 2200 mAh battery. If the motor draws 25 amps at full throttle, that’s approximately 11.4 times the 2.2 amps the battery is labeled. We know the battery can’t supply 25 amps for an hour, so long can it? An hour is 60 minutes. If we divide 11.4 into 60, we get 5.26. That means we could fly at full throttle (25 amps) for a little over five minutes. Of course, if we don’t stay at full throttle, but slow down, the battery will last longer since the motor won’t be drawing 25 amps during the entire flight.
There’s one more value labeled on the battery, “C”. C stands (kinda) for capacity. It tells us how many multiples of the rated current (mAh) the battery can supply. For instance, if the battery were labeled 1C it could only supply a maximum of 2.2 amps. It could never run that motor. But, if the label states 20C, the battery can supply 20 times 2.2 amps, or 44 amps. More current drawn than that will be harmful to the battery.
Then why have any battery rated for more than 20C? There two basic reasons. The higher the C rating, the more current can be supplied (30C times 2.2 amp is 66 amps). But we don’t need 66 amps, so why use a 30C battery?
We know the motor draws 25 amps at full throttle, but it can draw more current at startup or during aggressive maneuvers. Also, a higher C rating means the battery will run cooler and not suffer lower voltage at high currents. The higher C rating can also increase flight time since the battery isn’t working as hard.