Voltage / Cell Count
A LiPo cell has a nominal voltage of 3.7V.
About Nominal Voltages
Nominal voltage is the default, resting voltage of a battery pack. This is how the battery industry has decided to discuss and compare batteries. It is not, however, the full charge voltage of the cell. LiPo batteries are fully charged when they reach 4.2v/cell.
The voltage of a battery pack is essentially going to determine how fast your drone or wing is going to go. Voltage directly influences the RPM of the electric motor (brushless motors are rated by kV, which means 'RPM per Volt'). So if you have a brushless motor with a rating of 3,500kV, that motor will spin 3,500 RPM for every volt you apply to it. On a 2S LiPo battery, that motor will spin around 25,900 RPM. On a 3S, it will spin a whopping 38,850 RPM. So the more voltage you have, the faster you're going to go.
The capacity of a battery is basically a measure of how much power the battery can hold. Think of it as the size of your fuel tank. The unit of measure here is milliamp hours (mAh). This is saying how much drain can be put on the battery to discharge it in one hour. Since we usually discuss the drain of a motor system in amps (A), here is the conversion:
1000mAh = 1 Amp Hour (1Ah)
Discharge Rating ("C" Rating)
Voltage and Capacity had a direct impact on certain aspects of the vehicle, whether it's speed or runtime. This makes them easy to understand.
The C Rating is simply a measure of how fast the battery can be discharged safely and without harming the battery. Here's the way you find out the maximum safe continuous amp draw:
50C = 50 x Capacity (in Amps)
Calculating the C-Rating of our example battery: 50 x 5 = 250A
The resulting number is the maximum sustained load you can safely put on the battery. Going higher than that will result in, at best, the degradation of the battery at a faster than normal pace. At worst, it could burst into flames. So our example battery can handle a maximum continuous load of 250A.
The first reason that you need a LiPo-compatible charger is balancing. Balancing is a term we use to describe the act of equalizing the voltage of each cell in a battery pack. We balance LiPo batteries to ensure each cell discharges the same amount. This helps with the performance of the battery. It is also crucial for safety reasons - but I'll get to that in the section on discharging.
LiPo batteries come with a connector called a JST-XH connector on the balance tap. Most LiPo batteries need to be charged rather slowly, compared to NiMH or NiCd batteries. While we would routinely charge a 3000mAh NiMH battery at four or five amps, a LiPo battery of the same capacity should be charged at no more than three amps. Just as the C Rating of a battery determines what the safe continuous discharge of the battery is, there is a C Rating for charging as well. For the vast majority of LiPos, the Charge Rate is 1C. The equation works the same way as the previous discharge rating, where 1000mAh = 1A. So, for a 3000mAh battery, we would want to charge at 3A, for a 5000mAh LiPo, we should set the charger at 5A, and for a 4500mAh pack, 4.5A is the correct charge rate.
The SkyRC D100 v2 (pictured above) to be one of the best values on the market. It is included in the Gaterbx FPV charging box and it's a multi-chemistry charger, which means it can charge NiMH, NiCd, and Lead Acid batteries as well as LiPo batteries. It can even charge the newest LiFe batteries that some use for receiver packs in airplanes and cars. It has 2 built-in balancer that handles up to 6S LiPo batteries and can charge up to six amps.
Watts = Voltage x Amperage
See, wattage, voltage, and amperage are intertwined. You can convert the voltage to amperage, and vice-versa. This is important in determining what kind of charger you need. Let me show you how.
Let's say that I have a 6S 5000mAh LiPo battery, and I want to charge it at 1C, which would be 5A. If I have a D100 AC/DC Charger, I can set up the charger to charge at 5A for a 6S battery. But when I go to charge the battery, the most it ever charges at is around 3.5A. What gives? If we use the formula above, we can plug in our voltage (22.2V) and our Amperage (5A) and we get this:
22.2v x 5A = 111W
So the formula is saying that if we want to charge our 6S 5000mAh LiPo pack at 5 Amps, we would need a charger that is capable of delivering at least 111 Watts of power.
Lithium-Polymer batteries contain, quite obviously, lithium. Lithium is an alkali metal, meaning it reacts with water and combusts. Lithium also combusts when reacting with oxygen, but only when heated. The process of using the battery, in the sometimes extreme ways that we do in the R/C world, causes there to be excess atoms of Oxygen and excess atoms of Lithium on either end (be it the cathode or anode) of the battery. This can and does cause Lithium Oxide (Li2O) to build up on the anode or cathode. Lithium Oxide is basically corrosion, albeit of the lithium kind; not iron oxide, which is otherwise known as "rust". The Li2O causes the internal resistance of the battery to increase. Internal resistance is best described as the measure of opposition that a circuit presents to the passage of current. The practical result of higher internal resistance is that the battery will heat up more during use.
Higher Internal Resistance = Higher Operating Temperature
Heat causes the excess oxygen to build up more and more. Eventually the LiPo pack begins to swell (due to the oxygen gas build up). This is a good time to stop using the battery - its trying to tell you that it has come (prematurely or not) to the end of its life. Further use can, and probably will, be dangerous. After the pack has swollen, continued use can cause even more heat to be generated. At this point, a process called Thermal Runaway occurs.
However, even if you stop using the battery when it swells, you still have to render it safe. If you puncture a LiPo that has swollen and still has a charge, it can still catch fire. This is because the unstable bonds that exist in a charged battery are in search of a more stable state of existence. That's how a battery works; you destroy a stable chemical bond to create an unstable chemical bond. Unstable bonds are more apt to release their energy in the pursuit of a more stable bond.
The entire process of building up that lithium oxide usually takes around 300-400 charge/discharge cycles to reach a tipping point. That's a typical lifetime of a LiPo battery. But when we heat the batteries up during a run, or discharge them lower than 3.0 volts per cell, or physically damage them in any way, or allow water to enter the batteries (and I mean inside the foil wrapping), it reduces the life of the battery, and hastens the build up of Li2O.
A LiPo cell should NEVER be discharged below 3.0V
Proper LiPo Storage Voltage = 3.8V per cell