Voltage

A LiPo cell has a nominal voltage of 3.7V. For the 7.4V battery above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a "2S" battery pack - it means that there are 2 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on. 

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, and their minimum safe charge, as we will discuss in detail later, is 3.0v/cell. 3.7v is pretty much in the middle, and that is the nominal charge of the cell. 

Capacity

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)

The C Rating is simply a measure of how fast the battery can be discharged safely and without harming the battery. One of the things that makes it complicated is that it's not a stand-alone number; it requires you to also know the capacity of the battery to ultimately figure out the safe amp draw (the "C" in C Rating actually stands for Capacity). Once you know the capacity, it's pretty much a plug-and-play math problem. Using the above 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.

Proper Care & Treatment: Charging

Battery and Balance Board

It's important to use a LiPo compatible charger for LiPos. As I said in the Introduction, LiPo batteries require specialized care. They charge using a system called CC/CV charging. It stands for Constant Current / Constant Voltage. Basically, the charger will keep the current, or charge rate, constant until the battery reaches its peak voltage (4.2v per cell in a battery pack). Then it will maintain that voltage, while reducing the current. On the other hand, NiMH and NiCd batteries charge best using a pulse charging method. Charging a LiPo battery in this way can have damaging effects, so it's important to have a LiPo-compatible charger.

The second 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 .

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 safest charge rate for most LiPo batteries is 1C, or 1 x capacity of battery in Amps.

However, more and more LiPo batteries are coming out these days that advertise faster charging capabilities, like the example battery we had above. On the battery, the label says it has a "3C Charge Rate". Given that the battery's capacity is 5000mAh, or 5 Amps, that means the battery can be safely charged at a maximum of 15 Amps! While it's best to default at a 1C charge rate, always defer to the battery's labeling itself to determine the maximum safe charge rate.

Parallel charging can be very dangerous. 

Even experts from well-known battery manufacturers "consider parallel pack charging to be highly dangerous and should not be attempted even by experienced users". The problem with parallel charging (or even using your batteries in parallel) is that, when hooking up batteries in parallel, you are doubling the capacity of the batteries while, and this is important, maintaining the voltage of one of the individual batteries. What this means is that your charger, which normally monitors the battery while charging to prevent overcharging, cannot see all of the individual batteries' voltages - it can only see one. 

Proper Care & Treatment: Discharging (Using the Battery)

LiPo batteries offer plenty of power and runtime for us radio control enthusiasts, but that power and runtime comes at a price. LiPo batteries are capable of catching fire if not used properly - they are much more delicate than the older NiMH/NiCd batteries. The problem comes from the chemistry of the battery itself.

Lithium-Polymer batteries contain lithium, an alkali metal, which reacts with water and combusts. When heated, Lithium also combusts when reacting with oxygen. 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 (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 lithium corrosion, or lithium “rust”. The Li2O causes the internal resistance of the battery to increase. The practical result of higher internal resistance is that the battery will heat up more during use.

Higher Internal Resistance = Higher Operating Temperature

A LiPo cell should NEVER be discharged below 3.0V

The LVC works to cut-off the motor of the vehicle (or in some cases, pulse the motor) to alert you to a nearly-depleted battery pack. It uses the total voltage of the battery as its reference. Most LVCs cut-off around 3.2V per cell. For our two-cell example battery, that would be 6.4V. But if our battery isn't balanced, it's possible for the total voltage to be above the cutoff threshold, yet still have a cell below the 3.0V danger zone. One cell could be 3.9V, while the other could be a 2.8V. That's a total of 6.7V, which means the cut-off would not engage. The vehicle would continue to operate, allowing you to further degrade the battery. That's why balancing is so important.

Proper Care & Treatment: Storage

In the old days, we used to run our cars or airplanes until the batteries died, then just set the batteries on the shelf at home, waiting for the next time we could use them. We just stored them dead. But you should not do that with LiPo batteries. Nor should LiPo batteries be stored at full charge, either. For the longest life of the batteries, LiPos should be stored at room temperature at 3.8V per cell. Most modern computerized chargers have a LiPo Storage function that will either charge the batteries up to that voltage, or discharge them down to that voltage, whichever is necessary.

Proper LiPo Storage Voltage = 3.8V per cell

The most common problem people have with LiPo batteries is a direct result of improper storage. When a LiPo battery sits for a long period of time (and not at proper storage voltage), it tends to discharge itself. If it drops below 3.0V per cell, the vast majority of LiPo chargers will not charge it. Sometimes, batteries with this problem can be rehabilitated, but just as often, they are a lost cause. 

Lithium-Polymer batteries can be damaged by sitting fully charged for as little as a week. This doesn't mean they will get damaged every time you leave them for over a week. It just means they can, and I've seen it happen. So don't forget to put your LiPos at storage voltage when you're done using them. 

LiPo Battery Disposal

Before disposing of your LiPo, check to make sure the warranty has expired. Some companies have one-year replacement warranties that you can take advantage of, but most warranties will be voided if you perform the following steps before seeking a warranty replacement.

So you have a bad LiPo battery? No one really wants to keep them around (fire hazards that they are). So what is the process to get rid of a bad LiPo battery safely? Let's go through it.

1. Discharge the LiPo battery as far down as you safely can. You can do this a number of ways. Most computerized LiPo chargers have a discharge feature in them. If you don't have a charger with a discharge feature, you can run down the battery in your vehicle - keep in mind that you risk a fire and potentially damaging your vehicle doing this, so take care to have the necessary safety equipment around. Alternatively, you can build your own discharge rig with a taillight bulb and some wire. Simply solder a male connector of your choosing to the tabs on a taillight bulb, and plug the battery in. Make sure to have the battery in a fireproof container while doing this.

2. Place the LiPo in a salt water bath. Mix table salt into some warm (not hot) water. Keep adding salt until it will no longer dissolve in the water. Ensure that the wires are all entirely submerged. The salt water is very conductive, and it will essentially short out the battery, further discharging it. Leave the battery in the salt water bath for at least 24 hours.

3. Check the voltage of the LiPo. If the voltage of the battery is 0.0V, great! Move onto the next step. Otherwise, put it back in the salt water bath for another 24 hours. Continue doing this until the battery reaches 0.0V.

4. Dispose of the battery in the trash. That's right - unlike NiMH and NiCd batteries, LiPos are not hazardous to the environment. They can be thrown in the garbage with no problem.

There are some advantages of NiMH Batteries that make them great for certain applications. In drones, most often the low weight to capacity and discharge rate make LiPo the best choice today.