LiPo Batteries and Charging for your Model RC Airplane

The batteries and electrical aspects of this course are probably the most confusing of all the materials. While it helps to have a good understanding of electrical concepts, I will try and explain it in the simplest terms possible. I might end up over-generalizing and over-simplifying a bit, but that's ok. If you have some electrical background, or even took college level physics as an elective, there will be more comprehensive sources out there for you. Many of these other sources are extremely technical, and I've found the average person has a lot of trouble understanding them ... but that's ok. You don't have to become an engineer, you just have to have a basic grasp on the material. Simply put: the goal of this section is to give you enough knowledge not to burn your house down mishandling these batteries. It will also help you avoid frying your internal electronic components, but that will be covered in more depth in the next section. Its worth mentioning that there are many different types of batteries out there, like Nickel Cadmium (NiCd), Nickel Metal Hydride, Lithium (Li) and Lithium Polymer (LiPo). Since Lithium Polymer (LiPo) batteries are the current standard for electric aircraft, this section will focus exclusively on them.

There are four main factors to be aware of regarding batteries

  1. Number of Cells (Voltage)
  2. Capacity (mAh)
  3. Discharge Rate (C-rating)
  4. The Battery Connector

The Number of Cells / Battery Voltage for LiPo Batteries

Let's start with the number of cells. The LiPo batteries used in RC airplanes are actually several batteries rolled into one. They are all hooked up in series, and the number of batteries determines the operating voltage. The nominal voltage of a single LiPo cell is 3.7v. The number of cells is designated by its "S" rating. A one cell, 3.7v battery would be classified as "1S". For batteries with more than one cell, you will see them represent as so:

  • 2S (2 Cell):  7.4v  (max 8.4v)
  • 3S (3 Cell): 11.1v (max 12.6v)
  • 4S (4 Cell): 14.8v (max 16.8v)
  • 5S (5 Cell): 18.5v (max 21.0v)
  • 6S (6 Cell): 22.2v (max 25.2v)

With all other factors being equal, the higher the voltage, the more powerful it will be. If we were using car analogies, the voltage would equate to how much horsepower the car had. Just keep in mind, we aren't actually dealing with cars. Your plane was designed to work with a specific voltage battery, and changing to a higher battery won't supercharge it. Instead, it will probably burn out the engine and the attached electronics in the process. The lesson to take away from this is that planes that use a greater number of cells (higher voltages) will tend to have higher performance. Its also very important to remember that batteries with a different number of cells are not interchangeable. They are meant to operate in different planes and need to be handled differently. IF YOU ARE GOING TO BE AWARE OF ONE THING, BE AWARE OF THE DIFFERENT BATTERY VOLTAGES. NOT PAYING ATTENTION TO VOLTAGE REQUIREMENTS CAN START FIRES AND/OR RUIN YOUR EQUIPMENT.

When you are flying, 3.7v PER CELL (not total!) is probably the minimum voltage you should reach before you should switch out the battery. At 3.7v, you have roughly 30% of your power left, and going lower than this can shorten its usable lifetime. Going below 3.0v-3.3v is even worse, and can easily damage your battery. In fact, if you go below 3.0v, most chargers will refuse to even try and charge the battery due to the dangers involved. Due to this, its important to remember never to run your batteries all the way down. When you are flying, always err on the side of coming down too early. If you wait too long, a damaged battery might be the least of your worries... once the plane loses thrust from a dead battery, you are probably going to crater it.

You can measure the voltage with a special battery meter, or you can simply buy a multi-meter that they sell in any hardware store. The battery meters will connect to the balance plugs (little rectangular connectors) and give you the voltages of each individual cell. This can be very useful in determining if there is a problem with an individual cell. Since they are really just individual batteries stitched together, one battery can definitely "go bad" before the others. This can represent a fire hazard, so a special battery meter is definitely good to have. For quick voltage measurements at the field, the multi-meter is just fine though. Stick the red probe on the positive battery connector, and the black probe in the negative connector. You won't be able to see the voltage per cell this way, but you'd get the overall voltage of the battery. Dividing this voltage by the number of cells will give you an estimate of the per cell voltage (assuiming all your cells are working properly).

LiPo batteries have max voltages of 4.2v per cell. Going beyond that amount may damage the battery or shorten its usable life. The chart at the beginning of this section shows the maximum overall voltage for each type of battery. As previously mentioned though, it is very possible for individual cells to "go bad". Its important that you not only monitor the overall voltage while charging, but also monitor the voltage per cell. If one cell ceases to hold a charge, you can potentially overcharge the other cells in the battery. There will be more on this in the charging section lower down on this page..

You will find that most average park flying foam/electric airplanes will run on a 3 cell battery. Many micro sized flyers will use 2 cell batteries, and EDF jets and high performance planes will typically use 4 cell. Some of the really fast EDF setups will use 5S or even 6S setups, but they are relatively rare. In some instances, the electronics in the plane will actually accept two different types of batteries. For instance, one model airplane might accept 2S and 3S batteries, while another might accept 3S and 4S batteries. Its very important that you check with your plane's manufacturer to determine what you can use. In situations where you use a battery with too high a voltage, its very likely that you will burn out the engine... probably in mid flight, quickly destroying your entire airplane. Both the ESC and motor need to support the number of cells (i.e. the voltage) that you plan to use with them.

Battery Capacity

The battery capacity is rate in "mAh" or milliamps per hour. Using metric conversion, it can also be displayed in Amp-hours "Ah". 1000mAh = 1Ah. You will often see battery capacity listed as something like 2200 mAh, but it can also be expressed as 2.2 Ah. Its the same thing, with different units. Most brands will use mAh as their units, so we will work with that.

Going back to the car analogy, if the number of cells/voltage was like the horsepower, the capacity/mAh is like the size of the gas tank. This rating essentially represents how long you will be able to fly. The capacity measurement scales linearly, meaning that if you buy a new battery with twice the capacity of the old one, you will fly for twice as long. This can be seen in the chart above, which compares the time each battery lasted. The ones that lasted the longest were 9000 mah batteries, and the shortest appear to be 4600-5000 mah. Its worth noting that not all batteries are created equal. Some brands will clearly outperform others, and certain cheap brands like to overstate the ratings on their batteries.

You may wonder then: why not just buy one really big battery and fly for a long time instead of a bunch of little batteries. The simple answer is: weight. You can technically get as high of a capacity as you want, it will never harm the electronics in the RC airplane. Just keep in mind that higher capacity batteries have more of the LiPo gel inside of them to store the extra energy. This adds weight, and foam aircraft need to stay relatively light. Usually, the battery is the heaviest thing in the plane, by far, even at low capacities. Once you get too heavy you will fly slower, it will become very easy to stall and you may lose control of your aircraft. For this reason, I suggest you try to keep the size of the battery reasonable. If your airplane comes with a 1800 mAh battery, don't put a 4400 mAh battery in it. You might try a 2200 mAh or 2650 mAh, but be very careful before you go higher.

The Mysterious Discharge Rate (C rating)

The discharge rate, or C rating, is not as complicated as everyone makes it out to be. Let's do a quick review on capacity first. The capacity of every battery is rated in mAh or milliamps per hour, and it indicates how much energy is stored in the pack. Take notice of the fact that units of capacity also specify a period of time. Its not in milliamps - its in milliamps per hour. Now, hold that thought for a second as we go back to the discharge rate. The discharge rate is defined at how fast the battery can empty itself. You'll see it expressed in ratings like 10C, 20C, 30C. Now the C actually stands for capacity, referring to the mAh rating of the battery. Putting this all together, a 1C rating implies that your 2200 mAh battery can discharge the 2200 mA in no faster than 1 hour. At a sustained rate of 2.2A per hour, that is actually a pretty slow release of energy. What if we wanted to release that 2200 mAh faster? Say, in 6 minutes... Well, a 10C battery will discharge at 10 times the batteries capacity rating. Since this battery can dish out a current that is 10 times higher than its capacity rating, it will only last one tenth of the time the capacity is rated for. In other words, a 2200mAh battery discharging at the full 10C will last only 6 minutes. If we double that to 20C, we see that we can exhaust all of the energy in the battery in about 3 minutes. Now we are starting to roll! That is quite a lot of power in a short period of time. The EDF jets will actually draw this much current, and will consume a constant 40A-50A of power. A 2200 mah battery at 20C will barely keep up in this type of setup and it will be nearly drained in that 3 minutes.

Now keep in mind, this fast discharge is not necessarily what the battery WILL DO, its what it CAN DO. With a 30C battery, you will be able to output a lot of power at once, but it doesn't mean it has to discharge anywhere close to its maximum rate. How fast your battery dishes out the power is completely dependent on how much power the motor tries to draw from it. Think about it - how much energy is your battery discharging when its disconnected? None. Its perfectly happy not releasing any energy. Its usually best if your battery never reaches its maximum discharge rate. In fact, the smart thing to do is to buy a battery with a higher C rating than you need. This not only ensures that you will have the power you need, but it also prevent the battery from being overtaxed (and damaged) by a power hungry motor. If you buy too high of a C rating, you will NOT do any harm to your plane. You want to have C's to spare. The only drawback a high C rating, aside from cost, is that more C's = more weight. I personally like to give a decent amount of overhead, so I'd recommend 20C batteries for slower park flyers and 30C batteries for higher performance airplanes like EDF jets. If you have several different types of planes, or plan to upgrade, its smarter to just buy 30C batteries. It doesn't increase weight as much as a higher capacity battery would, and you don't have to buy new batteries when you upgrade airplanes.

If we were to return to our car analogy one more time, the C rating would be similar to how fast the car's fuel pump could supply fuel to the engine. Bigger fuel tanks would naturally come with bigger & better fuel pumps.. but whenever you bought a fuel tank of a certain size, you'd have the option of several different speeds of fuel pumps. Faster fuel pumps would be able to supply the faster engines with the gas they need, while slower ones would not be able to help the engine fully utilize its potential. If your fuel pump was too slow, it might even burn out (i.e. damage the battery) trying to keep up with the engine. If your fuel pump was faster than your engine needed (maybe you have a slow engine), then the fast fuel pump would simply be underutilized. Nothing bad would happen, other than the fact that you've added a little extra weight to your car.

If you are still confused, that is ok. Here are the important points to take away: The higher the C rating, the more power your battery can put out per second. This means you can power faster equipment. Again, it does NOT mean the battery HAS TO put out more power, only that it can put out the power if it needs to. Higher C ratings also mean more weight to your plane, but the difference in weight will usually not be extreme. You may want to take note that higher capacity batteries don't need to have nearly as high of a C rating. A 40C 1100mah battery will be able to sustain the same power output as a 10C 4400mah, just for a much shorter time period. The math: 40C x 1100 mah = 44Ah ; 10C x 4400 mah = 44Ah . This means that if you have a bigger battery then you don't need to care about the C rating as much. On the other hand, with very small batteries, it will be very easy to under power your engine with a low C rating!

Battery Connectors


Without a doubt, one of the biggest pains of RC batteries are the varying connectors. I really wish there was a standard, but unfortunately there is not. The connector we are dealing with here is the primary connector for the battery. It is what you will use to supply the plane with power. In the past, the deans connector was very popular (image on the left), but they seem to have fallen out of favor. The reason is mostly due to the fact that Deans connectors are technically only rated for 50A, making it a poor choice for high power applications. Many EDF jets will easily pull 50A, if not more. Deans connectors are also more difficult to solder to low gauge wire. The EC3 (shown in the center photo),EC4 and EC5 connectors are still very popular and can be found on a lot of parkzone and hobbyzone planes. In the image on the far right, you will see 4mm bullet connectors, which are also very popular today.

The XT60 connector, pictured above is a relatively new connector that is often featured on HobbyKing products. Most people feel its very easy to work with and provides a good connection for the battery. Believe it or not, there are actually many more types of battery connectors. Due to this, its incredibly likely that at some point you will want to buy a plane that uses a different connector than your batteries. When this happens you have two choices: buy new batteries with the right connector OR learn to solder new connectors onto the plane's wiring. I actually prefer to standardize all of my planes with the same connector, and have soldered the 4mm bullet connectors on all of the connections. Soldering does take some practice, so you might want to buy a few extra connectors and do some test runs on some spare wire before you do anything to your plane.

Fortunately, there is one connector that is mostly standardized across the batteries you buy. The JST-XH balance connector (shown above) is usually found on the majority of LiPo batteries. There are other types of balance connectors, but I've only seen a few airplane batteries that carry them. So, what is a balance plug? The balance plug is used exclusively during the charging process as a way to ensure all cells inside the battery are charged evenly. Its also the preferred connection for battery meters, as it provides a direct measurement of the voltage for each cell. The JST-XH balance connectors will vary in size, depending on how many cells you have. A 2S battery will have a 3 pin connector, while a 6S batter will have a 7 pin connector.

Charging The Battery

Charging the battery is one of the most dangerous things about this hobby. The goal here is to educate you enough to ensure that you are fully capable of charging your batteries without starting a fire. If you charge your battery at the wrong voltage, or charge it too fast, then its very possible that your battery will explode. While this explosion won't be the C4 induced kind you see in the movies, there will definitely be flames that could start a major fire in your house. Here is a good video on youtube, illustrating what might happen. We will revisit battery safety after this section, so lets return to going over how to charge your battery.

The first thing we are going to do is to plug in your charger to a power source. This might be a car battery, or it could be a DC adapter. Some chargers even plug directly into an AC wall socket. Once this is done, we are going to connect both the balance charger board and the battery connector. The balance board is shown in the picture above, towards the top right. As you can see, it will have a JST-XH balance connector for batteries of varying cell size. The charger will often come with several different battery connectors, and you will have to find the one that matches your battery. In the picture, we see the battery connector on the bottom right.

The battery connector will always have a red and a black wire that needs to be plugged into the charger. Its very important that you plug this connector into the charger BEFORE you connect the battery. If you connect the battery first, then there is a chance the red and black connectors may touch. This would short the battery, and potentially start a fire. After you are done charging, the same principle applies. You'll want to remove the battery before disconnecting these wires from the charge. If you remove the wires from the charger first, they will still be connected to the battery and again have the opportunity to make contact with one another.

So once you have all your connectors in, now plug in the battery. You should connect both the main battery connector and the balance plug. Technically, you can charge the battery without the balance plug, but there is little reason to do this. Using the balance plug and the "balance charge" option on your charger, the charger will ensure that all cells are evenly charged. If you do not use the balance plug, its entirely possible for the cells to charge unevenly. Without the balance plug, the charger is only trying to reach the desired max voltage. If you have a cell that is damaged and won't charge, then your 3S battery might charge with cell voltages like this: (5.0v, 3.0v, 5.0v) instead of this: (4.2v, 4.2v, 4.2v). Overcharging the cells could lead to a fire, so for this reason I always recommend that you charge using the balance plug.

When we go into the charger menu you will be prompted with two variables: Voltage/Cell Count and Amperage. The voltage/cell count sets the voltage you will charge your LiPo battery to. If this is set too high, it will start a fire. Setting it too low will probably just result it the charger thinking its finished as soon as it starts. The main point to take away is to make sure you choose the right number of cells. Good chargers will perform a check for you, but its best not to rely on that.

The second option is the Amperage. This determine how fast your battery will charge. The setting for this should be the same (or lower) than the capacity of your battery. If you have a 2200 mAh battery, you should charge at 2.2 Ah (remember that 1 Ah = 1000 mAh). If you don't understand the conversion, you should just try to remember that you should divide the number on the battery by 1000. Using too high an amperage during charge can result in damaging the battery or a fire. Its ok to use a number that is lower than your capacity rating. In fact, its actually better for your battery. If you charge at full capacity, the charger will take anywhere from 45 minutes to 1 hour to charge. Chargers usually beep when they are done, and then stop charging... but you should not assume this will happen properly every time.

You may have noticed I mentioned the word fire a lot. It can happen very easily, and the last thing you want is to endanger lives and property. For this reason, its a good idea to keep a watchful eye on the batteries while they are charging. In most cases, its not necessary to sit there and stare at it, but its a good idea to check on it every few minutes. If you insist on leaving it unattended, I recommend you do it outside and away from anything flammable (just watch out for rain!). When charging inside, I recommend you place your batter inside a container - preferably something flame retardant. I keep mine in a LiPo bag, inside a metal pot. Others have used ceramic pots, and even bags of sand.

After you fly your plane, and come home with a mostly dead battery, your work is not yet done. To properly preserve your battery, you now must put it to its optimum storage capacity of 3.8v. Almost all chargers have a "LiPo Storage" function, and it operates almost identically to that of the charging function. Connect the battery the same way you do when you charge it. Like with charging, you must select voltage/cell count and amperage. The voltage must match that of your battery, otherwise you are risking a fire. The amperage setting operates a little different in storage mode, as you will usually be limited to about 1A. This means it will generally be a slower process. What storage mode will do is either raise or lower the voltage of your battery to 3.8v per cell. If you came from the field with an empty battery, storage mode will charge each cell to 3.8v and then stop. If you have a fully charged battery you didn't use, storage mode will discharge each cell in the battery until it reaches 3.8v, then it will stop.

LiPo Handling and Safety

While we covered most of the safety tips related to charging, lets now go over some other general information about safely handling batteries. One of the most important things is to have your LiPo in some sort of protective case/container at all times. There are many different bags out there made specifically for LiPo batteries. I often use these bags when transporting the batteries to and from the flying field. You should be especially careful with batteries that have been involved in a crash. Its very easy to damage the cells this way, and the damage may not be readily apparent. Take extra precaution in this case, and ensure your battery is properly contained. Its also a good idea to monitor the cells carefully next time you put it in the charger.

When the LiPo aren't being used, its still a good idea to keep them protected. I use a combination of a LiPo bag and a pressure cooker to store my batteries. As mentioned, other people use clay pots, fire proof safes, and bags of sand to store their LiPo's. Keep in mind, that if one battery spontaneously goes up in flames, any adjacent batteries will surely catch fire too. That being said, the chance of starting a fire with stationary LiPo's at their storage voltage (3.8v) is incredibly low. Just remember to keep them away from any heat sources, like fireplaces or your kitchen stove.

Just a quick recap on LiPo safety:
  • Never overcharge your battery
  • Never drain your battery all the way down
  • When flying, try to monitor the voltage of all the cells to prevent damage
  • Remember to never let the charging connector leads touch when your battery is connected
  • Only use chargers meant for LiPo batteries
  • Try to use a safe and quality charger
  • Charge your battery at the right voltage
  • Charge your battery at the right charge rate (the capacity rating), or lower
  • Use the balance charging feature
  • Stay near the battery while it charges
  • Use safe containers or bags that would help contain a fire if it should happen
  • Have a fire extinguisher handy
  • Use the storage mode of your charger before you store your batteries - don't leave it charged or drained for extended periods of time
  • Keep LiPo's away from heat sources.
  • Be smart and responsible with them!

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Supplemental Videos From the Web

LiPo Fire Risks

If you follow proper safety precautions, LiPo batteries are perfectly safe. This is an example of just how dangerous LiPo batteries can be if they are misused.

Using your LiPo Charger

This video shows you how to use the Thunder Power AC6 charger, which happens to use the same programming as many other chargers. This is a good walk through of the proper charging procedure.

Soldering on New Connectors for your Battery/ESC

Here we see how to add a new connector to your battery or ESC. If you solder a new connector on your battery, its important that you secure the wire you aren't working on. You do not want the two exposed battery wires touching each other. Also, make sure you solder and install the connector with the right orientation - there is only one way it should go on..

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