Lithium Batteries and Chargers
Battery technology has improved so much in the last 10 years that most RC flyers have switched to electric planes. The standard battery of the industry is Lithium-Polymer (Li-Po, or “lipo”) batteries. They have drastically dropped in price over the last 3 years. Lipo batteries that used to cost $50 now cost less than $15.
Lipo batteries store a lot more power for their size than Nicads or NiMH batteries and are able to release it at higher amperage giving you more available power. These are not like a household battery. You need to understand how they work to be safe. Do not let your lipo battery voltage get too low or store the battery fully charged or the battery life will be seriously affected. Most ESCs are designed to prevent over discharge of your batteries.
Lipos come in all sorts of sizes and cell counts. The size of battery, type of motor, and style of plane determine how many amps the system will draw. Electronic speed controllers are rated for different max amperage, so you need to make sure your combination of battery, ESC, motor and servos are all rated to work together.
How many batteries do you need? Most flyers like to have at least 3-5 batteries. Flying time on each battery depends on the size of your battery and how much amperage your motor uses. Charging time is based on the type of charger and C-rating of the batteries you are using. Some flyers recharge some of their batteries at the flying field or off their car battery while they are flying so they get by with fewer batteries. Recharging a hot battery right after flying reduces battery life but if you are careful you can use a single battery several times a day. If you have 3-5 batteries and a good field charger you can fly continuously while batteries are recharging.
Battery Ratings
Lithium batteries have multiple cells, just like AA or C batteries. The differences are that the lipos have 3.7 volts per cell, rather than 1.5, and they have the ability for a much higher discharge rate for the same weight. Remember weight is everything on a plane. The number of cells on a lipos is the “S” rating. For example, a 3S lipos have three cells and 11.1 volts (3.7 x 3).
The entire battery will also have a capacity rating, measured in milliamp-hours (mAh). This is the load that could (hypothetically) be placed on the battery so that it would be fully discharged after one hour. 1 amp = 1000 milliamps, so for a 2200 mAh battery, it would take one hour’s worth of a 1.2 amp draw to discharge the battery.
Third, lipos have a “C” rating which tells us how fast they can be discharged safely. The C rating is a multiple of the capacity, so a 20C battery can be discharged at 20 times the capacity (mAh) of the battery. For example, a 1500 mAh 20 C battery could produce 30 amps (30,000 milliamps) of current. A 1500 mAh 40 C battery could produce 60 amps of current. This does not mean you want to run a battery at its maximum output for a whole flight. If you do your battery may overheat and may be damaged. Always check your battery temperature when you land to see how hot the battery is. If your battery is too hot you either need to use a smaller propeller or a bigger battery.
If all of this is slightly confusing to you, don’t worry. You will learn more over time, but for now, read the rest of this article so you know how to safely use lipos, then follow the recommendations for your specific plane, so you know which battery, motor, and ESC to buy. Just remember, some less expensive batteries are heavier or have a lower C rating, so they aren’t as good of a deal as they may look. Compare the ratings before you buy.
A 3.7 V lipo cell will actually give an open circuit voltage reading of 4.2 V when you test them with a battery monitor, like the one we recommend from Hobby King. So, a fully charged 3S lipo will have a total voltage of 12.6 V. The battery monitor helps you to see if one of the cells is damaged by giving a voltage reading for each cell in the battery.
You never want to discharge your batteries too far. A reading of 3.2 V per cell is usually as far as you should go. Some chargers have a safety setting that won’t even allow you to charge a battery below a certain voltage, so be careful not to go past the point of no return. A dead lipo battery cannot be saved.
Get a battery charger with built in battery balancer and discharging circuit.
I did a review of the IMAX B6AC lipo charger that has a built in power supply for a friend.
Most quality chargers will also balance the cells on the battery while it charges. If you have a three cell battery it will make sure all of the cells have the same charge before it is done. This is a very nice feature to have, so check before buying. If yours is not a balance charger, you’ll need a separate balancer.
There are also chargers that discharge your batteries to a safe storage voltage. Using this feature will prolong the life of your batteries. I have ruined batteries before by leaving them fully charged for too long. It is best to charge batteries before you fly not after you have flown. It is best to store your batteries with the voltage about 3.8 volts per cell.
There are a lot of different chargers out there. Many of them only charge one battery at a time. If I had 10 batteries, I would be charging all day. I have gone with the multi-chargers that will charge 4 batteries at once and have not been disappointed;
There are some AC/DC charger/balancers that have their own power supply build in, but most chargers require a stable 13.8 volt to 18 volt DC power supply which normally isn’t built in with the charger. Check to see if the charger’s manufacturer sells a matching power supply, or at least recommends one. One advantage to a separate power supply is that you can charge from your car battery in the field. Make sure the car is running while you charge, though. It is the same as leaving a light on in the car, it runs the battery down.
Each cell of the battery needs to be balance. Balancing lipos helps to prolong their life. Batteries produce power with a chemical reaction, and chemical reactions are always sensitive to heat. The middle cell of a 3 cell lipo will run hotter because it can’t ventilate. Temperature changes the power draw on the cell so the cells are producing slightly different amounts of power. When you charge the battery, the different cells at different voltages will respond differently to the charge, increasing the problem. A battery balancer corrects the cell voltage after use. When you plug your battery into the balancer, it returns the cells to identical voltages.
Electronic Speed Controllers (ESCs)
An ESC does several things: It plugs into the battery and provides power to both the radio and and motor. It watches over the battery and is designed to cut the power to protect the battery from over discharge, which can ruin a lipo battery. It also cuts the power to the motor before the power drops so low the radio won’t work. It also helps protect the pilot from unwanted accidental start ups when the radio is turned on. Lastly, it controls the speed of the motor by translating the movement in your radio’s throttle into rapidly changing polarity of magnets that surround the motor.
You need to know the power limits of each of the components in your power system, so you don’t exceed their limits. Each battery, ESC and motor has a rating that tells you how much power it can handle. You have to base the power you are using on the lowest rated part of your system.
You need to trim in the ESC just like you trim in other components of the radio. If the ESC is beeping trim the throttle down on the transmitter till it stops. If your prop starts to turn when you turn on your radio you need to trim the throttle down. If your ESC and throttle aren't trimmed properly you won't be able to get your maximum RPM because the ESC will not go full throttle.
Each ESC is rated for a different maximum amperage. If you are flying a low output motor you may only need a 25 A ESC. If you are planning on getting a high output motor for your flying wing, you may want to get a 35 or 45 amp ESC for the same plane. You can use a big ESC to power a small plane, but an ESC that’s too small cannot fly a big plane. Many less expensive brands of ESCs are appearing on the market, and in most cases they seem to be working well. I recommend you read the reviews on any motor, battery, or ESC before you buy. There are too many to make a list here. The other customers will tell you when they don’t like something.
After all of the research and advice, it all comes down to the propeller. We adjust the amperage the system will use by the size and pitch of the propeller we put on our motors. Small propellers don’t take much power to turn. If we use too big of a propeller, we will love the extra power but sooner or later it will destroy, kill, maim, or greatly reduce the lifespan of your battery, ESC, and motor. If you are using too many amps, or your batteries are getting hot, try a smaller propeller.
Multiple Batteries
Lipos also can be wired in series or parallel. In series will double your voltage, while parallel will keep the voltage the same, but double the capacity. Occasionally I will fly with two 3S batteries in parallel to give me longer flight times without changing the voltage supplied to my motor/ESC.prop combo. You can buy or make plugs that will connect two batteries to one ESC battery connector in series or parallel.
Here is a list of a few things I do to make sure my lipo batteries are safe and maintained properly:
I have charged thousands of batteries and never had a fire but I am careful.
I design and build my EPP planes to protect the batteries in a crash.
I don’t run them hot by drawing too many amps with big motors and props that overheat the batteries or ESCs .
I have my ESC set so they won’t drop below a dangerous voltage level.
I remove the battery from the plane at the field, so they don’t accidentally get left plugged in and die.
I avoid charging my batteries off my car battery, even though I am set up to do so.
I check the batteries voltage when I get home and store them from 3.7 to 3.9 volts a cell to extend their life.
I wait for lipos to cool before charging. I don’t charge hot lipo batteries.
I use a charger made to charge and balance lipo batteries, set at a slower charge rate.
I always charge in a battery bunker in case a battery decides to burn while charging.
I triple check my chargers, and set safety features to alarm if batteries are not charging normally.
I balance my lipo cells every charge.
I don’t over-charge or over-discharge.
I charge multiple batteries at once so I can stay with them. This is my build time.
I keep my chargers 2 feet from the batteries with wire extensions.
I have no combustible materials near the charging batteries.
I have a fire alarm specifically for the chargers mounted directly above the charge site.
I have a fire extinguisher just outside the room where I charge the batteries.
I throw away damaged lipo batteries. (More details later)
Lipo Fires
I am aware of at least 5 house fires in Utah that started while charging batteries. I have never had a battery burn unintentionally, but I put my batteries in a bunker and treat them as if they might burn every time I charge them. You’ve heard about computer batteries catching fire? Well, the same problem exists with model airplane batteries. I recommend that you use cinder blocks as a battery bunker. They are cheap and can easily handle the heat if there is a battery problem. I still charge in my house, but I have a set up that will contain any fire that could occur. I have charged thousands of times over the years and have never had a battery catch fire on the charger. Part of that is because I am meticulous on battery care and dispose of questionable batteries. Most charging problems happen on batteries that were already known to have damage or cells that wouldn’t balance. Don’t take the risk of using damaged batteries.
I’m a “Mythbusters” fan, and I thought it would be a good project to separate myth from truth about the lipos. I have heard a lot of different things, most of which are based on some truth. The obvious way to prevent lipo fires is by being careful, but what happens that one time in 10,000 that being careful isn’t enough?
To test this out I got some used lipos that could be destroyed for the sake of science (and fun) and asked around to see what bunkers people were using so I could test a few of the ideas. Our local club tends to be pleasantly paranoid about the dangers and have been quite willing to offer suggestions and donate older batteries rather than throwing them away.
I have heard about people charging lipos in bath tubs, showers, buckets, fish tanks, cinderblocks, brinks boxes, amo cans, metal garbage cans, and in planes. Did I miss any?
Wouldn’t it be fun to try all of them and make a video while doing it? Well, we changed the settings on a charger to force a fire on purpose. Here are some of the results so far:
Lipo batteries store a lot more power for their size than Nicads or NiMH batteries and are able to release it at higher amperage giving you more available power. These are not like a household battery. You need to understand how they work to be safe. Do not let your lipo battery voltage get too low or store the battery fully charged or the battery life will be seriously affected. Most ESCs are designed to prevent over discharge of your batteries.
Lipos come in all sorts of sizes and cell counts. The size of battery, type of motor, and style of plane determine how many amps the system will draw. Electronic speed controllers are rated for different max amperage, so you need to make sure your combination of battery, ESC, motor and servos are all rated to work together.
How many batteries do you need? Most flyers like to have at least 3-5 batteries. Flying time on each battery depends on the size of your battery and how much amperage your motor uses. Charging time is based on the type of charger and C-rating of the batteries you are using. Some flyers recharge some of their batteries at the flying field or off their car battery while they are flying so they get by with fewer batteries. Recharging a hot battery right after flying reduces battery life but if you are careful you can use a single battery several times a day. If you have 3-5 batteries and a good field charger you can fly continuously while batteries are recharging.
Battery Ratings
Lithium batteries have multiple cells, just like AA or C batteries. The differences are that the lipos have 3.7 volts per cell, rather than 1.5, and they have the ability for a much higher discharge rate for the same weight. Remember weight is everything on a plane. The number of cells on a lipos is the “S” rating. For example, a 3S lipos have three cells and 11.1 volts (3.7 x 3).
The entire battery will also have a capacity rating, measured in milliamp-hours (mAh). This is the load that could (hypothetically) be placed on the battery so that it would be fully discharged after one hour. 1 amp = 1000 milliamps, so for a 2200 mAh battery, it would take one hour’s worth of a 1.2 amp draw to discharge the battery.
Third, lipos have a “C” rating which tells us how fast they can be discharged safely. The C rating is a multiple of the capacity, so a 20C battery can be discharged at 20 times the capacity (mAh) of the battery. For example, a 1500 mAh 20 C battery could produce 30 amps (30,000 milliamps) of current. A 1500 mAh 40 C battery could produce 60 amps of current. This does not mean you want to run a battery at its maximum output for a whole flight. If you do your battery may overheat and may be damaged. Always check your battery temperature when you land to see how hot the battery is. If your battery is too hot you either need to use a smaller propeller or a bigger battery.
If all of this is slightly confusing to you, don’t worry. You will learn more over time, but for now, read the rest of this article so you know how to safely use lipos, then follow the recommendations for your specific plane, so you know which battery, motor, and ESC to buy. Just remember, some less expensive batteries are heavier or have a lower C rating, so they aren’t as good of a deal as they may look. Compare the ratings before you buy.
A 3.7 V lipo cell will actually give an open circuit voltage reading of 4.2 V when you test them with a battery monitor, like the one we recommend from Hobby King. So, a fully charged 3S lipo will have a total voltage of 12.6 V. The battery monitor helps you to see if one of the cells is damaged by giving a voltage reading for each cell in the battery.
You never want to discharge your batteries too far. A reading of 3.2 V per cell is usually as far as you should go. Some chargers have a safety setting that won’t even allow you to charge a battery below a certain voltage, so be careful not to go past the point of no return. A dead lipo battery cannot be saved.
Get a battery charger with built in battery balancer and discharging circuit.
I did a review of the IMAX B6AC lipo charger that has a built in power supply for a friend.
Most quality chargers will also balance the cells on the battery while it charges. If you have a three cell battery it will make sure all of the cells have the same charge before it is done. This is a very nice feature to have, so check before buying. If yours is not a balance charger, you’ll need a separate balancer.
There are also chargers that discharge your batteries to a safe storage voltage. Using this feature will prolong the life of your batteries. I have ruined batteries before by leaving them fully charged for too long. It is best to charge batteries before you fly not after you have flown. It is best to store your batteries with the voltage about 3.8 volts per cell.
There are a lot of different chargers out there. Many of them only charge one battery at a time. If I had 10 batteries, I would be charging all day. I have gone with the multi-chargers that will charge 4 batteries at once and have not been disappointed;
There are some AC/DC charger/balancers that have their own power supply build in, but most chargers require a stable 13.8 volt to 18 volt DC power supply which normally isn’t built in with the charger. Check to see if the charger’s manufacturer sells a matching power supply, or at least recommends one. One advantage to a separate power supply is that you can charge from your car battery in the field. Make sure the car is running while you charge, though. It is the same as leaving a light on in the car, it runs the battery down.
Each cell of the battery needs to be balance. Balancing lipos helps to prolong their life. Batteries produce power with a chemical reaction, and chemical reactions are always sensitive to heat. The middle cell of a 3 cell lipo will run hotter because it can’t ventilate. Temperature changes the power draw on the cell so the cells are producing slightly different amounts of power. When you charge the battery, the different cells at different voltages will respond differently to the charge, increasing the problem. A battery balancer corrects the cell voltage after use. When you plug your battery into the balancer, it returns the cells to identical voltages.
Electronic Speed Controllers (ESCs)
An ESC does several things: It plugs into the battery and provides power to both the radio and and motor. It watches over the battery and is designed to cut the power to protect the battery from over discharge, which can ruin a lipo battery. It also cuts the power to the motor before the power drops so low the radio won’t work. It also helps protect the pilot from unwanted accidental start ups when the radio is turned on. Lastly, it controls the speed of the motor by translating the movement in your radio’s throttle into rapidly changing polarity of magnets that surround the motor.
You need to know the power limits of each of the components in your power system, so you don’t exceed their limits. Each battery, ESC and motor has a rating that tells you how much power it can handle. You have to base the power you are using on the lowest rated part of your system.
You need to trim in the ESC just like you trim in other components of the radio. If the ESC is beeping trim the throttle down on the transmitter till it stops. If your prop starts to turn when you turn on your radio you need to trim the throttle down. If your ESC and throttle aren't trimmed properly you won't be able to get your maximum RPM because the ESC will not go full throttle.
Each ESC is rated for a different maximum amperage. If you are flying a low output motor you may only need a 25 A ESC. If you are planning on getting a high output motor for your flying wing, you may want to get a 35 or 45 amp ESC for the same plane. You can use a big ESC to power a small plane, but an ESC that’s too small cannot fly a big plane. Many less expensive brands of ESCs are appearing on the market, and in most cases they seem to be working well. I recommend you read the reviews on any motor, battery, or ESC before you buy. There are too many to make a list here. The other customers will tell you when they don’t like something.
After all of the research and advice, it all comes down to the propeller. We adjust the amperage the system will use by the size and pitch of the propeller we put on our motors. Small propellers don’t take much power to turn. If we use too big of a propeller, we will love the extra power but sooner or later it will destroy, kill, maim, or greatly reduce the lifespan of your battery, ESC, and motor. If you are using too many amps, or your batteries are getting hot, try a smaller propeller.
Multiple Batteries
Lipos also can be wired in series or parallel. In series will double your voltage, while parallel will keep the voltage the same, but double the capacity. Occasionally I will fly with two 3S batteries in parallel to give me longer flight times without changing the voltage supplied to my motor/ESC.prop combo. You can buy or make plugs that will connect two batteries to one ESC battery connector in series or parallel.
Here is a list of a few things I do to make sure my lipo batteries are safe and maintained properly:
I have charged thousands of batteries and never had a fire but I am careful.
I design and build my EPP planes to protect the batteries in a crash.
I don’t run them hot by drawing too many amps with big motors and props that overheat the batteries or ESCs .
I have my ESC set so they won’t drop below a dangerous voltage level.
I remove the battery from the plane at the field, so they don’t accidentally get left plugged in and die.
I avoid charging my batteries off my car battery, even though I am set up to do so.
I check the batteries voltage when I get home and store them from 3.7 to 3.9 volts a cell to extend their life.
I wait for lipos to cool before charging. I don’t charge hot lipo batteries.
I use a charger made to charge and balance lipo batteries, set at a slower charge rate.
I always charge in a battery bunker in case a battery decides to burn while charging.
I triple check my chargers, and set safety features to alarm if batteries are not charging normally.
I balance my lipo cells every charge.
I don’t over-charge or over-discharge.
I charge multiple batteries at once so I can stay with them. This is my build time.
I keep my chargers 2 feet from the batteries with wire extensions.
I have no combustible materials near the charging batteries.
I have a fire alarm specifically for the chargers mounted directly above the charge site.
I have a fire extinguisher just outside the room where I charge the batteries.
I throw away damaged lipo batteries. (More details later)
Lipo Fires
I am aware of at least 5 house fires in Utah that started while charging batteries. I have never had a battery burn unintentionally, but I put my batteries in a bunker and treat them as if they might burn every time I charge them. You’ve heard about computer batteries catching fire? Well, the same problem exists with model airplane batteries. I recommend that you use cinder blocks as a battery bunker. They are cheap and can easily handle the heat if there is a battery problem. I still charge in my house, but I have a set up that will contain any fire that could occur. I have charged thousands of times over the years and have never had a battery catch fire on the charger. Part of that is because I am meticulous on battery care and dispose of questionable batteries. Most charging problems happen on batteries that were already known to have damage or cells that wouldn’t balance. Don’t take the risk of using damaged batteries.
I’m a “Mythbusters” fan, and I thought it would be a good project to separate myth from truth about the lipos. I have heard a lot of different things, most of which are based on some truth. The obvious way to prevent lipo fires is by being careful, but what happens that one time in 10,000 that being careful isn’t enough?
To test this out I got some used lipos that could be destroyed for the sake of science (and fun) and asked around to see what bunkers people were using so I could test a few of the ideas. Our local club tends to be pleasantly paranoid about the dangers and have been quite willing to offer suggestions and donate older batteries rather than throwing them away.
I have heard about people charging lipos in bath tubs, showers, buckets, fish tanks, cinderblocks, brinks boxes, amo cans, metal garbage cans, and in planes. Did I miss any?
Wouldn’t it be fun to try all of them and make a video while doing it? Well, we changed the settings on a charger to force a fire on purpose. Here are some of the results so far:
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April 1, 2007 – I just saw a garage that had $25,000+ of damage from a local flyer that had a fire while fast-charging a NiMH battery without a bunker. I believe a simple cinder block bunker would have been able to contain the fire.
July 4th 2008 – “Hot Pants” new definition. This past 3 weeks I have had two different flyers show me lipo batteries that have had balancer wires melt while being carried in a pocket. This concerns me because I often carry batteries in my pockets with my car keys in the same pocket.
In one instance one of the wires melted when the micro deans plug he was using came in contact with one of the tiny but visible wire connector in the balancer plug. We could tell that it had made contact with the micro deans because it had burned the end on the plug too.
The other melted coming in contact with car keys or something else in the pocket and melted two wires. I got looking at my battery balancing plug and the exposed wire connections. Shoe Goop is fairly liquid, dries fast and easy to work with, so I put a thin layer over the exposed connectors on the side of the balancing plugs, making sure not to get any in the plug. I wiped off the excess so that the plugs would still fit in the balancer.
Disposal
To dispose of old lipos batteries, soak them overnight in salt water to fully discharge them, and then punch holes in them, soak them some more, then throw them in the regular trash.
-Lee
July 4th 2008 – “Hot Pants” new definition. This past 3 weeks I have had two different flyers show me lipo batteries that have had balancer wires melt while being carried in a pocket. This concerns me because I often carry batteries in my pockets with my car keys in the same pocket.
In one instance one of the wires melted when the micro deans plug he was using came in contact with one of the tiny but visible wire connector in the balancer plug. We could tell that it had made contact with the micro deans because it had burned the end on the plug too.
The other melted coming in contact with car keys or something else in the pocket and melted two wires. I got looking at my battery balancing plug and the exposed wire connections. Shoe Goop is fairly liquid, dries fast and easy to work with, so I put a thin layer over the exposed connectors on the side of the balancing plugs, making sure not to get any in the plug. I wiped off the excess so that the plugs would still fit in the balancer.
Disposal
To dispose of old lipos batteries, soak them overnight in salt water to fully discharge them, and then punch holes in them, soak them some more, then throw them in the regular trash.
-Lee
