Read full article here: http://www.rchelicopterfun.com/rc-lipo-batteries.html
LiPo RC Battery Ratings
Now that I have bored you to death on RC LiPo battery basics, time to get into the main topics at hand. First are ratings, specifically voltage and capacity. These are the two main numbers you will need when going battery shopping.. There is a third number you will also need to be aware of which I will get to in just a bit.
Unlike conventional NiCad or NiMH battery cells that have a voltage of 1.2 volts per cell, LiPo battery cells are rated at 3.7 volts per cell. The benefit here is fewer cells can be used to make up a battery pack and in some cases on smaller micro sized RC aircraft like Blade mCX's, mSR/X, mCPX, or 120SR one 3.7 volt cell is all that is needed to power the model.
Other than the smallest of electric RC models, RC LiPo battery packs will have at least two or more cells hooked up in series to provide higher voltages. For larger RC models that number can be as high as 6 cells and even more for larger birds or HV (high voltage) applications. Here is a list of LiPo RC battery pack voltages with cell counts. If you are wondering what the 2-12S in parenthesis means; it is a way the battery manufacturers indicate how my cells hooked in series(S) the battery pack contains.
- 3.7 volt battery = 1 cell x 3.7 volts (1S)
- 7.4 volt battery = 2 cells x 3.7 volts (2S)
- 11.1 volt battery = 3 cells x 3.7 volts (3S)
- 14.8 volt battery = 4 cells x 3.7 volts (4S)
- 18.5 volt battery = 5 cells x 3.7 volts (5S)
- 22.2 volt battery = 6 cells x 3.7 volts (6S)
- 29.6 volt battery = 8 cells x 3.7 volts (8S)
- 37.0 volt battery = 10 cells x 3.7 volts (10S)
- 44.4 volt battery = 12 cells x 3.7 volts (12S)
I should point out you may run across packs or cells hooked up in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 2S2P would indicate two, two celled series packs hooked up in parallel to double the capacity (2S2P is actually a popular configuration in high capacity LiPo receiver packs).
So, those are the voltages you need to know and each RC model or more specifically, the motor/speed controller combination will indicate what voltage is required for correct operation/RPM. This number has to be followed to the letter in most cases since a change in voltage equates to a change in RPM and will require changing the gearing but more likely the motor to a higher or lower Kv rating - not something I want to get into in this write-up. If a model calls for a 3 cell (3S) 11.1 volt battery – lets just say that is what has to be used unless you want to open a whole new can of worms.
A quick word on motor ratings...
Many people new to electric flight get confused by brushless electric motor ratings, specifically the Kv rating thinking Kv = kilo-volts (1 kV = 1000 volts). This is not the case at all. The Kv rating of a brushless motor refers to how many RPM it turns per volt. An example might be something like a 1000 Kv motor with a voltage range of 10 - 25 volts. That would mean this motor will turn at about 10,000 RPM @ 10 volts up to around 25,000 RPM @ 25 volts.
I don't want to start into motor ratings; battery ratings are plenty to get through... I just thought I would make mention of it since I do get that "Kilo-Volt" question often.
Capacity indicates how much power the battery pack can hold and is indicated in milliamp hours (mAh). This is just a fancy way of saying how much load or drain (measured in milliamps) can be put on the battery for 1 hour at which time the battery will be fully discharged.
For example a RC LiPo battery that is rated at 1000 mAh would be completely discharged in one hour with a 1000 milliamp load placed on it. If this same battery had a 500 milliamp load placed on it, it would take 2 hours to drain down. If the load was increased to around 15,000 milliamps (15 amps); a very common current drain in a 3S powered 450 sized RC helicopter while hovering - the time to drain the battery would be only about 4 minutes.
As you can see, for a RC model with that kind of current draw, it would be very advantageous to use a larger capacity battery pack such as a 2000 mAh pack. This larger pack used with a 15 amp draw would double the time to about 8 minutes till the pack was discharged.
The main thing to get out of this is if you want more flight time; increase the capacity of your battery pack. Unlike voltage, capacity can be changed around to give you more or less flight time. Naturally because of size & weight restrictions, you have to stay within a certain battery capacity range seeing that the more capacity a battery pack has, the larger and heavier it will be.
Remember that third number I was talking about when you go RC LiPo battery shopping? Yes, discharge rate is that number. This one is probably the single most over rated & miss understood of all battery ratings.
Discharge rate is simply how fast a battery can be discharged safely. Remember that ion exchange thing further up the page? Well the faster the ions can flow from anode to cathode in a battery will indicate the discharge rate. In the RC LiPo battery world it is called the “C” rating.
What does it mean?
Well Capacity begins with “C” so that should give you a pretty good idea. A battery with a discharge rating of 10C would mean you could theoretically & safely discharge it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C pack = 20 times more, and so on.
Using our 1000 mAh battery as an example; if it has a 20C discharge rating, that would mean you could pull a maximum sustained load up to 20,000 milliamps or 20 amps off that battery (20 x 1000 milliamps = 20,000 milliamps or 20 amps). From a purely theoretical time stand point, this equals 333 mAh of draw per minute so the 1000 mAh pack would be completely exhausted in about 3 minutes if it's exposed to the maximum rated 20C discharge rate the entire time. Calculation as follows: 20,000 mA divided by 60 minutes = 333 mAh which is then divided into the 1000 mAh capacity of the pack giving us 3.00 minutes).
Most RC LiPo Battery packs will show the continuous C rating and usually a maximum burst C rating as well. A burst rating indicates the battery discharge rate for short bursts (a few seconds maximum) of extended power. An example might be something like "Discharge rate = 25C Continuous/50C Bursts".
The higher the C rating, usually the more expensive and even slight heavier the battery gets. This is where you can save some money, and maybe even a little weight. Getting an extremely high discharge rated pack when there is no way you could possibly pull the full amount of power is not required but it won't hurt either. The most important thing is you can't go with too low a discharge C rating or you will damage your battery and possibly your ESC (electronic speed control).
So how do you know what C rating to get when purchasing your LiPo RC Battery Pack? The easy answer most will give is to get the largest C rating you can... If money is not an object I agree with that 100%; but for most folks, especially beginners & intermediate or scale fliers who won't be performing power hungry 3D maneuvers and drawing much current - stretching your RC battery budget by purchasing lower C rated packs when you're first learning so you can get a few extra packs makes much more sense in my opinion.Same goes for multi-rotors as they generally don't pull as much current.
As a very general guide line, 25C to 30C discharge rated packs are the norm for most 250-400 size electric helicopters with general to light sport flying in mind. For larger birds, 25C to 30C discharge rated packs are a safe bet (again for normal to light sport). Once up to aggressive sport or 3D, that is where the 35C and up discharge rated packs come into play.
All this said, RC LiPo packs are coming down in price all the time. If you find a 35C pack for the same price as a 25C when that is all you need, go for the 35C pack - it will run cooler and have a longer life span. Like most things, pushing a Lipo pack hard close to its limits will wear it out and reduce it's useful capacity in very short order. If however you get a pack with a C discharge rating at least double of the maximum you intend to pull out of it; with proper care, there's no reason you shouldn't be able to get at least 400 charge and discharge cycles out of it with average degradation.
One interesting point I should mention about selecting discharge ratings seeing that HV (high voltage) electric RC aircraft (usually defined as using LiPo packs over 8S) are becoming more and more common place is the reduced current that HV provides. This of course is another topic, but for many HV applications, you can get away with lower C ratings since the models won't pull as much current as a similar size/powered model running on a lower voltage pack. The flip side of course is most folks who are running HV birds are also pushing them to the limits and will still need high discharge rates... I just wanted to point out why higher voltage can be advantageous (less current = less heat).
Lastly, taking a temperature reading of your packs after running them is another good way to gauge if you're using a high enough C rating. I'm afraid to say it, but just because a pack says it is rated at 30C doesn't necessary mean it is in real world applications. Realistically, C ratings are somewhat meaningless because they are rarely verifiable. On top of that, as packs age the internal resistance gets higher making them run warmer and as your flying ability improves, chances are you will be pulling more current.
The general rule is if you can't comfortably hold a LiPo pack tightly in your hand after using it, it's way too hot. This equates to anything higher than about 50C (122F). That is even way too warm as far as I'm concerned. Nothing higher than 40C (about 104F) is what I consider safe and I rarely have my packs go much past 35C (95F) unless it's also very hot outside as well. So - if you find your packs are getting warmer than this, it's a good bet you should consider moving up to a higher discharge rating for your next LiPo pack.
Leaving your packs in the car on a hot sunny day can certainly heat them up well past 40C as well. Internal or external heat - both have the same negative effect, hot LiPo's are miserable and they won't last long.
OVER DISCHARGING - THE NUMBER ONE KILLER OF LIPO'S!!!
The other thing that will heat a pack up fast and irreversibly damage it is pushing it right down to or lower than 3.0 volts per cell under load. Even if you have a 60C pack and can only draw one quarter that amount of power, if you push it hard right down to 3 volts per cell - it will become very warm/hot and will shorten its life substantially.
THE 80% RC LIPO BATTERY RULE TO THE RESCUE!
A very good rule to follow here is the "80% rule". This simply means that you should never discharge a LiPo pack down past 80% of it's CAPACITY to be safe. For example, if you have a 2000 mAh LiPo pack, you should never draw more than 1600 mAh out of the pack (80% x 2000). This is assuming a healthy pack as well that has the full 2000 mAh capacity (as packs age, their capacity drops).
This again is where computerized chargers pay for themselves many times over so you can see how much capacity the battery takes allowing you to adjust your flight times accordingly to stay within that 80% rule to get the most life out of your pack.
If you don't have a computerized charger to confirm the amount of capacity, another good indicator is to measure the open circuit voltage (no load voltage) of the pack or individual cells right after a flight/drive with a digital volt meter or other similar digital voltage measuring device. An 80% discharged LiPo cell, will give an approximate open circuit voltage of about 3.72 to 3.74 volts. A 3S LiPo pack therefore would show about 11.2 volts after a flight when it's about 80% discharged, a 6S pack would be in the 22.4 volt region. The longer you wait after the flight/drive, the less accurate this voltage method of determining an 80% percent discharge works because as the pack rests after the flight, the resting open circuit voltage recovers slightly, perhaps up to 3.76 volts or so. Remember, states of charge in any battery are based on capacity, not voltage for the simple reason voltage drop in a battery is non-linear.
I for instance use these little inexpensive LiPo battery monitors after most flights to gauge my flight times to ensure I'm not over discharging my packs much past 80%. These ones I use work with 2S to 6S LiPo packs.
They are also very useful to quickly identify fully charged and discharged packs when you get them mixed up by mistake so you don't accidentally put a discharged pack in your machine thinking it was fully charged. Not sure about you, but when I go out for a full day of flying, I can easily have a couple dozen LiPo's on the go and it doesn't take much more than a simple interruption or memory lapse to get packs mixed up.
You just plug the little rascal into your balance plug on your LiPo battery after the flight (or drive) and it will show the voltage of each individual cell in sequence, followed by the full voltage of the LiPo battery pack.
You can see in the photo above I have plugged the little monitor into this particular 5000 mAh 6S pack's JST-XH balance plug in a Bell 206L scale heli that I just finished building to get an idea of flight times to correctly set my flight timer. All cells in the pack were showing about 3.74V after this 8 minute flight which again is pretty close to an 80% discharged state. I might be able to push it to 8:30 minutes, but to keep things safe, I set the timer to 8:00 minutes and it's working great (confirmed by charging the pack on a computerized charger to see how much capacity it takes). It should take about 4000 mAh of charge (80% x 5000 mAh).
I have at least half a dozen of these little monitors and take at least two or three out to the flying field to be sure I can always easily place my hands on one. As I said, they are very inexpensive and I personally could not live without them now.
Another rating??? Yep, the first 3 are industry standards and as was mentioned with that last one (C discharge ratings), is used by the manufacturers to market their product or justify a higher price and realistically can't be verified, but they are still a good general guide line when choosing a pack.
Internal resistance to the rescue! This one is verifiable and one of the best ways to monitor your RC LiPo batteries condition. Most decent higher capacity and higher discharge rated LiPo cells will have roughly 2 to 6 milliohms (0.002 to 0.006 ohms) of internal resistance when brand new. To calculate the total internal resistance of a series wired pack, you would then add these numbers together so a 4S pack with each cell having 4 milliohms of resistance will show a total internal resistance of about 16 milliohms (0.016 ohms).
As I mentioned, as packs age, the internal resistance goes up and the warmer they run. Lower discharge rated packs and small capacity packs will generally have higher internal resistance readings. It is not unusual to measure internal resistance numbers in the region of 200 milliohms on smaller 100 to 200 mAh micro park flyer LiPo packs when they are brand new for example.
So the best way to use internal resistance (if your charger supports this very useful function) is to take an IR reading of your LiPo/s when it/they are brand new.
As seen here, I will then write that number (or the IR of all the cells in the pack) somewhere on the pack with a permanent marker so I will always have a brand new IR base reference for that particular battery. As the packs age, I can simply reference how the resistance is increasing or if one cell for some reason is getting bad.
How do you measure internal resistance? This again is where good computerized chargers come into play. The good ones that support this feature with built in balance boards will check the "IR" of each cell as well as the entire pack. Pictured above I am taking the IR reading of each cell in this new 6S Turnigy LiPo. It is hard to make out in the photo, but the IR of cells 1-6 are 2,2,1,1,1,2 milliohms each giving a total IR for the entire pack of 9 milliohms - pretty respectable!
Internal resistance really opens up a huge and complex topic of how to accurately calculate voltage drop in the pack and the total amount of watts being expended in the form of heat within the pack. I am not going to get into those calculations here for the simple reason I am not qualified enough to explain them.
Charging RC LiPo Batteries
Charging RC LiPo Batteries is a topic in itself. LiPo, LiIon, and LiFe batteries obviously have some very different characteristics from conventional RC rechargeable battery types. Therefore, charging them correctly with a charger specifically designed for lithium chemistry batteries is critical to both the life span of the battery pack, and your safety.
Maximum Charge Voltage and Current
A 3.7 volt RC LiPo battery cell is 100% charged when it reaches 4.2 volts. Charging it past that will ruin the battery cell and possibly cause it to catch fire. This is important to understand once I start talking about Balancing RC LiPo batteries, so keep that in the back of your head for right now.
It is critical that you use a charger specified for LiPo batteries and select the correct voltage or cell count when charging your RC LiPo batteries if you are using a computerized charger. If you have a 2 cell (2S) pack you must select 7.4 volts or 2 cells on your charger. If you selected 11.1V (a 3S pack) by mistake and tried to charge your 2S pack, the pack will be destroyed and most likely catch fire. Luckily, all the better computerized chargers out there these days will warn you if you selected the wrong cell count.
All LiPo battery chargers will use the constant current / constant voltage charging method (cc/cv). All this means is that a constant current is applied to the battery during the first part of the charge cycle. As the battery voltage closes in on the 100% charge voltage, the charger will automatically start reducing the charge current and then apply a constant voltage for the remaining phase of the charge cycle. The charger will stop charging when the 100% charge voltage of the battery pack equalizes with chargers constant voltage setting (4.2 volts per cell) at this time, the charge cycle is completed. Going past that, even to 4.21 volts will shorten battery life.
RC LiPo Battery Charging Current
Selecting the correct charge current is also critical when charging RC LiPo battery packs. The golden rule here used to be "never charge a LiPo or LiIon pack greater than 1 times its capacity (1C)."
For example a 2000 mAh pack, would be charged at a maximum charge current of 2000 mA or 2.0 Amps. Never higher or the life of the pack would be reduced. If you choose a charge rate significantly higher than the 1C value, the battery will heat up and could swell, vent, or catch fire.
Times are a changing...
Most LiPo experts now feel however you can safely charge at a 2C or even 3C rate on quality packs that have a discharge rating of at least 20C or more safely and low internal resistances, with little effect on the overall life expectancy of the pack as long as you have a good charger with a good balancing system. There are more and more LiPo packs showing up stating 2C and 3C charge rates, with even a couple manufacturers indicating 5C rates. The day of the 10 minute charge is here (assuming you have a high power charger and power source capable of delivering that many Watts and Amps).
Once again, the four main things that shorten LiPo battery life are: HEAT, OVER-DISCHARGING (voltage & current), OVER CHARGING (voltage & current) & INADEQUATE BALANCING.