Li-Po Battery Information.
Risks and Limitations
- All Li-Ion cells expand at high levels of state of charge (SOC); if uncontained, this may result in delaminating, and reduction of reliability and cycle life; the case of cylindrical cells provides that containment, while pouch cells, by themselves, are not contained. Therefore, to achieve the rated performance, a battery composed of pouch cells must include a strong external casing to retain its shape.
- Overcharging a Li-poly battery can cause an explosion or fire.
- During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V per cell (used in a series combination), or else the battery will subsequently no longer accept a full charge and may experience problems holding voltage under load. Li-poly batteries can be protected by circuitry that prevents over-charge and deep-discharge.
- Compared to the lithium-ion battery, Li-poly has a greater life cycle degradation rate.
- Lithium polymer-specific chargers are required in order to avoid fire and explosion.
- Explosions can also occur if the battery is short-circuited, as tremendous current passes through the cell in an instant. Radio-control enthusiasts take special precautions to ensure their battery leads are properly connected and insulated. Furthermore fires can occur if the cell or pack is punctured.
- While charging the lithium polymer batteries, the individual cells in the pack should be charged evenly. For this purpose, the cells are to be charged with special chargers. This entails special care while charging the batteries in addition to incurring expenses on procuring the chargers specific to lithium polymer batteries.
· LiPoly batteries must be charged carefully. The basic process is to charge at constant current until each cell reaches 4.2 V. The charger must then switch to a constant voltage mode, and the charging current will gradually reduce while the charger holds the cell voltage at 4.2 V. This will continue until the charge current has dropped to a small percentage of the initial charge rate, at which point the battery is considered fully charged. Some manufacturers specify 2%, others 3%, but other values are also possible. The difference in achieved capacity is minute.
· Balance charging simply means that the charger monitors the voltage of each cell in a pack and varies the charge on a per-cell basis so that all cells are brought to the same voltage.
· Trickle charging is not recommended for lithium batteries. Most manufacturers claim a maximum and minimum voltage of 4.23 and 3.0 volts per cell, respectively. Taking any cell outside these limits can reduce the cell's capacity and ability to deliver full rated current.
· Most dedicated lithium polymer chargers use a charge timer for safety; this cuts the charge after a predefined time (typically 90 minutes).
· As of the beginning of 2013, charging rates of up to 15C (i.e., 15 times the capacity of the battery, or approximately 4-minute charge times) are possible in the relatively new (circa 2009) breed of nanowire technology LiPo batteries. This, however, is the exception to the rule, as the more common 1C charge rate still stands as the recommended standard among RC users. It is also important to note that regardless of the charge rate that a battery can handle, using the lower 1C charge rate will always increase the longevity of any RC LiPo battery.
· LiPo RC Battery Ratings
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.
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)
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).
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 – let’s just say that is what has to be used unless you want to open a whole new can of worms.
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 400 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 until the pack was discharged.
The main thing to get out of this is if you want more sailing time; increase the capacity of your battery pack. Unlike voltage, capacity can be changed around to give you more or less running 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. 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 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.
Let's use our 1000 mAh battery as an example; if it was rated at 10C that would mean you could pull a maximum sustained load up to 10,000 milliamps or 10 amps off that battery (10 x 1000 milliamps = 10,000 milliamps or 10 amps). From a time stand point, this equals 166 mAh of draw a minute so the 1000 mAh pack would be exhausted in about 6 minutes.
This is calculated by first determining the mAh per minute of the pack. 1000 mAh divided by 60 minutes = 16.6 mAh's per minute. You then multiply that number by the C rating (10 in this case) = 166 mAh of draw per minute divided into the packs capacity (1000 mA) = 6.02 minutes.
How about a 20C rating on a 2000 mAh battery? 20 x 2000 = 40,000 milliamps or 40 amps. Time wise, a 40 amp draw on this pack would exhaust it in about 3 minutes (2000/60= 33.3 multiplied by 20c = 666 mAh per minute - divided into the packs capacity of 2000 mA = 3 minutes). As you can see, that is a pretty short run and unless you are drawing the maximum power for the entire run, it is unlikely you would ever come close to those numbers.
Most RC LiPo Battery packs will show the continuous C rating and some are now indicating a burst rating as well. A burst rating indicates the battery discharge rate for short bursts of extended power. An example might be something like “Discharge rate = 20C Continuous/40C Bursts”
The higher the C rating, the more expensive the battery. This is where you can save some money. 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 you’re 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 people - 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.
As a very general guide line, 25C to 30C discharge rated packs are the norm. 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 its 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 little degradation.
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.
The general rule is if you can't comfortably hold a LiPo pack tightly in your hand after using it, its way too hot. This equates to anything higher than about 50C (122F). That is even way too warm. Nothing higher than 40C (about 104F) is considered safe. 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 is if you push it right down to or lower than 3.0 volts per cell under load. Even if you have a 40C pack and can only draw one third that amount, if you push it hard right down to 3 volts per cell - it will become very warm/hot and will shorten its life substantially.
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 its 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, you can see how much capacity the battery takes allowing you to adjust your run time 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 run 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 3.72 to 3.74 volts. A 3S LiPo pack therefore would show about 11.2 volts after a run 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 will increase slightly, perhaps up to 3.76 volts or so.
Many people 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.
RC LiPo Battery Storage
How you store your LiPo’s between uses will greatly affect their life span. As I mentioned, a LiPo cell that drops below 3 volts under load is almost always & irreversibly damaged (reduced capacity or total inability to accept a charge). 3 volts under load is generally equates to about 3.5 volts open circuit resting voltage, so if your batteries are stored for any period of time after you use them at close to that magic 3.5 volt per cell number, you risk damage.
As batteries sit, they will naturally self discharge. LiPo’s are actually very good in this respect and self discharge much slower than most other rechargeable battery types, but they still do loose voltage as they sit. If you leave them for a number of weeks or months at close to 3.5 volts per cell, chances are they will drop below that and may be irreversibly damaged.
You must store them charged, but not fully charged either – that will also degrade/oxidize the cell matrix. Basically, the speed at which a LiPo pack ages (during storage) is based on both storage temperature and state of charge. You are likely ok to store a fully charged RC LiPo battery at room temperature for up to 4 days without doing much damage. Never store a LiPo in a hot car fully charged for an extended time; that will certainly cause damage. For optimum battery life, store your RC LiPo batteries at room temperature and at about 40-60% charged. That equates to around 3.85 volts per cell (open terminal resting voltage). The actual storage range is likely a little broader than this (I have heard some say numbers as high as 20-80% is fine, but since computerized chargers set the storage charge at 50% (3.85 volts per cell) that's what’s recommend .