This how you should look after them properly.
Bruce has years of experience dealing with Lipo's and dealing with the gross mis-use of them. Some of the mis-use was quite exciting......
Lipo Batteries; Ratings, Usage & Safety #80
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Lipo Batteries; Ratings, Usage & Safety #80
Thursday, 23 October 2014 04:03
At our last Club Night, Bruce Abbott gave a very informative talk about the why's, wherefore's, do's and don'ts of Lithium Polymer batteries. For those that missed it and also those who want it as a reference, it is reproduced here below, well worth a read.
LiPo Batteries: Ratings, Usage & Safety.
Voltage and “S” Rating
Unlike conventional NiCad or NiHm battery cells that have a voltage of 1.2 volts per cell, LiPo battery cells are rated at 3.7 volts per cell nominal and 4.2 volts when fully charged. The benefit of higher voltage is that fewer cells are needed to make up a battery pack, and in micro sized RC aircraft a single 3.7 volt cell may be all that is needed to power the model.
Other than for the smallest of electric RC models, LiPo battery packs have two or more cells hooked up in series to provide higher voltages. For larger RC models that number can be as high as 12 cells.
Here is a list of LiPo RC battery pack voltages for the most common cell counts;
•3.7 volt battery = 1 cell x 3.7 volts (1S), min 3V max 4.2V
•7.4 volt battery = 2 cells x 3.7 volts (2S), min 6V max 8.4V
•11.1 volt battery = 3 cells x 3.7 volts (3S), min 9V max 12.6V
•14.8 volt battery = 4 cells x 3.7 volts (4S), min 12V max 16.8V
•18.5 volt battery = 5 cells x 3.7 volts (5S), min 15V max 21V
•22.2 volt battery = 6 cells x 3.7 volts (6S), min 18V max 25.2V
If you are wondering what the number in parenthesis means, it is how battery manufacturers indicate how my cells hooked in series “S” the battery pack contains.
Battery packs can also be wired in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 3S2P would indicate 2 x three celled series packs hooked up in parallel to double the capacity.
Capacity or mAh rating
Capacity indicates how much power the battery pack can hold, and is indicated in milliamp hours (mAh). This is a way of showing how much current (measured in milliamps) can be taken out of the battery for 1 hour (by which time it will be fully discharged).
For example a battery that is rated at 1000mAh would be completely discharged in one hour with a 1000 milliamp (1 Amp) load placed on it. If this same battery was discharged at 500mA, it would take 2 hours to drain down. If the load was increased to 20A, it would drain in under 3 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 2000mAh or even higher. This larger pack used with a 20A draw would double the run time to about 6 minutes.
The main thing to get out of this is if you want more running time, increase the capacity of your battery pack. Unlike voltage, capacity can be changed to give you more or less running time. Of course because of size restrictions and weight you have to stay within a certain battery capacity range (since the more capacity a battery pack has, the larger and heavier it will be).
Capacity @ Voltage (IA-Flyer test).gif
"The amount of charge left in a Lipo battery can be accurately determined from its resting voltage. At 3.7V a Lipo cell has only about 10% capacity remaining."
Discharge Rate or “C” Rating
This is probably the single most over rated and miss understood of all battery ratings. Discharge rate is simply how fast a battery can be discharged safely. 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?
A battery with a discharge rating of 10C would 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 mA of draw per minute, so a 1000mAh pack would be exhausted in about 6 minutes.
This is calculated by first determining the mA per minute of the pack. 1000 mAh divided by 60 minutes = 16.6mA per minute. You then multiply that number by the C rating (10 in this case) = 166mA of draw per minute divided into the packs capacity (1000mA) = 6.02 minutes.
How about a 20C rating on a 2000mAh 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= 33.3mA minutes multiplied by 20C = 666mA per minute - divided into the packs capacity of 2000mA = 3 minutes). As you can see, that is a pretty short running time and unless you are drawing the maximum power for the entire flight, it is unlikely you would ever come close to those numbers.
Most RC LiPo Battery packs will show the continuous C rating and some also indicating a burst rating as well. The 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”.
Usually the higher the “C” rating, the more expensive the battery. Depending on what application you are using the battery for, this is where you could save some money by understanding all the ratings on a Lipo battery. Getting a 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 not to go with too low a discharge “C” rating, or you may damage your battery.
So how do you know what ‘C’ rating to get when purchasing your battery pack? The easy answer is to get the largest C rating you can. It will run cooler and most likely last a little longer. However if you are drawing less than half the ‘C’ rating then you could safely use a lower rated battery, which may be cheaper and perhaps a bit lighter weight.
Of course, as with most performance figures, some manufacturers C ratings are more ‘honest’ than others. A battery should be able run continuously at its maximum rating, but it will get hot - perhaps too hot. The general rule is if you can't comfortably hold a LiPo pack tightly in your hand after using it, it's too hot. This equates to anything higher than about 50⁰C. If you find your packs are getting warmer than this, it's a good bet that you need a higher ‘C’ rating.
"Lipos require a 'constant voltage constant current' (CVCC) charge regime. The battery is charged at constant current until the voltage reaches 4.2V/cell, then current must be progressively reduced to stop the voltage from rising any higher. The battery is fully charged when current drops to zero."
Charging RC LiPo Batteries
LiPos have very different characteristics from conventional (Nicad and NIMH) rechargeable battery types. Therefore, charging them correctly with a charger specifically designed for LiPo batteries is critical to both the lifespan of the LiPo battery and your safety.
Maximum Charge Voltage and Current
A 3.7 volt LiPo cell is 100% charged when it reaches 4.2 volts. Charging past that voltage will destroy the cell, and possibly cause it to catch fire. This is very important to note and keep in mind at all times. A computerized charger will stop the charge process when the battery reaches 4.2V per cell. A balancing computerized charger will do this for each individual cell.
It is critical that you use a charger specified for Li-Po batteries and select the correct voltage or cell count when charging your 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 would be destroyed and most likely catch fire.
Most good LiPo battery chargers use the constant current / constant voltage charging method (cc/cv). What 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. The charger will stop charging when the 100% charge voltage of the battery pack equalizes with the charger’s constant voltage setting (4.2 volts per cell). The charge cycle is then complete. Going even a little bit above 4.2V will shorten battery life.
Selecting the correct charge current is also critical when charging RC LiPo battery packs. The golden rule here is "never charge a LiPo or Li-Ion pack greater than 1 times its capacity (1C)”. For example a 2000mAh pack would be charged at a maximum charge current of 2000 mA or 2.0 amps.
But times are changing and many battery manufacturers allow higher charging “C” current. Most LiPo experts feel that 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, with little effect on the overall life expectancy of the pack, as long as you have a good charger with a good balancing system.
Remember, the three main things that shorten LiPo battery life are:
What is balancing and why is it important?
As mentioned, each cell in a Lipo battery pack is 4.2 volts when fully charged. Balancing is required on any LiPo battery pack that has more than one cell, since the charger only sees the whole pack voltage and doesn’t know if one cell might be overcharged even though the total voltage of the pack is good.
For example, consider a 3 cell LiPo battery pack (three LiPo cells hooked in series or 3S). This would be an 11.1V battery pack (3.7 volts per cell x 3 = 11.1 volts). The 100% charge voltage of this LiPo pack is 12.6 volts (4.2 volts x 3 = 12.6 volts). Our trusty charger set up for an 11.1 volt RC LiPo battery pack will then stop charging at 12.6 volts.
But what would happen if one of those three cells is charging a bit faster than the other two? Two cells might only get to 4.1 volts while the other one gets overcharged to 4.4 volts, before the charger stops at 12.6 volts. That would ultimately damage the overcharged cell, and even perhaps cause the battery to catch fire.
That kind of voltage difference between cells is unlikely with a healthy pack, but even a 0.1V difference between cells can cause damage over time.
At the other end of the spectrum if there is one cell in the pack that is not reaching full charge when the pack is charged, and then gets discharged below 3.0 volts in your model (even though the 3 cell battery pack is indicating a voltage of 9 volts or higher), this could also damage it.
Balancing ensures that all cells are always within about 0.01-0.05 volts per cell. This prevents your battery pack from being damaged by one or more cells getting over charged or discharged, or worse becoming a safety issue.
Balancing can be done either by having a cell balancer working alongside your charger, or by using a computerized charger with built in balance function. How this works is the charger (or balancer) will check each cell in a Lipo battery pack as it is charging/ discharging, and adjust the voltage so that they all charge/discharge equally to end up at the same voltage.
Most balancers do this by bleeding a small current of about 0.2A from any cells which have a higher voltage. If the battery is being charged at a high rate then the balancer may not be able to remove enough current to equalize it. Therefore, if you have a battery that is grossly out of balance then you should charge it a low rate (eg. 0.5A) until it gets back into balance.
There are plenty of warnings on safety measures needed when charging Lipo batteries. Lipo batteries are made from volatile materials and if not looked after and charged / discharged at correct “C” rating as well as charge voltage, they can heat up and in some cases explode.
Heat and LiPo’s are usually a bad combination, sometimes very very bad. Resulting in explosion or fire. There are plenty of videos on YouTube with LiPo packs exploding and releasing toxic gases. You should always look after you pack. Charge them in suitable condition and never overcharge, over discharge or allow them to heat up.
•Charge batteries in a fire resistant container. Fire proof bags are a great item to get, and are also useful for safe battery transport and storage.
•After use, let the pack cool down for at least 15 minutes before recharging it. This prolongs the life of the pack and prevents possible overheating and damage.
•Never leave the house or leave a Lipo unattended when charging.
•Install an inexpensive smoke detector close to where you charge your batteries.
The other thing that will heat a pack up fast is if you drag it right down to 3.0 volts per cell under load. Even if you have a 40C pack and only draw half that amount, if you work it hard right down to 3 volts per cell, it may become very hot.
A good rule to follow is the "80% rule". This simply means that you should never discharge a LiPo pack down past 80% of its capacity. For example, if you have a 2000mAh LiPo pack, you should never draw more than 1600mAh out of the pack (80% x 2000). This is assuming a healthy pack as well that has the full 2000mAh capacity (as packs age, their capacity may drop).
This again is where computerized chargers pay for themselves many times over as you can see how much capacity the battery takes, allowing you to adjust your running times accordingly to stay within that 80% rule to get the most life out of your pack.
Breaking In RC LiPo Batteries
Breaking in a new LiPo pack is a good practice, even though there is mixed opinion on whether you have to do this. Just like a new engine, not pushing your new LiPo to the maximum limits the first time out may give it added life and performance over the years. The general break in method is very simple; for the first few uses (perhaps 4 or 5), don't stress the pack, keep the charge rates low (1C or lower), and don't discharge down past 50% of the battery's capacity.
"Nicad and NiMH chargers use either temperature rise or voltage depression (negative deltaV) to detect end of charge. Note that NiMH batteries have less voltage drop, and so may need a more sensitive setting on the charger."
LiPo Battery Storage
How a LiPo battery is stored between uses can greatly affect its life span. As 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 generally equates to about 3.5 volts open circuit resting voltage, so if your batteries are stored for a long period of time below 3.5 volt per cell, you run the risk of permanent 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 many months at close to 3.5 volts per cell, they could drop below the 3 volt threshold and be damaged.
However you should not store them fully charged either. At higher voltage the plates oxidise faster, increasing internal resistance and reducing maximum power output. Increased temperature also has a similar effect.
For optimum battery life, store your LiPo batteries at room temperature and at about 40-60% charge. That equates to around 3.85 volts per cell (open terminal resting voltage). A good computerized charger will have a setting where you can charge or discharge your pack for storage.