Learn about LiPo batteries

All radio controlled scale models need electricity to feed the radio controlled system and the motors, both at the same time. Since many years we had different generations of batteries, Nickel-Cadmium (NiCd), then Nickel-Methal-Hydride (NiMh) and today the Lithium-Polymer (LiPo) batteries are the most used.

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What is a Lithium battery ?

The polymer hype of the early 2000s is still going strong, however, most users cannot distinguish between a regular Li-ion and one with polymer architecture. Lithium-polymer differs from other battery systems in the type of electrolyte used. The original polymer design dating back to the 1970s uses a solid (dry) polymer electrolyte that resembles a plastic-like film. This insulator allows the exchange of ions (electrically charged atoms) and replaces the traditional porous separator that is soaked with electrolyte. A solid polymer has a poor conductivity at room temperature and the battery must be heated to 50–60°C (122–140°F) to enable current flow.The much anticipated “true plastic battery” promised in the early 2000s did not materialize; the conductivity could not be attained at ambient temperature.
To make the modern Li-Polymer battery conductive at room temperature, gelled electrolyte is added. All Li-ion polymer cells today incorporate a micro porous separator with moisture. The correct term is “Lithium-ion polymer” (Li-ion polymer or Li-Polymer for short). Li-Polymer can be built on many systems, such as Li-cobalt, NMC, Li-phosphate and Li-manganese. For this reason, Li-Polymer is not considered a unique battery chemistry. Most Li-Polymer packs for the consumer market are based on Li-cobalt.
With gelled electrolyte added, what then is the difference between a normal Liion and Liion polymer? As far as the user is concerned, the lithium polymer is essentially the same as the lithium-ion battery. Both use identical cathode and anode material and contain a similar amount of electrolyte. Although the characteristics and performance of the two systems are alike, the Li-Polymer is unique in that a micro porous electrolyte replaces the traditional porous separator. The gelled electrolyte becomes the catalyst that enhances the electrical conductivity. Li-Polymer offers slightly higher specific energy and can be made thinner than conventional Li-ion, but the manufacturing cost increases by 10–30 percent. Despite the cost disadvantage, the market share of Li-Polymer is growing.
Li-Polymer cells also come in a flexible foil-type case (polymer laminate or pouch cell) that resembles a food package. While a standard Li-ion needs a rigid case to press the electrodes together, Li-Polymer uses laminated sheets that do not need compression. A foil-type enclosure reduces the weight by more than 20 percent over the classic hard shell. Furthermore, thin film technology liberates the format design and the battery can be made into any shape, fitting neatly into stylish cell phones and laptops to make them smaller, thinner and lighter. Li-Polymer can be made very slim to resemble a credit card. Read about the Pouch Cell.                
Charge and discharge characteristics of Li-Polymer are identical to other Li-ion systems and do not require a special charger. Safety issues are also similar in that protection circuits are needed. Gas buildup during charge can cause some Li-Polymer in a foil package to swell, and equipment manufacturers must make allowances for expansion. Li-Polymer in a foil package may be less durable than Li-ion in the cylindrical package. Li-Polymer is not limited to a foil package and can also be made into a cylindrical design.

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Carateristics of the LiPo batteries

Voltage:
The voltage of a battery is also determined by the cell arrangement (series), and there are a few common voltage measurements worth noting:
Charged - the voltage of a fully-charged LiPo cell is 4.20V, and charging above this will damage the cell.
Nominal - this can be considered a sort of "half-charged" voltage, as it is 3.70V, in between charged and discharged.  Nominal voltage is what manufacturers use when describing the voltage of their batteries.
Discharged - the voltage of a discharged LiPo cell is 3.00V, and discharging below this will definitely damage the cell.
Because the battery shown has a 3S arrangement, it is marked with its nominal voltage of 11.1V (3.70V*3 cells).  A fully charged 3S pack is 12.60V and a fully discharged 3S pack is 9.00V.

Capacity:
Usually measured in mAh (milliamp hours), this is determined by the cell arrangement (parallel) and tells you how long you can expect the battery to last on a charge (although it's not quite that simple).  2600mAh as shown on the battery in the picture is equal to 2.6Ah (amp hours), a format you may be more familiar with on larger batteries, like the SLA (sealed lead acid) one in your car, which is probably around 50Ah.  A capacity of 2600mAh means that the battery can discharge at 2.6 amps for one hour (hence "amp hours"), 1.3 amps for 2 hours, etc., before it runs out of "juice."  Because the battery shown has a 1P arrangement, each cell has a capacity of 2600mAh.

The C value:
In the late 1700s, Charles-Augustin de Coulomb ruled that a battery that receives a charge current of one ampere (1A) passes one coulomb (1C) of charge every second. In 10 seconds, 10 coulombs pass into the battery, and so on. On discharge, the process reverses. Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.
Most portable batteries are rated at 1C, meaning that a 1,000mAh battery that is discharged at 1C rate should under ideal conditions provide a current of 1,000mA for one hour. The same battery discharging at 0.5C would provide 500mA for two hours, and at 2C, the 1,000mAh battery would deliver 2,000mA for 30 minutes. 1C is also known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is a half-hour discharge.
The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. The analyzer discharges the battery at a calibrated current while measuring the time it takes to reach the end-of-discharge voltage. An instrument displaying the results in percentage of the nominal rating would show 100 percent if a 1.000mAh test battery could provide 1.000mA for one hour. If the discharge lasts for 30 minutes before reaching the end-of-discharge cut-off voltage, then the battery has a capacity of 50 percent. A new battery is sometimes overrated and can produce more than 100 percent capacity; others are underrated and never reach 100 percent even after priming.
When discharging a battery with a battery analyzer capable of applying different Crates, a higher Crate will produce a lower capacity reading and vice versa. By discharging the 1,000mAh battery at the faster 2C, or 2,000mA, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same as with a slower discharge since the identical amount of energy is being dispensed, only over a shorter time. In reality, internal resistance turns some of the energy into heat and lowers the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over two hours, will likely increase the capacity to above 100 percent.
To obtain a reasonably good capacity reading, manufacturers commonly rate lead acid at 0.05C, or a 20-hour discharge. Even at this slow discharge rate, the battery seldom attains a 100 percent capacity. Manufacturers provide capacity offsets to adjust for the discrepancies in capacity if discharged at a higher Crate than specified

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Choose a battery

With so many choices out there it is difficult to decipher what is marketing hype, what is brand
loyalty, and what is outright lies. Battery manufacturers are constantly trying to one up one another. While capitalism can drive prices down, it also can give cause to false claims about products.
One great way to find out what the best battery is, is to look at graphs of the batteries performance. Looking at how low the voltage of the cell drops at various amperages will give you a metric to compare that battery to similar size/weight batteries.
If graphs aren't your thing then simply look at what other people are using in successful setups that are similar to your application. If a lot of people are reporting long flight times and lots of power from airplane X, with power system Y, and battery Z and you do the same, then if your setup is similar the same battery will probably work well for you.
It pays to learn something about Watts, Volts, and Amps. Understanding these concepts is beyond the scope of this document, but can serve you well in not only figuring out what battery is best but also in your electric aircraft hobby.
I'm not convinced that a 30C battery is really any better than a 10 or 20C battery. Sure a higher C rating means it can discharge faster. But at the same time a battery discharged at 20C continuously will be empty in 3 minutes. Do you really only want to use the battery for 3 minutes? I love having burst power in helicopters and boats, but in almost all other applications actually running a battery at or above 20C is useless to me. I prefer to run batteries at 8-10 C and have a little headroom if I need it.
A final note on choosing a battery. Don't cheap out. Confirm that your batteries are capable of running that the amperage level you plan to use them at. Running a cell at a higher C rating than the battery can handle can not only damage your batteries, but it can also damage your speed control. Castle Creations has an excellent article on how using a weak battery can destroy a perfectly good speed control of any brand. Better to buy a bit better battery than you need than to destroy your electronics.

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Specifications of an element :

The specifications of an element are :
A LiPo element 2200 mAh 20C/30C, means, LiPo = 3,7 V voltage, his capacity is 2200 mAh (2,2 Ah), the continuous maximum consumption is 20 x 2,2 of 44 A and the peak consumption is 30 x 2,2 of 66 A.

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What about the charging current ?

The best is to load the LiPo batteries with maximum 1C. 1C means 1x the capacity. A 5000 mAh battery can be loaded with 5A, een 3000 mAh battery with 3A.
The old generation LiPo batteries were not allowed to be loaded with a higher current, because then they could plop, but the new generation does not The latest generation as our TCR LiPo batteries can be loaded with a much higher current, 2 C is no problem. The manufacturer gives even that you can load them with 5 C without plopping, but it’s not good for the battery.
In the technical data sheets that we have of the real battery manufacturers (there are many vendors who have their own name on a battery paste, but there are only a few factories that they make) the top of the batteries is descripted as follows.
These batteries can best be loaded at 1 to max. 2 C without loss of life span or performance. If desired they can be loaded with 5 C, but the life span runs then dramatically back.

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The "packs"

Only the very small models have enough with 3.7 V but in most cases models are fed with battery blocks, which are called "packs". These consist of multiple elements that are in series and/or parallel.

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Specifications of the packs

Described using the format xSyP (where x and y are integers), this tells you how the cells in the battery are wired up.  Batteries are made up of cells, whose voltage is determined by cell chemistry and whose capacity is determined by energy density and physical size of the cell.  S stands for series and P stands for parallel.  As you may know, series adds the voltage of the cells and parallel adds the capacity of the cells, so a combination of cells in series and parallel results in a battery.  The battery shown in the second image reads that it has an arrangement of 3S1P, meaning it has 3 cells that are all in series with no parallel wiring.  This may seem confusing because it says "1P," but think of the arrangement as a grid.  By multiplying the 3 and the 1, you get the total number of cells in the battery, which in this case is 3.  If it were a 3S2P battery, there would be 2 sets of 3 series-wired cells in parallel, resulting in 6 cells total.  Often times the parallel arrangement is omitted when discussing batteries, because most packs are 1P (so instead of saying you're using a 3S1P pack, you may as well just say 3S).

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Using more than 1 pack

Battery packs achieve the desired operating voltage by connecting several cells in series, with each cell adding to the total terminal voltage. Parallel connection attains higher capacity for increased current handling, as each cell adds to the total current handling. Some packs may have a combination of serial and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two strings of these 4 cells in parallel (for a pack total of 8 cells) to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning 4 cells are in series and 2 strings of these in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short. The foil also shields against heat transfer should one cell get hot.
Most battery chemistries allow serial and parallel configuration. It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.
Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

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Balancing

To balance the pack you need a balancing charger, or a separate Li-Po balancer used in conjunction with a charger. The charger needs to be specific for lithium polymer batteries, you cannot use an NiMH or NiCD charger on Li-Po packs because the charging algorithms are different between the chemicals. Trying to charge and balance a Li-Po battery pack with anything other than a Li-Po specific charger presents a potential and real fire hazard, so do not try it.There are many many different battery chargers available, some very basic and some very complex. The most basic ones often sold with RTF aircraft may only feature a socket for the balancing connector and will charge through that, while others will charge the pack through the main ESC leads and balance the cells via the white connector at the same time.
When it comes to charging and balancing your Li-Po battery packs always follow your specific charger instruction manual carefully and don't take risks.

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Charging 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.

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Storage of LiPo batteries

Well, you now know how a LiPo battery works, the safety concerns, what to look for when purchasing one, how to charge and balance one and why it’s important, what more is there???
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) due to cell oxidation. 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 lose 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 as the cells oxidize.
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 okay to store a fully charged RC LiPo battery at room temperature for up to 4 days without doing too much damage. Never store a LiPo in a hot car fully charged for an extended time, that will certainly cause damage as I explained earlier, but it worth repeating.
For optimum battery life, store your RC LiPo batteries (in a cool room if possible - slows down chemical reaction) at about a 40-60% charged state. 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 I recommend and what I follow myself.
You can actually extend the fully charged storage time from days to weeks by storing your batteries in the fridge (not freezer) close to 0 degrees Celsius (32F); again, that helps slow down the chemical reaction that oxidizes the cathode in the cells. I have started doing this with my smaller packs seeing that I will often find myself wanting to go flying with little lead time and it is very convenient having packs all ready fully charged.
If you do store your fully charged LiPo's in the fridge, pack them in a zip-lock freezer bag and squeeze out all the air before sealing the bag. This will prevent condensation forming on the battery packs when you take them out of the fridge as they warm up. You should allow the LiPo pack to warm up after removing from the fridge before using it of course.
I only store in cold temps if I know I will be flying within a 2-3 week time frame. Once winter hits and I know my flying days will be limited, I once again store at 50% charge @ room temp. It wouldn't hurt to store at 50% charge capacity in the fridge all the time either; but it takes up precious beer chillin' real estate (priorities you know).

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The Definitive Guide to Proper Disposal of Lithium-Polymer (LiPo) Batteries

When I was a newcomer I had concerns. I didn't want to do something dumb that could backfire on me, my club, or the hobby in general. Misuse of Lipo could have been just that dumb thing. Once when I had a lipo puff on me, I placed it on the ground several feet frommy car in case of fire. I enjoyed the rest of the day of flying, packed up and went home,only to realize I had left the puffed lipo at the field! The drive back to find it, in my mind,was absolutely necessary. What if some child had stumbled across it and picked it up right when it blew? That would have been a disaster, not only for the child but for myself, my club, and for aeromodeling in my community. Thankfully I found the Lipo and brought it back home. That event made an impression on me so I resolved to learn proper disposal so I would never have to worry about my actions causing anyone harm ever again. Little did I know that I was going the need that high school chemistry after all... Most newcomers will agree that the information I was seeking was difficult to locate and confusing at best if any was found and certainly not authoritative. This document will provide you with everything you will need to sleep peacefully at night knowing you have done the right thing concerning the disposal of bad lipo's...

Step 1 – Gather materials
You will need: A multimeter to measure DC voltage, wire cutter/stripper – blue painters tape – a 12 volt automotive light bulb, a disposable plastic container, salt, an insulated hobby knife, the broiler pan from your oven and most importantly a fireproof container. Learn what is actually fireproof before assuming that old metal ammo box is fireproof (which it most definitely is not). Here I use a ceramic crock pot, which is resistant to high temperature and is truly flame proof. Painted metal is neither. Ask any veteran you know. Clay flower pots at the local hardware store work too but be cautious of the hole in the bottom which could become a liability in the event of an emergency. Placing the flower pot on a flameproof surface (like a brick walkway or concrete garage floor) will reduce the danger.

Step 2 – Hack into the battery
Cut both the positive and negative wires leading to the main connector. DO NOT cut the wires at the same time. Cut one at a time, ensuring that no contact between + and - is made during the process! Cut them to the same length. Save the connector you have just removed for future use.

Step 3 – Strip the wires
Strip about ¼ inch from each lead. Again, be damn careful to avoid shorting the wires.

Step 4 – Begin the slow drain process
Using the blue tape, carefully tape one lead to the outer casing of the automotive bulb and the other lead to the bottom contacts. When the bulb lights – you're there. Immediately place the assembly into your fireproof container. Enjoy the glow. At this point the battery is discharging rapidly. Watch carefully at first for any bad reaction. You should see the glow in the bulb gradually fade away. In the event of sudden expansion or even worse, flame, immediately cover your fireproof container with the broiler pan and back away. The likelihood of this is slim, so chill and just enjoy the light show. Here's the easy part: Let the battery/bulb assembly sit in the fireproof container for 24 hours. The bulb will grow dim and eventually dark long before this – DO NOT proceed for 24 hrs, ok? Trust me.

Step 5 – Test remaining voltage
Remove the bulb from the leads and test the remaining voltage with your multimeter – You may be surprised to see over 2 volts remaining in the battery after 24hrs connected through a near short like a light bulb, yet it can happen. Usually you will see much smaller remaining voltages. Less than ½ volt is ideal. As long as the remaining voltage is less that 2 volts you are ok to proceed. If its a lot over 2 volts, call that guy in the club that knows all about lipos and ask his advice so you don't burn your house down. If there's noone to call – reassemble the battery/light bulb and let it sit another day or even two – It's not like your bad battery has a hot date anywhere. That's an important thing to remember – the battery has gone bad and you've already decided to dispose of it properly – there's no rush. Let the thing sit draining for a week if it makes you feel better, (which I have done). None of these steps has to happen quickly. In fact, the more time you take to be examine the situation the better. Keep draining and measuring until you're ok. Something you will observe at this point I call 'Voltage Rebound' – which shows up on your meter as a slow steady increase in voltage after disconnecting the drain mechanism. This is normal and as long as the voltage increase is not too fast (more than around 200mv per second or so) everything is as it should be.

Step 6 – Down the drain
Once you have a satisfyingly low voltage level, its time to throw it all away. It's time to short the leads of the battery together! This can be scary the first time you do it, but so was your first landing and you lived through that. Twist the leads together, (yes, with your fingers,) and place the battery back in your fireproof container. Let sit for another 24 hrs..

Step 7 – Measure Voltage Again
Your initial reading should show something less that 50 millivolts. The 'Voltage Rebound' you will see at this point will be very small – less than 3 millivolts per second. If the values you read are in this area – you're ok to proceed. If your voltage measurements are higher than 1/8 volt per cell, or the voltage rebound seems extremely fast, (like >50 mV/sec), something is wrong – call the club expert and ask for help.

Step 8 – Remove the outer covering
Using scissors, a hobby knife, a razor blade, etc. - Cut the outer covering in a safe place ¼ to ½ inch. This is enough to then peel the remaining plastic outer covering from around the battery. This is also the proper time to remove the balance connector if you wish to save it for use with another battery, or to give it to your friend at the club that has weird batteries with weird balance connectors that he wants to change out...

Step 9 – Separate the cells
Some Lipos will have an inner covering as well as the outer covering – if yours has one, cut and peel that as well – we're trying to gain access the the individual cells, thats the goal. Once any inner coverings are removed, pry the individual cells apart.

Step 10 – Score the cells
Using a plastic handled (or otherwise insulated) hobby knife – Slice at least two DEEP scores into BOTH SIDES of EACH CELL. Put your weight into it! You want to cut deeply into the cell at least as much as half it's thickness. The chemical reaction necessary to neutralize a Lipo will not occur effectively if your cuts are not deep! Gouge the cell deeply on both sides!

Step 11 – Prepare Salt Bath
Use a disposable container! After you're done, the container will be contaminated with Lithium, which is a bad thing. Pour a generous amount of ordinary table salt into the container. ½ inch deep covering the bottom should be enough. Add water until the container has enough water to cover ½ of the batteries height. Swirl vigorously! Mix the salt and water until no crystals of salt remain. You will notice the water become opaque. If you don't see this, you don't have enough salt. Swirl vigorously! When you are satisfied that the salt has completely dissolved, add enough water to cover the batteries height by about an inch.

Step 12 – Drown the battery
Drop the battery into the solution lead end first. The first thing you will see is bubbles rising from the scores cut into the surfaces of the cells – thats a good thing – the chemical reaction that neutralizes the lithium has begun. Depending on a lot of different factors, the bubbling may be very vigorous and result in a foam forming at the top of the container, or it may just bubble slowly and gently. Both results are ok. The important thing at this point is to leave the battery in the salt solution until there's no more bubbling. I've had lipos bubble for a week and then show signs of being completely neutralized, and I've had lipos bubble slowly for a month. To help the process – every day when you walk by the container, give it a swirl to stir up the salt solution and help the solution penetrate the polymer layers, then put it back and ignore it until the next day. Keep doing that until you see no more bubbles.
For those with a bent towards chemistry, here's why this works. The Lithium in Lipos is an ionic form of Lithium, hence the term Lithium-Ion Battery. Salt dissolved in water is no longer salt, but becomes Sodium Ions and Chlorine Ions. Mixing Chlorine Ions and Lithium Ions causes an exothermic reaction between the two which results in Lithium-Chloride (LiCL) which is a hydrate salt crystal. This is why we see the cells in the above picture swelled and bloated. The crystallization process makes each Lithium molecule larger when it combines with a chlorine ion to become a molecule of Lithium-Chloride, a stable hydrate salt that won't explode. Hydrate salts don't have the nasty tendency to catch fire like Lithium Ions do! In fact, Class D fire extinguishers, used to put out metal fires, actually shoot a powdered form of ordinary table salt. Bet you didn't know that!
Ahh, safe at last...

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Life time of LiPo batteries

If you respect the conditions of use may the life time can be 2 to 3 years, even if some packs last longer, or have a hundred charge-/decharge cycle. After that, the pack will still work, but the performance will be less because the internal resistance increases (due to the aging of the electrolyte).
Reminder of the conditions of the use of LiPo accus

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Reminder of the conditions of the use of LiPo accus

1. Use only a charger approved for lithium batteries. The charger may be designed for Li-Ion or Li-Poly. Both batteries are charged in exactly the same. Some older cell phone chargers may charge the batteries .1 volt to low (4.1 vs 4.2), but that will not harm the battery. However, inexpensive lithium chargers are widely available and the use of cellphone chargers is highly discouraged.
2. Make certain that the correct cell count is set on your charger. Watch the charger very closely for the first few minutes to ensure that the correct cell count continues to be displayed. If you don't know how to do that, get a charger that you do know how or don't charge the batteries.
3. Use the Taps. Before you charge a new Lithium pack, check the voltage of each cell individually. Then do this after every tenth cycle there after. This is absolutely critical in that an unbalanced pack can explode while charging even if the correct cell count is chosen. If the cells are not within 0.1 volts of each other then charge each cell individually to 4.2 volts so that they are all equal. If after every discharge the pack is unbalanced you have a faulty cell and that pack must be replaced.
   Taps are provided on most new lithium packs. Taps give you the ability to check individual cell voltages and charge one cell at a time. Make sure and get the appropriate connector to go into your taps. Don't try to stick you volt meter probes in the taps to measure voltage. They could slip and short your cells. Don't try to charge more than one cell at a time from the taps. Unless you have an isolated ground charging system, you'll short your batteries out. Refer to your individual cell maker for tap pin-outs.
4. NEVER charge the batteries unattended. This is the number one reason for houses and cars being burned to a crisp by lithium fires.
5. Use a safe surface to charge your batteries on so that if they burst into flame no damage will occur. Vented fire safes, pyrex dishes with sand in the bottom, fireplaces, plant pots, are all good options.
6. DO NOT CHARGE AT MORE THAN 1C unless specifically authorized by the pack vendor. I have personally had a fire in my home because of violating this rule. Todays highest discharge batteries can supposedly be safely charged at greater than 1C, however so far in all cases doing so shortens the life of the pack. Better to buy 3 packs than to try to charge 1 pack 3 times quickly. This may change in the future but as of Winter 2005 1C is still the recommended charge rate.
7. DO NOT puncture the cell, ever. If a cell balloons quickly place it in a fire safe place, especially if you were charging it when it ballooned. After you have let the cell sit in the fire safe place for at least 2 hours. Discharge the cell/pack slowly. This can be done by wiring a flashlight bulb of appropriate voltage (higher is voltage is ok, lower voltage is no) up to your batteries connector type and attaching the bulb to the battery. Wait until the light is completely off, then throw the battery away.
8. If you crash with your lithium cells they may be damaged such that they are shorted inside. The cells may look just fine. If you crash in ANY way carefully remove the battery pack from the aircraft and watch it carefully for at least the next 20 min. Several fires have been caused by damaged cells being thrown in the car and then the cells catch fire later and destroys the car completely.
9. Charge your batteries in a open ventilated area. If a battery does rupture or explode hazardous fumes and material will spew from the battery.
10. Keep a bucket of sand nearby when you are flying or charging batteries. This is a cost effective way to extinguish fires. This is very cheap and absolutly necessary.
11. It can happen to you, do not think to yourself that “it won't happen to me” as soon as you do that it you'll be trying to rescue your kids from your burning house or car. I'm very serious about this.