Introduction to Lithium Polymer (Li Poly) Batteries
Lithium Batteries are found in many modern electronic devices, such as laptops and cell phones. They are significantly lighter than NiMH or NiCD batteries, and have higher capacity for the same size. Over the last five years they have worked their way into the hobby market, for use in electric RC, boats, cars, and particularly Heli. Their light weight and high capacity make them ideal for long flight times, while also providing more power.

Voltage, Cell Count And C Rating:
The nominal voltage of each cell is 3.7V, but can go down to 3.3V during discharge, and up to 4.2V when fully charged. A battery pack is usually composed of two or more cells put together in series for increased voltage, or in parallel for increased capacity. The C rating denotes how quickly a battery can be discharged (a rating of 1C continuous would mean that a 2000mAh battery should not be discharged any faster than 2000mA or 2A, which would take one hour). A 2000mAh pack rated at 12C continuous would be able to discharge at 12 times its capacity (12 x 2000mA = 24000mA or 24A) at which rate it would discharge in 1/12th of an hour. If you know how much continuous current you will be drawing and the capacity of the pack you want to use, you can easily determince what C rating you require. If you are drawing 5A from a 1320mAh pack, just take the current and divide by the capacity: 5A = 5000mA, 5000mA / 1320mAh = 3.8C. Using a pack with a higher C rating than you require will leave some headroom, and extend the life of your battery. Batteries are also given a C rating in terms of burst, which is how quickly the battery is able to discharge for a short period. A burst rating of 20C would mean a 2000mAh battery could supply 20 x 2000mA = 40000mA or 40A for a few seconds at a time.

Naming Conventions:
Lithium Batteries are also know as LiPo, Li-Po, LiPoly, or Li-Poly. The pack configuration is denoted by the number of cells in series and the number of cells in parallel. A 3s2p pack would have three cells in series, and 2 cells in parallel, using a total of 3×2=6 cells. A 4000mAh 3s2p pack would have a capacity of 4000mAh, and a voltage of 11.1V (3 x 3.7V). It would internally consist of six 3.7V 2000mAh cells. The cells would be doubled up (the 2p part of 3s2p) to get 4000mAh, and there would be three in series (the 3s part of 3s2p) to get 3 x 3.7V = 11.1V.

Which Battery Is Best For Me?:
To select a battery, you first need to know what voltage you require and how much current you will be drawing continuously. If you have a motor that works with 11.1V, you would need a 3 cell battery. If you need to draw 20A, and you would like to have a 10 minute (1/6th of an hour = 6C) flight, you would need a battery with 20A / 6 = 3.3A = 3300mAh. This means you would need a 3s 3300mAh battery with a C rating of 6 or higher.

Safety Precautions:
Never charge a lithium battery if it is below 3.0V per cell, puffed up, or damaged. Always place on a fire-proof surface when charging. Only use chargers designed to work with Lithium Polymer batteries. Never leave your battery unattended while charging.

Lithium Polymer Battery Balancer Information and Benefits
Lithium polymer batteries have made R/C flying more fun. The performance they offer is revolutionary and remarkable. But, they are susceptible to damage and need to be treated carefully. One important device that protects and maintains your lithium polymer battery is the balancer. Let’s examine how that is.
Lithium polymer batteries are damaged by:

• Mechanical deformation
• High temperature (above 60degC / 140degF)
• High voltage (above 4.2V/cell)
• Deep discharge (below 3.0V/cell in use, 3.3V/cell recovered, or 3.7V/cell storage)
• Rapid discharge (shorting cells or using above recommend discharge rates)

Of these, high voltage is the most difficult to control. Batteries made of lithium polymer cells wired in series will naturally, with time and cycles, become imbalanced. That is, each cell will be at a slightly different voltage.
This leads to high voltage while charging, as the charger can only read the average cell voltage. The charger will bring the pack to an average of 4.2V / cell, but one or more cells are above 4.2V / cell. Slight imbalance is always present -even in a battery that is new and balanced by the factory - and does no harm, but large imbalance does harm and is prevented by a balancer.
A balancer connects to a battery via cell taps. Cell taps are small wires soldered onto each cell’s tabs. The balancer thus has direct access to every cell, allowing it to adjust individual cell voltages by dissipating the energy as heat, or re-routing it to cells of lower voltage.
The tangible benefits of this include:

• Stronger performance throughout the battery’s life
• Longer battery life (more cycles)
• Ability to eliminate one cause of cell combustion, namely cell over voltage while charging due to imbalance

Though every brand and model of cell balancer works a little different, the general procedure for their use is: Connect the balancer to the battery before charging, charge, and remove balancer when charge is complete. Regarding frequency of use, a balancer can be used with every charge, though this isn’t necessary if the battery is in good condition. Most users feel the extra effort of using a balancer with every charge is worth the added safety features.

Guide to Charging Lithium Polymer Batteries
Lithium polymer batteries are expensive. How you handle yours will have a big effect on how long they last. Therefore, it is worth taking the time to learn about proper charging procedures.
Lets say you just finished using your lithium polymer battery. It can be charged again without delay, as long as it isn’t hot. If it is hot, you will need to wait until it has cooled down; preferably to below 35° Celsius (95° Fahrenheit).
Your battery isn’t hot, so let the charging begin. The first steps are to power up your charger and connect your battery charge cable to the charger. You should plug in the charge cable before connecting it to the battery, because charge cables usually terminate in exposed bullet / banana connectors, which are easy to short to each other directly or via anything metal (i.e. the charger). Now you can connect the battery to the charger via the charge cable.
It’s time to set the charger’s settings. Whatever the input method, there are two important settings: charge voltage and current. The correct voltage setting is determined by the lithium polymer battery you are charging. Most batteries say their nominal voltage, with the common ones being 7.4V and 11.1V. This number is calculated as
Correct Voltage Setting = 3.7V * Number of Cells in Series
On small to medium sized lithium polymer batteries, the number of cells in series is simply the number of cells in the battery. However, on large batteries, some of the cells are wired in parallel and as such do not contribute to the correct voltage setting. The standard for writing the number of cells in series is Xs. For a small battery, like the Thunder Power Pro Lite 3s 1320mAh, we see that X=3. For this battery, correct voltage setting = 3.7V * 3 = 11.1 V.
Correct voltage setting now determined, all that remains is current setting. Current setting is largely up to the user. The most common and always correct setting is:
Standard Current Setting = Battery Capacity / 1h
Continuing with the above example, the battery capacity is 1320mAh. Therefore, standard current setting = 1320mAh / 1h = 1320mA = 1.32A. This is known as a 1C charge, because C is defined as 1 / 1h.
Some like to charge slower than 1C to be gentle with their batteries. However, there is no scientific evidence of slower charging leading to longer battery life.
With those two settings set, you are ready to start the charge. Once the charging process has started, the charger takes care of everything. It will end the charge when the battery is full, at which time you can use it again. There is no need to wait between the end of a charge and the start of a discharge.
There are two ways of increasing the safety of your charges. One is to use a balancer. The balancer will fight cell imbalance, and provide some a warning if the cells are dangerously imbalanced. The other is to place your battery somewhere nonflammable. Ask yourself, if this battery ignites, will anything nearby ignite as well? Leaving the battery on a wood counter or in a wood airplane is more risky than isolating it on a bare cement floor.