Rechargeable Battery: How Lithium-ion polymer batteries works in remote controlled planes and helicopters

Lithium-ion polymer batteries are rechargeable (secondary cell) batteries. They are especially used for flying planes and helicopters. How Lithium-ion polymer batteries work

The array of electrochemical cells in the batteries are wired together in a series of parallel. Each of these electrochemical cells acts as a device that can “pump” charge. The individual cells are wired together to create an efficient device that can move electrical current in the desired manner. Each cell consists of 2 “half cells” that are connected by a line refered to as the “salt-bridge.” This salt-bridge is an electrolyte that allows ions to pass through it. The material used to construct these half cells allows electrons to travel from one half cell to another. Electrons enter the cell by flowing through at the cathode (positive electrode) and then exits (flows out) at the anode (negative electrode). This flow of electrons between the half cells takes place via the salt bridge, resulting in a potential difference between each cell terminal.

Electric potential is the amount of energy contained in a certain ion and is measured in volts. It takes one volt of energy to correspond to one joule of energy per coulomb of charge. Coulomb refers to the number of electrons (6.241506 x 10^18) which are the negatively charged particles responsible for carrying current. In addition to maintaining the potential difference between the cathode and anode (positive and negative) terminals, batteries also need to be able to maintain this potential difference for a certain amount of time.

Batteries require charging because overtime each cell reaches its equilibrium, at which stage ions do not want to travel between the half cells. When this happens, no potential difference is created. This process is reversed with rechargeable batteries, as current is sent through the cells making it usable again.

The capacity of a battery refers to the amount of charge it can provide and is usually given in milliamp hours (mAh). An amp is the measurement of the rate at which charge travels. It is easier to distinguish the strength or capacity (C rate) of a battery using the mAh. The voltage/potential difference and capacity are the two primary defining quantities of a battery, and these are the numbers used for battery shopping. The voltage measures the amount of energy the battery provides per unit charge while the capacity measures the amount of current the battery can provide, or the length of time the battery can operate while a specific amount of current is being drawn from it.

Advantages over other types of RC batteries

LiPo's are popular today for several reasons.

1. They are lightweight and have large energy capacities.

2. They have a high discharge rate that powers the most demanding electric motors.

3. They provide high energy storage to weight ratios and come in various shapes and sizes.

4. Lithium polymer batteries can produce up to 4.2 volts while other battery designs can only give a maximum of 1.2 volts.

5. Fewer lithium polymer cells than NiCd or NiMh cells are needed to produce the same voltage.

Disadvantages

1. Expensive when compared to NiCad and NiMH although the price continues to decrease.

2. Charge cycles can be short.

3. The electrolytes in LiPo’s are volatile and can catch a fire and explode, creating a safety issue.

4. Requires unique and proper care if they are to last any time.

Charging, Balancing and Storage

The charging, discharging and storage of LiPo batteries affects their lifespan. A simple mistake can destroy the battery. The three main things that shorten the life span of a LiPo battery are heat, overcharging and inadequate balancing.

• Use the correct charger at all times for your safety and to protect the life span of the battery.

• Charging the battery beyond its maximum will destroy it and can cause it to explode.

• For computerized chargers, ensure that the correct voltage or cell count is used.

• Select the correct charge current and never charge the battery greater than 1 times its capacity. Note that as batteries improve it may be possible to charge up to 2C or even 3C greater on quality packs with a discharging rate of at least 20C.

• The best way to balance and charge a LiPo battery is to use a computerized charger that comes with a built-in balance circuitry.

• Store batteries while they are partially charged. Fully charged storage time can be extended if the battery is stored in the fridge (not freezer) at 0 degrees Celsius (32F). Place the battery in a zip-lock freezer bag and remove all the excess air before placing in the fridge.

• Charge and store batteries in a fire-safe container and away from flammable items.

In the case of a crash, carefully remove the battery pack and observe it carefully for at least 20mins. Keep a bucket of sand close by while flying or charging your battery. The sand will quickly absorb any gas leaks and quickly stop a fire. After discharging, place it in a covered container of salt water outside. Let it stand for approximately two weeks. If the ballooned or its covering is torn skip the discharge and place in salt water as quickly as possible. Once fully discharged LiPo’s can be discarded in the trash as they are non-toxic.

Here are some good brands to try.

Turnigy LiPo batteries from HobbyKing.com.

Hyperion G3 Polymer batteries

Thunder Power RC’s G6 battery series