How Lithium Polymer Batteries Work – Your Source For Lithium Battery Info

Lithium polymer batteries are one of the best things that ever happened to electric RC flight. With their huge capacities, high energy density, and low weight, they’re a huge step forward from the NiCd and NiMh batteries used before. Lithium polymer batteries are great, but how exactly do they work? A modern Thunder Power lithium polymer battery is the result of over a hundred years of technological innovation, and it’s useful to have an understanding of what goes in to these amazing devices.

Batteries In General – How They Work

Before taking a look at what makes lithium polymer batteries special, we need an understanding of how batteries work in general. So, whats a battery? A battery is an array of electrochemical cells, wired together in series or in parallel. Each electrochemical cell can be thought of as a device which can “pump” charge. By wiring the individual cells together, we obtain an efficient device which can move electrical current in the way we need it to. Each electrochemical cell actually consists of 2 “half-cells”. The half cells are connected by what is called a “salt-bridge”, an electrolyte which allows ions to travel through it. The cell is constructed out of materials such that electrons want to travel from one half cell to another. Eelectrons flow into the cell at the cathode (positive electrode) and flow out at the anode (negative electrode). The electrolyte permits this travel, and the net result is whats called a potential difference between the cell terminals.

Electric potential is really just a fancy word for a simple concept: the amount of energy that a certain amount of charge contains. Think of it like this: if the wires to each cell were water pipes, and the cell was a pump, the potential would be the pressure in the pipes. Potential is usually measured in a unit called the volt. One volt corresponds to one joule of energy (about the energy needed to lift an apple 3.2 feet), per 1 coulomb of charge. A coulomb is simply a number of electrons (6.241506 × 10^18), the negatively charged particles which carry current.

It’s here that we see the first important advantage that lithium polymer batteries have over other types. Each lithium polymer cell can produce up to 4.2 volts. Other battery designs, like nickle metal hydride (NiMH) and nickel cadmium (NiCd) only give a maximum of 1.2 volts. From this we can see that you need fewer lithium polymer cells than NiCd or NiMh cells to produce the same voltage.

Besides maintaining a potential difference between the positive and negative terminals, a battery also
has to be able to maintain that potential difference for a set amount of time. The reason that we have to charge batteries is that over time, each chemical cell reaches equilibrium. At equilibrium, no ions want to travel between the half cells, and so no potential difference is created. With rechargeable batteries, sending a current through the cell reverses this process, so it can be used again.

Every battery has a capacity associated with it. Capacity measures the amount of charge that the battery can provide, usually given in milliamp hours. Think of amps as measuring the rate that charge travels at. 1 Amp corrosponds to 1 coulomb of charge moving per second. Because a coulomb is such a large measure of current (comparable to that found in lightening strikes), RC aircraft batteries use milliamps (thousandths of an amp) to measure current. Because amps measure the rate of charge, multiplying the current in amps by the time the battery operates gives us the total charge that the battery can move. This is expressed as follows:

If the capacity is known, we can find the time the battery can operate by dividing by the current. From the above equation:

In summary, batteries have two primary, defining quantities. The voltage, or potential difference, measures how much energy per unit charge the battery provides. The capacity measures how much current the battery can provide, or the time that the battery can operate while a given current is being drawn.

Lithium Polymer Batteries – Some Terms Explained

There are a few naming conventions and terms that the RC community has adopted, which express the number of cells in a pack, how they are wired, and how fast the pack can be discharged. The “C” is a common unit, which tells how long it takes to discharge the battery in fractions of an hour. For example, discharging a pack at 1 C would take 1 hour, at 2 C it would take a half and hour, and so on. If we take a 3000 mAh battery and discharge it at a rate of 3000 mA, it will take 1 hour and the battery will be discharging at a rate of 1 C. You get the charge / discharge rate in C by dividing the capacity by the current.

Every lithium polymer battery consists of cells wired in series and parallel, and the number of each is specified on the pack. The naming convention looks like “XSYP” where X is the number of cells in the pack, and Y is the number wired in parallel. As an example, a 4S3P battery would have 4 cells in series, and 3 in parallel, giving 12 total.

RCToys.com has a huge selection of lithium polymer batteries, and the chargers, balancers, and power supplies needed to use them.

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