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 Battery Basics

By Ed Anderson ("aeajr" on the forums) 

April 2, 2005

All RC planes use battery packs to operate their electronics. Planes that don't have electric motors must still have batteries called receiver packs. Receiver Packs power the receiver (RX) and the receiver then distributes the power to the servos and other electronics in the plane. However, for electric planes, we also use the batteries to power the motor. They are the chemical fuel tanks that store the energy we use to fly. Each has advantages and disadvantages.


Battery Types

At the time of this writing, April 2005, there are three commonly used rechargeable types of cells. They vary by the chemical mix that is used to hold and deliver the electricity to the motor of the plane. 

Nickel Cadmium, NICAD, have been in around the longest. 

Nickel Metal Hydride, came in to use later and are very popular today. 

Lithium cells are typically lithium polymer, Li-Poly or LiPo, and the less commonly used Lithium Ion. These are the newest breed of chemical cells.

NICAD packs have the lowest power to weight ratio. That is to say that, for a given capacity they will weigh the most of the three types. However they have the ability to be charged faster than the other two and will give up their power fastest. While still in common use, the are dropping in popularity as the other two types are improving and gaining on NiCad's advantage of quick charge and quick discharge. Each NICAD cell is rated at 1.2 volts.

Nickel Metal Hydride, NIMH, packs hold about 40-70% more capacity per ounce than NiCad's. So, for example, a 900 mAH NICAD pack might weigh 6 ounces while an equivalent capacity NIMH pack might be 3.5-4 ounces. However NIMH packs can't quite match NiCad's for how fast they can deliver their electricity or how fast we can charge them, but they are catching up. There used to be a big gap, but the gap is closing fast. NIMH are far more popular today then they were just a few years ago, and probably have passed NiCad's in usage. Each NIMH cell is rated at 1.2 volts, the same as NICAD cells.

Lithium packs are the lightest for their capacity. They typically hold 4 or more times as much electricity per ounce as compared to NICAD packs. So that same 900 mAH NICAD pack at 6 ounces would compare to a 900 mAH Li-Poly pack at 1.5 ounces. However these packs have been slower to charge and slower to deliver their power, but over time they are improving. They are growing in popularity as the discharge rates improve and the prices come down. Each Lithium cell is rated at 3.7 volts. Lithium Ion cells are more commonly used in smaller aircraft as they do not give up their power as fast and typically are not a good choice for driving large electric motors.

Pack Configuration

Unless stated otherwise, we join the cells into packs by joining them in series. In series we add the voltage of each cell so that a 6 cell NICAD or NIMH pack will be rated at 6 X 1.2 volts or 7.2 volts. With lithium packs, which are rated at 3.7 volts per cell, it would take two cells to create a comparable 7.4 volt pack. Before Lithium batteries came along,  you would hear people talk about 4 cell, 6 cell or other pack cell count and you knew they where usually talking about, either NICAD or NIMH cells. However, with the rise of Lithium cells, you should ask to be certain that they are not talking about lithium cells. 

Clearly if your instructions say that your motor can use a 7 cell pack, it would be important to know if that is 7 NIMH or NICAD cells or 7 Lithium cells as the voltages would be very different. A 7 cell NIMH or NICAD pack would be 8.4 volts. A 7 cell Lithium pack would be 24.9 volts.

While it is unusual to combine NICAD or NIMH packs in parallel to increase capacity, it is quite common with Lithium packs. This has spawned the xS xP designation, were the first x is how many cells are connected in series and the second is how many groups of these cells are connected in parallel. So a 3S2P pack would have two groups of 3 cells. This allows us to deliver higher amperages at the same voltage, or to provide more capacity for longer flights at the same voltage. The xSxP designation is most commonly used with Lithium packs.

Battery Chargers

When charging your battery packs you MUST use the right kind of charger or you will damage the cells. Using the wrong charger, especially with lithium cells, can actually lead to a fire or an explosion. So be sure that you have the right charger for the kind of cells you are charging. Some chargers are made for one kind of cell, some can charge two kinds of cells and some can charge all three. Make CERTAIN you know before you charge or you could put your model, your care, your home or your personal safety at risk.

For more on volts, amps, C ratings, read the FAQ that covers that topic.

I hope this has been helpful. Below are some additional resources for further reading.

Excellent overview and safety information on Lithium Batteries
http://www.rchobbies.org/lithium_battery_breakthrough.htm

More on Batteries
http://www.modelaircraft.org/mag/FTGU/Part8/index.html