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Old 02-23-2008, 05:31 AM
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AEAJR
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BATTERY BASICS
by Ed Anderson
aeajr on the forums
Revised January 2007

All RC planes use battery packs to operate their electronics. On planes
that don't have electric motors we call these receiver packs as they power
the receiver 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 and
fuel pumps that store and deliver the energy we use to fly.

These battery packs are made up of cells which act as chemical storehouse
for electrical energy. When multiple cells are joined together we call this
a
battery or battery pack. There are a variety of battery types. Each has
advantages and disadvantages that we will discuss.


Battery Types

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

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

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

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

NiCd packs have the lowest power to weight ratio. That is to say that, for
a given electrical 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 fast. While still in common use, the are dropping
in popularity as the other two types are improving and gaining on NiCd's
advantage of quick charge and quick discharge. Each NiCd cell is rated at
1.2 volts.

Nickel Metal Hydride, NIMH, packs hold about 40-60% more capacity per ounce
than NiCds. So, for example, a 800 mah NiCd pack might weigh 6 ounces while
an equivalent capacity NIMH pack might be 4 ounces. Except for very high
performance, NIMH packs can't quite match NiCds 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 NiCd in usage. Each
NIMH cell is rated at 1.2 volts, the same as NiCd cells.

In many ways NiCd and NiMh cells are very similar in their application. So,
as a shorthand, I am going to start to refer to NiMH and NiCd as NiXX when
what I am saying applies to both. I hope this does not lead to confusion on
the reader's part.

Lithium packs are the lightest for their capacity. They typically hold 4 or
more times as much electricity per ounce as compared to NiCd packs. For
example a 6 cell, 7.2V 2100 MAh NiCd pack might weigh 12 ounces while a 2
cell 7.4V Lithium pack of the same capacity will be about 4 ounces.

Because much of our RC electronics have been based on 4-5 cell NiXX packs
they are tuned for 4.8-6V receiver packs. However Lithium packs are 3.7V so
one cell is a bit low and two cells at 7.4V is a bit high. So Lithiums have
not been in common use for receiver packs used in gliders or glow powered
planes. Some micro plane electronics systems have been designed for 1 cell
lithium packs and the newer generation of electronics for the rest of the
market are being retuned to accept 1-2 cell Lipo receiver packs.

As a result, Lithiums have been used primarily as motor packs. Up until
recently, Lithium packs have been slower to charge and slower to deliver
their power. The newest generation Lipos can now deliver high currents but
still need to be charged at 1/3 the rate of NiCd or 1/2 the rate of NiMH
motor packs. However over time they are improving. They are growing in
popularity as the charge/discharge rates improve and the prices come down.
Each Lithium cell is rated at 3.7 volts.


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 NiXX
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. When you hear people talk about 4 cell, 6 cell,
however many cells today, they are usually talking about NiCd or NIMH cells.
However, with the rise of Lithiums, 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 NiXX cells or 7 Lithium
cells as the voltages would be very different. A 7 cell NIMH or NiCd
pack would be 8.4 volts. A 7 cell Lithium pack would be 24.9 volts.

While it is unusual to combine NiCd or NIMH packs in parallel to increase
capacity, it is quite common with Lithium packs. This has spawned the xSyP
designation, were x is how many Lithium cells are connected in series and y
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. I don't recall ever seeing this used with NiXX
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 specific to one kind of cell while some can charge two kinds
and
some can charge all three. Make CERTAIN you know before you charge or
you could put your model, your car, your home or your personal safety at
risk.

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
http://www.rcgroups.com/forums/showthread.php?t=209187

Lithium Battery Balancers and Chargers
http://www.rcgroups.com/forums/showthread.php?t=599287

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

A123 CELLS - This is an emerging cell for large
electric plane use.
http://www.rcgroups.com/forums/showthread.php?t=618071#post6636651

The Battery Clinic
http://www.rcbatteryclinic.com/
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