BATTERY BASICS
by Ed Anderson
aeajr on the forums
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 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, NICD, 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, 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 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 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 40-70% more capacity per ounce
than NICDs. So, for example, a 900 mah NICD pack might weigh 6 ounces while
an equivalent capacity NIMH pack might be 3.5-4 ounces. However 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.
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. So that
same 900 mAH NICD pack at 6 ounces would compare to a 900 mAH LiPoly 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.
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 NICD 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. 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 NIMH or NICD 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 xS xP
designation, were the first x is how many cells 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 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.*********.org/lithium_bat...eakthrough.htm
http://www.rcgroups.com/forums/showthread.php?t=209187
More on Batteries
http://www.modelaircraft.org/mag/FTGU/Part8/index.html
New Electric Flyer FAQs
http://www.ezonemag.com/pages/faq/a105.shtml
A series of posts on electric power system basics
http://www.wattflyer.com/forums/showthread.php?t=1933
http://www.rcgroups.com/forums/showthread.php?t=417868