BATTERY BASICS
by Ed Anderson
aeajr on the forums
Revised March 2014
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. Still used for receiver
packs but are no longer commonly used as motor packs.
Nickel Metal Hydride, NiMH came in to use later and are still in use
today for receiver packs. While still used for motor packs their popularity
is fading compared to the lithium based cells. .
Lithium cells are typically lithium polymer, LiPoly or LiPo, or the Lithium Ion
cells. These are the newest breed of chemical cells. They are starting to
be used as receiver packs but probably represent the most popular battery
type for powering electric airplane motors.
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.
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. 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. For this reason they have become
the standard for most electric airplane applications. They typically hold 3 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 13 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 to power most receivers 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. But be careful.
Your receiver might be able to handle a 7.4V 2 cell lipo pack but your servos may not.
The newest generation Lipos and LiOn packs can now deliver high currents. And
many lithium packs rated 20C or higher can be charger higher than the traditional
1C maximum that has been standard for Lithium packs. 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.
BATTERY CHARGING SAFETY
Any battery pack can fail or overheat so always charge your batteries in a fire proof area.
While most cautions are directed to Lithium type packs, I have never had a problem with a
Lithium pack. But I have had a NiMh pack catch fire. So no matter what you are charging
take precautions.
A popular aid to charging is the LiPo charging sack. Here are some examples:
http://www3.towerhobbies.com/cgi-bin...&I=LXZKA2&P=ML
http://www3.towerhobbies.com/cgi-bin...?&I=LXCBTN&P=7
There are also Lipo bunkers
http://flitetest.com/articles/LiPo_Charging_Bunker
If the links have gone out of date, just do a yahoo or Google search on Lipo Sack and you
will find a variety of offerings. Their design is to contain any flame or burning that might result
from a failed pack during charging.
If you follow proper charging procedures and use a
quality charger then the chances are very slim that you will have a problem. But just like the
seatbelts in your car, you put them on to be prepared for the unexpected. And use these for
all battery packs, not just lipos. I keep a coffee can on my work bench and all batteries get
charged in the coffee can or in a lipo sack, just in case.
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.
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 xSyP 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 additional resources for your
further reading.
Lipo Batteries and charging for your model airplane
http://www.longislandelectricrcairpl...nbatteries.php
The Battery Clinic
http://www.rcbatteryclinic.com/
AMA – battery basics – the article is a bit old but still useful
http://www.modelaircraft.org/mag/FTGU/Part8/index.html
Battery Basics
http://www.redrockethobbies.com/RC-A...sics-s/263.htm