| At
this year's F3A Aerobatic World Championships held in Poland,
history was made. For the first time in its over-30-year existence,
an electric powered model was competitive against the most powerful
O.S. and Y.S. glow-powered pattern airplanes in an FAI world
championship format. At the start of the event, contestants and
spectators alike were skeptical, believing that competing with an
electric model was more of a publicity stunt than a serious
competition attempt. But after the first flight, all skeptics were
silenced and by the end of the preliminary rounds, a 4th place
position transferring into the finals proved that an
electric-powered model had the right stuff to compete against the
most powerful glow-powered aircraft, as flown by the best pilots in
the world.
The technology
that's made this possible is also changing the face of electric
modeling at local fields and indoor flying sights across the
country. Lithium Polymer batteries are the latest technology and
offer several times the capacity/run time of previous generation
Nickel Cadmium and Nickel Metal Hydride batteries, with reduced
weight and size. Plus the latest generation LiPo (short for Lithium
Polymer) cells have a low internal resistance, so they're able to
provide high amp draw for powering high-performance and even
large-scale models.
RaidenTech.com
is in to stock
tons of RC LiPo batteries. Imagine flying a GWS Tiger Moth or FD
stick for over an hour on a single charge. Or imagine an indoor 3-D
model that you can hover in a small office for over 15 minutes. With
LiPo batteries you no longer have to imagine, as this type of
performance is now a reality.
Understanding LiPos
Lipo batteries are very different from previous generation batteries
and understanding how they work, and especially how to charge them,
is the key to getting the best performance.
Series
Individual Lithium Polymer cells have a nominal voltage of 3.7 volts
(vs. 1.2volts per cell for Ni-Cds). Cells are wired in series to
give the following pack voltages:
1 cell = 3.7 volts
2 cells in series = 7.4 volts
3 cells in series = 11.1 volts
Parallel
Unlike Ni-Cd and Ni-MH cells that self-discharge when wired in
parallel, LiPo cells can be hooked up, charged and discharged in
parallel with no detrimental effect. Wiring two LiPo cells in
parallel doubles the capacity (more run time), plus an important
advantage of wiring in parallel is that each cell only sees half the
total current.
3S 2P
A battery pack that has three cells in series (giving 11.1 volts)
and 2 of these 3-cell packs are wired in parallel is commonly
referred to as a 3S, 2P (3 series, 2 parallel).
C Rating
LiPo cells are also commonly given a C or current rating. This is
the maximum average recommended discharge current for the cell. For
example, lets look at a 1900mAh packs with a 6C rating. To determine
the maximum recommended discharge rate multiply the capacity times
the C rating. 1900mAh x 6C = 11,400. So the maximum recommended
discharge rate would be 11,400mA or 11.4 amps. If your application
has a higher amp draw, remember that LiPo cells can be wired in
parallel, and with 2 cells in parallel each cell sees half the total
current. With 3 cells in parallel, each cell see one third the
current.
By wiring packs in
a combination of series to get the voltage and parallel to achieve
the capacity and individual cell current to an acceptable level,
LiPo cells can be used to power nearly every type and size of model.
Charging
Charging LiPo batteries requires a very different charge method than
other types of cells. It's imperative to use a charger designed
specifically for LiPo batteries, and with most LiPo-specific
chargers it's necessary to correctly select the cell count (1, 2 or
3 cells) manually.
Serious Safety
Issues
If LiPo batteries are improperly charged they can cause an explosive
fireball. Our staff recently tested the effect of improperly
charging LiPo cells and the results were dramatic! We cannot
over-emphasize the importance of using a charger specifically
designed to charge LiPo batteries, and to be positive that the
manual cell count (if the charger has one) is correctly set for the
specific battery being charged. If you use a good quality LiPo
charger, these batteries are totally safe.
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LiPo Do's and Don'ts
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Do:
Only use a charger that is specifically designed to
charge LiPo batteries. Using other types of chargers can
cause an extreme fire hazard!
|
Do:
LiPos don't develop memory or voltage depression
characteristics like Ni-Cds. Do charge them without the
worry of cycling or discharging them.
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Do:
Store LiPos at least partially charged. LiPos will
maintain their performance levels over time, even during
non-use, much better than Ni-Cds and there's no need to
cycle them.
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Don't:
Use any type of charger that is not specifically
designed to charge LiPo batteries. Using non-LiPo-specific
chargers can cause an extreme fire hazard!
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Don't:
Fully discharge your LiPo battery pack. Most speed
controller have a voltage cut off that will prevent
over-discharging the battery. Discharging a LiPo beyond
it's critical minimum voltage will cause damage to the
battery.
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Don't:
If you have a crash and the battery is damaged, don't
put the battery in your car or house immediately after a
crash. It's possible that a chemical reaction can take
place in the damaged battery that could cause a fire.
Put the battery in a safe place for a least one hour.
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