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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.
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, the Thunder Power
1900mAh packs have 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.
Fortunately, several good chargers are available through
FMA. Astro Flight, plus many other companies including
E-Flite and Thunder Power, will be releasing inexpensive
LiPo-specific chargers within the upcoming months.
These chargers give a constant current charge rate at
75% the cell capacity until the pack voltage reaches 3.6
volts per cell. This charges the pack to about 80% of total
capacity. At this point the charger switches to a constant
voltage charge rate of 3.6 volts per cell to top off the
battery. To charge a fully depleted pack typically takes
about one and a half hours.
Serious Safety Issues
If LiPo batteries are improperly charged they can cause
an explosive fireball. 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.
The games should be over by 7:30pm. There are also a
few U-14 boys and girls games on Saturdays on fields 9
&10 that should be over by 3:00pm.
There was a lot of excitement at our last meeting. A lot of
talk about holding a major raffle to raise money for the
purchase of land, and holding our major fun fly later in the
summer. I hope to see many of you out at Leon’s for our
next fun fly/meeting.
-- Kevin
Minutes
Meeting called to order by President. Rusty gave
Treasurer’s report. Club has $5,466.00 in cash assets.
Discussion about the clubs cash assets and its purpose
was discussed, i.e. the money was/is being saved for the
purchase of land.
Members discussed that this is not enough money to
purchase enough land to fly off of. Rusty and Darin
brought up the idea of a raffle to raise money for the
purchase of land. Wayne made a motion to have the club
host a raffle, Rusty made a second motion and everyone
was in favor of having a raffle.
Raffle to consist of: Grand Prize drawing, a $500.00
shopping spree at Grampa’s Train Shop, or $400.00
cash. 2nd place drawing will be $150.00 shopping spree
at Grampa’s Train Shop, or $100.00 cash, Third prize will
be either $50.00 or $75.00 in a gasoline giveaway. Rusty
to follow up on raffle license, Darin to follow up on tax id #
for the club. Details of raffle to be finalized by Rusty and
Darin. The raffle will likely be held in the beginning of
September during a fun fly at moon lake park, after the
Eau Claire Fun fly and New Richmond Fun Fly.
Members broke out into a work session after the meeting
and leveled the shed in its new location, added the
frequency board to it, and set up the clubs newly finished
rollaway airplane stands like the ones in the AMA
magazine. The orange flight line fence was also installed.
Understanding Lithium Batteries
Lithium Polymer, or 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