| Many
thanks are due to Lew Weaver for his invaluable technical
help and support in compiling this article. Thanks Lew. |
BDMFC
electric flight - Batteries
For the average Club sport
flier there are typically three main battery cell types currently in
use.
Nickel Cadmium (NiCad) -
although produced from an old technology manufacturing process,
NiCad cells continue to play a useful role in electric flight
packs. Although it's always dangerous to abuse any battery, NiCads
are able to withstand some mishandling and still provide a reliable
low cost source of power. A major disadvantage of NiCad packs is
their ability to 'remember' previous partial charge levels and they
will sometimes stop at this level during charge without being fully
charged. When this happens it is necessary to 'recycle' the pack
by discharging (to around 1v per cell) and recharging several times
to re-establish the fully charged level. This is normally all that
is required to revitalise a NiCad pack with a 'memory' problem.
Lithium
Polymer (LiPoly)
- New technology manufacturing technique with a major improvement
in capacity per cell compared with NiCad and NiMH cells. Much more
sensitive to mistreatment, over-discharging can cause terminal damage.
With a fixed voltage of 3.7v per cell a 7.4v pack can be made from
only 2 cells with a significant saving in weight over equivalent NiCad
or NiMH packs. Very little loss out of use after charging so Li-Poly
packs can be charged and left for a week then used without recharge.All
Li-Poly packs are sold with explicit safety instructions for use due
to the volatile nature of the materials used in their construction.
As a new technology, high performance product, Li-Poly packs are rapidly
reducing in price and this will no doubt continue as these packs become
more widely used.
Note:
it is very important that newcomers to this technology do not attempt
to charge these cells without a dedicated lithium charger and to
charge at the correct voltage and current settings. To do anything
else will result in disaster.
Basic battery
terminology
Voltage
(volts) -
voltage of a flight pack is determined by the number of 1.2v
cells wired in series (+ connected to - on each cell). e.g. a
7 cell pack will have a nominal voltage of 8.4v. Increasing the
voltage to an electric motor will normally increase the shaft
speed and therefore thrust from a well matched propeller.
Capacity
(mA/h) -
the capacity of a flight pack regardless of voltage is a measure
of how many Amps the pack will provide (current flowing to the
motor) over a given period of time (usually an hour). For example,
a flight pack rated at 1000mAh will provide 1000mA (1 Amp) for
an hour. As the typical current used by a motor is around 5 -
10 Amps then this means the pack could provide 10 Amps for 1
hour divided by 10 or 6 minutes. This is a very approximate measurement
as the current used by motors can vary significantly. For example,
fitting a very large propeller will cause the motor to use more
current (Amps) which will result in a corresponding reduction
in duration (flying time).
Power
(watts) -
not normally measured but this is a measure of the power produced
by the pack and is calculated by multiplying voltage at the motor
by current ( volts x amps). This
measurement is the best way of finding out how much power the
motor/battery is producing and is almost essential for estimating
the required power for an own-design electric model or conversion.
A Wattmeter is one of the better investments for
a dedicated electric model flier.
Resistance
(ohms) -
again not normally measured but flight pack cells which have
a low resistance rating will be capable of supplying more power
to the motor. Best example of
this would be the GP3300 NiMH cell - you can draw >100A if you
need to from this cell - that should warm your hands in the winter
!!
C rating (Current) - There has been quite a bit of debate
on this, but the C-rating is a figure you see quoted by most Lithium
manufacturers.
The easiest way to understand this rating
is for "C" to
stand for current, and this relates to the current that can be
drawn from the cell, e.g. if a 1000mAH(1AH) cell is rated at 10C
it means you can safely draw 10x1Amp=10amp. Cells are often rated
as say 10-15C which in the case above would mean 10amp continuous
with 15A for short bursts.
It is important not to exceed these
figures or the life of the battery will be shortened.
Useful measuring tools for the electric flier
1) Wattmeter, will measure Watts,
amps and volts. Indispensible for the dedicated electric builder,
flier and for tuning motor/battery/model combinations.
2) Multimeter - handy for general
purpose use.
3) Ammeter - not much use unless
the 'clamp' type.
Useful
electric flight inks - suppliers of batteries, motors, ESCs, accessories
and kits
