The Battery Clinic - 2011/05

92 MODEL AVIATION
Red Scholefield | redscho@The Battery Clinic bellsouth.net
Also included in this column:
• Let MAMA watch your A123
flight pack
• Hobbico LiFe charger and
LiFe discussion
• Reader’s thoughts about Li-
Poly balancing
• Robinette Micro Grey Ghost
offers flexible payload
options
Models you don’t see everyday
In addition to charging its LiFeSource
receiver packs, the Hobbico LiFeSource
balancing charger can handle other one-,
two-, or three-cell LiFePO4 packs. Shown is an alternative to using LiFe packs.
Several models at the Zephyr Electric Fly-In Festival in Florida caught the author’s eye.
Shown are just a few of the wide range of aircraft at the event.
THE ZEPHYR Electric Fly-In Festival in
Florida, hosted by the Zephryhills Can-Am
Flyers, gets bigger every year. The 2010
edition featured some great flying in almost
perfect weather, with no zephyrs.
Several models at the event caught my
eye. A huge twin-engine Double Ugly
Monster Stick by Bill Witson also caught a
tall tree. It was rescued, but not without a
fair amount of damage. I don’t know if they
got one of the dishwasher-sized Hacker
motors out of the tree or not.
Another twin was an 8-foot-span PBY.
According to the owner, the only problem
was that the limited propeller clearance
made it a challenge to use enough propeller
to fly the airplane.
Honors in the beauty department went to
Don Heath’s Percival Mew Gull. Designed
by Seagull Models as a 120-size ARF, it had
been converted to electric power.
Neil Harris of Graves RC in Orlando
created the twin-ducted-fan-powered Ugly
Hog Wart Stick. You haven’t seen a hobby
shop until you’ve visited Graves. If you are
ever in the area, it is well worth the visit.
SLK Electronics has added to its line of
battery-care products an item that will monitor
your A123 (LiFePO4) flight pack: the
MilliAmp Monitor/Analyzer, or MAMA.
Following are a few of its features.
• Keeps track of current in and out of a pack.
• Is compatible with all two-cell A123
(LiFePO4) packs.
• Gives visual indication of remaining
capacity.
• Remote indicator (option) can be located for
easy viewing.
• Low-throttle blips (option) warn of low
remaining capacity.
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May 2011 93
Left: Mike Robinette’s Micro Grey Ghost uses the wing from a
ParkZone Super Cub.
Below: The modular fuselage of the Micro Grey Ghost allows
equipment to be shifted, making balancing at the CG a breeze.
• Handles up to 10 amps continuous and 25
amps peak.
• No need to unplug; can remain connected
for several months.
• Automatically adjusts to pack capacity.
• Keeps track of partial charging.
• Draws 5 milliamps with receiver on and
fewer than 100 microamps at idle.
• Measures 2.05 x 1.10 inches and weighs 12
grams (less optional 16-gauge wires).
The company’s Web site has more details
about this product and is a goldmine of
battery information.
Hobbico has released a neat little balancing
charger that, in addition to charging its
LiFeSource receiver packs, can handle other
one-, two-, or three-cell LiFePO4 packs. The
cover of the unit provides a good idea of what
it can do.
This charger operates from either 120
volts AC or 12 volts DC, making it useful in
the shop or at the field. An automatic-start
function with status LED and audible tones
make it user-friendly. Solid-state polarity and
current-overload safety devices ensure
trouble-free operation.
An adapter is included for the two-cell
receiver pack. In addition to the balancing
port, connection to the pack is either via a
Deans Male Ultra Plug or a socket that
accepts normal receiver connectors.
The question still remains: Are LiFe packs
too much for our receivers? Letters from
users suggest that there are no problems. Nick
Gaynor offers those who are concerned an
option.
He wrote:
“I’ve been using some 2S LiFe packs that
I made as directly connected receiver power
for a Reactor. So far the results have been
excellent.
“I’m using a Futaba R617FS 7-Channel
2.4GHz FASST Receiver and five Futaba
S9650 Digital servos. I never let the battery
get below 6 volts before recharging.
“The reason I write is that it occurred to
me that a diode (rectifier) can be used to drop
the LiFe pack voltage by 0.5 to 0.8 volts or
so. That might satisfy those afraid to connect
the fully charged (7.2 volt) LiFe batteries
directly to their receivers. Have you heard
about anyone else doing that?
“I connected a LiFe battery through a
diode to a wattmeter with a 10 ohm load. At a
550mA load current the diode dropped 0.79
volts. The resulting voltage from the 6.6 volt
LiFe is shown in the attached photo as 5.81
volts.”
Darren Braun is much more cautious about
using LiFe packs without some form of
voltage reduction. He wrote:
“I have enjoyed your column for many
years now. I may be able to help shed some
light on the issue of applying ‘overvoltage’ to
various microelectronics, based on what I do
for a living.
“Futaba states that it is not recommended
to supply voltages greater than 6v (unless
noted otherwise i.e. some servos only 4.8v) to
their products and this simply has to do with
reliability.
“The ICs [integrated circuits] themselves
inside Futaba products are fabricated on
process nodes at foundries that are designed
and characterized for specific ranges of supply
voltages. Operating outside these conditions
does not guarantee reliable operation.
“Take for example the timing of a digital
circuit. Hold time gets worse as voltage
increases.
“There is no guarantee there is enough
margin to satisfy hold time in the severely
overvoltage condition. And timing for any
voltage greater than 10% over nominal
voltage will not be checked by the IC
designer. However, they will check
and ensure it operates within a range (usually
+/- 10%) around nominal IC supply voltage
which corresponds to the characterization
range.
“So you may find your receiver works
fine at 6.5v or even 7v but there is no
guarantee all will do so and for how long.”
More Reader Letters: Earl Haury wrote the
following about “sick” Li-Poly packs.
“Let me add my thoughts/observations as
to why things cannot be all good with
charging/balancing related to pattern packs
and the common chargers we use.
“So what about ‘charge abuse’? My
observations suggest that balancing
equipment doesn’t always do what is
expected. Many balancers function by
drawing charge current away from the
high cell(s) which allows the low cells to
‘catch up.’
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“Part of the problem lies with the
balancer’s capacity to divert that charge
current—usually 300-500mAh—which isn’t
sufficient when charging at 5A. If the
charger primarily terminates on total pack
voltage, the high cells are pushed too high
until the average reaches the target pack
voltage.
“This will often leave the weak cell low
and some too high and won’t be noted
unless the individual cell voltages are
observed. (The way to charge packs with a
large imbalance is at a rate that doesn’t
exceed the diversion capacity of the
balancer—takes time.)
“On a good note, most systems will
terminate charge (with notification) if the
high cells are pushed too far—still not good
for them (especially without consideration
for temperature) but an important safety
feature that should prevent charging
eruptions as well as providing notification
that something isn’t right.
“Then there are chargers that balance by
either charging each cell independently or
by adding current to the low cells to help
them ‘catch up.’ This is a good strategy and
that appears ideal except for issues with
connectors/leads.
“A common problem is that both balance
methods can be compromised by the balance
leads/connectors themselves which can
result in the balancer actually misbalancing
the pack.
“The connectors are small and of
materials that will oxidize over time. They
also tend to lose tension with use. Both the
looseness and oxidation result in a poor
connection. A marginal connection will
prevent the balancer from ‘seeing’ the
correct voltage and will become worse as
balance current flows.
“Ever have a ‘balance lead not
connected’ or ‘incorrect cell count’ report?
These are indications that the connectors are
not doing their job. This all is 10X true if
charging at 5A is attempted through these
faulty connections!
“Additionally, the balance leads
themselves are small gauge wire of low
strand count (not very flexible) which often
results in several strands breaking from the
connector with use. Eventually the wire
might break off—in the meantime it’s a
resistor in the balancing circuit! I’ve also
seen extensions (have all the same
problems) where only a couple of strands
were captured in crimping to the connector!
“The good news is that much of this can
be avoided with maintenance and
observation. Contact cleaners will help keep
the connections clean, then some female
connectors can be removed from the
housing (one at a time if on the battery) and
‘bent’ back closed when they loosen.
“Check connections on new leads/
extensions and always fix the problem if
some lead ‘wiggling’ is needed to make the
charger happy.”
Mike Robinette has done a lot of work
with first-person-view flying and has had
some innovative ideas for inexpensive
airframes; one he calls the “Micro Grey
Ghost.”
He explains:
“The wing is from a ParkZone Super
Cub, 48 inches. I strap 3M Extreme tape
to the wing before attaching the balsa/
basswood booms. The fuselage is balsa
and can be interchangeable with other
fuses for other purposes.
“This is a trainer version, normally
94 MODEL AVIATION
there would be a CCD camera mounted
in the nose and 900 MHz video gear
installed to fly FPV (First Person View).
It flies super stable and travels with the
wing attached quite well in a smaller
vehicle.
“It is powered by a Turnigy 3530/
1100kv motor swinging a 10 X 5 APC
prop and Turnigy 30 amp ESC, all from
HobbyKing. I use a Zippy 3S 3000mAh
pack in this version for long training
flight times, normally I would use a
2200mAh 3S pack with the video gear
installed.
“The twin boom design is perfect for
FPV and makes balancing at the CG a
breeze.”
Flying season is upon us; give those
batteries a good preseason check.
Measure the open circuit voltage; if it is
not at least 1 volt per cell, the pack (Ni-
Cd or NiMH) is suspect.
Slow-charge the batteries, discharge
to check capacity, charge again, and
check again. If they won’t deliver at
least 80% of the rated values, use them
in a model you don’t care about. MA
Sources:
Graves R/C
(407) 294-5699
www.gravesrc.com
SLK Electronics
www.slkelectronics.com
LiFeSource
www.lifesourcebatteries.com
Tower Hobbies
(800) 637-6050
www.towerhobbies.com
Nick Gaynor
[email protected]
Darren Braun
[email protected]
Earl Haury
[email protected]
Mike Robinette
[email protected]
Red Scholefield
12219 NW 9th Ln.
Newberry FL 32669
www.hangtimes.com/redsbatteryclinic.h
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