new market trends are lowering prices, most of us consider
the cost of Lithium batteries to be a significant part of
our electric flight setup. If we had been previously flying
with glow or gas engines, suddenly paying for fuel "up
front" can be a real paradigm shift. When we buy a battery
pack, we are essentially paying for hundreds of flights all
are perishable products that start deteriorating right from
the moment they leave the factory. Much like cars and computers,
our flight packs loose value after they are purchased, even
if we don't use them. As our batteries loose capacity over
time, or from abuse, we start to realize that it is important
to maximize our investment.
I prepare for a long cold winter here in upstate, NY, my focus
in this month's issue of AMP'D is to show some simple preventive
measures that anyone can apply to slow the aging process of
Lithium packs, especially during the off-season months when
they may not even be used.
Storing Lipo Packs
University states that the recommended storage temperature
for most batteries is 15°C (59°F). While lead-acid
batteries must always be kept at full charge, nickel and lithium-based
chemistries should be stored at 40% state of charge. This
level minimizes age-related capacity loss, yet keeps the battery
in operating condition even with some self-discharge.
capacity loss during a battery's life cannot be eliminated,
following a few simple guidelines will minimize the loss of
batteries in a cool and dry storage area. Refrigeration
is recommended but freezers should be avoided. When refrigerated,
the battery should be placed in a plastic bag to protect
not fully charge lithium and nickel-based batteries before
storage. Keep them partially charged and apply a full charge
before use. Store lithium-ion at about 40% state-of-charge
(3.75-3.80V/cell open terminal). Lead-acid batteries must
be stored fully charged.
not store lithium-ion fully depleted. If empty, charge for
about 30 minutes (at a 1 x Capacity rate) before storage.
Self-discharge on a depleted battery may cause damage and
prevent a recharge.
not stockpile lithium-ion batteries; avoid buying dated
stock, even if offered at a reduced price. Observe the manufacturing
date, if available.
best possible capacity retention for LiPos is obtained when
you store them about half-charged and keep them cold around
0° (C) or 32° (F). This results in approximately 2%
loss of capacity per year. The next best is a 4% loss per
year when stored half-charged at 25° (C) or 75° (F).
winter storage of LiPos in upstate NY, my garage stays around
the freezing temperatures for most of the "building"
season. The remaining task is to get my packs at around the
half-charged voltage and I accomplish this in several ways.
Once the pack is around half charge, I usually place a 50%
circle sticker on it so that I remember which packs were done.
The packs are then sealed in plastic bags, with as much air
squeezed out as possible to reduce condensation, and placed
in a box to be stored in the garage.
Discharging Packs for Storage
discharging my packs for off-season storage, I first organize
the packs in various cell counts, capacities, and discharge
rating. My larger more expensive packs get first attention
and I seldom discharge packs that are smaller than 1500mAh.
Once the packs are sorted, there are a number of ways to discharge
them to 50%. Although discharging packs does not have the
potential for hazard associated with over-charging, it is
important to stay nearby and not let your packs get discharged
below 3v per cell for an extended period.
way to get my packs at the half-charge voltage of 3.8v/cell
is to charge them after my last flight to only half way. Some
chargers currently on the market display a fuel gauge to help
make this process easier. Once the pack is at 50% fuel level,
you stop the charge. The FMA Direct Cellpro
Product Line of chargers, like the 4s shown on the left,
have a built-in STORE feature that automatically stops the charge
at 50%. This is a great feature to help automate your battery
way I used to get my packs at the half-charge voltage was
to use a series of light bulbs like the BC015 8-bulb Battery
Dumper that Hobby Lobby
used to sell.
used a wattmeter in series with the BC015 Battery Dumper to
monitor the voltage for a half-charged state and maintain
a reasonable current draw for the pack. The number of bulbs
can be varied to suit the pack voltage and battery discharge
rating. This process typically took about 10-15 minutes per
pack so you had to keep a close eye on it.
bulbs produced plenty of heat so I would typically do this
process on the cement floor in the garage. After letting the
pack sit unloaded for a few minutes, I would then verify the
pack voltage using a simple voltmeter. This inexpensive approach
to discharging batteries required skill and attention.
current procedure to discharge packs to 50% uses several of
my old Astro
Flight Lithium 109 Charger/Dischargers. It discharges at
a much slower rate than the bulb technique so a fully charged
5AH pack takes a few hours to reach half charge. The procedure
is as follows:
Plug pack to be discharged into a powered-down Astro Lithium
Apply power to the charger and it should go into a slow
The display will read pack voltage and capacity removed
in mAh. Let the pack discharge until you approximately see
the appropriate value below. An exact 50% discharge is not
critical but it is still a manual process.
Voltage Value for Various Packs
- 3s = 11.4v
- 4s = 15.2v
- 5s = 19v
- 6s = 22.9v
disconnecting the pack from the charger, you can let it sit
for a few minutes or so and confirm the pack voltage using
battery users may consider using the Computerized Battery
Analyzer (or CBA) from West
Mountain Radio. In addition to performing professional
quality battery tests and analysis, the CBA has a user selectable
discharge test current setting from 0 to 40 Amps. It automatically
shuts off the discharge at a safe pre-determined minimum battery
CBA is much more than a simple battery voltage tester or a
battery load tester. Unlike a simple load tester the CBA will
test virtually any type or size of battery, any chemistry,
any number of cells up to 55 volts. The software supplied
with the CBA is easy and intuitive to use. It provides automatic
sensing of the battery cell count and recommends a safe maximum
discharge current and minimum safe cutoff voltage for your
Gas Gauging LiPo Packs
OEM applications of LiPos such as cell phones use a "gas
gauge" to inform the user of the status of the battery.
Since most R/C applications are price sensitive, the added
cost of individual pack gauging is not practical. However,
if the user has a simple digital voltmeter, a good gas-gauging
emulation can be had by using a gas gauge load or using the
RC system as the load.
receiver, servos, and ESC combined current without the motor
running approximate the 0.5C load at which the cell is rated
as shown in the 0.5C curve (right). The discharge curve for
the pack is nominally 3.7v/cell x (# of cells at 0.5C). Recognize
that this is a nominal value and will vary with the specific
pack, how close the current is to 0.5C and how old the pack
LiPo packs are shipped from the manufacturer at half-charge
and checked prior to assembly to measure exactly 3.81v or
is well to check this voltage before storing the pack or charging
it in order to "calibrate" your setup. For the example
curve: if your DVM says 3.5V under the light load, the 2150mAh
cell has been depleted to 3.68V which equals ¼ capacity
remaining. That is about 80% of capacity and time to stop
flying if you wish to maximize cycle life or minimize risk
of an ESC cut-off. The last ¼ pack capacity is sometimes
referred to as the "zone of temptation" as it can
be used to maximize flight time but often leads to premature
loss in pack capacity.
the most accurate results, keep track of pack voltage after
each flight, track it against the discharge curve, and then
calibrate when the pack is recharged. Also, keep a record
of flight time. Record the capacity it takes to charge the
pack. Divide the capacity needed for charge by flight time
in minutes to get the average capacity consumed per minute.
Now you have it "gas-gauged" and can use the DVM
to see what is left after each flight or use the average consumption
per minute to estimate capacity used.
most cases, I use a subset of the gas-gauging technique just
described as a simple field tool. I print out a copy of the
example voltages below that I extracted from the chart and
keep it in my field box. I then measure the pack voltage after
flight using a multi-meter. For best accuracy, this can be
done with the motor stopped and the receiver and servos still
energized, but, I often do it on an unloaded pack as well.
After I roughly determine the remaining capacity of the pack,
I may decide to adjust the flight timer on my transmitter.
Eventually, this iterative process allows me to hone into
each model's flight duration so it stays in memory with the
rest of my programmable settings. In this manner, I can successfully
"gauge" dozens of setups and keep the results in
my transmitter. Some of my planes are not fun to do a "dead
Using the chart below and a simple voltmeter, you can determine
the capacity left in a LiPo pack.
NiCad and NiMH packs, LiPo batteries can be stored for several
months without significantly losing charge. However, if
storing for long periods, you can minimize the loss of capacity
by discharging the battery to 40% or 50% and keep it refrigerated
but not frozen. By sealing the packs in plastic bags during
cold storage, you can eliminate condensation from forming
that could damage the pack.
This month's issue of AMP'D we learned that the best possible
capacity retention for LiPos is obtained when you store
them about half-charged and keep them cold but not frozen.
This simple process is not hazardous and results in only
about 2% loss of pack capacity per year and is a great way
to maximize your investment in electric flight.
to keep a few packs handy for some fun in the snow! Let
the packs warm to room temperature overnight and then charge
them up. Keep them warm until you are ready to fly for best
you fly electric, fly clean, fly quiet, and fly safe!
thanks for contributions by:
"Papa Jeff" Ring and Lynn Bowerman
section of AMP'D cover some of the questions that our
readers have sent in and I thought would be interesting
am interested in your Fly Fly BAe Hawk on RCU.
Did you run the Fly Fly 90mm fan with an Ammo-36-56-1800
motor and 6-cell Lipo 22.2V? How good is it with
this set up? Have you tried with Midi 90mm rotor?
best combination for me has been using the Midi
90mm rotor, FlyFly DF, Ammo 36-56-1800
motor, 60-80amp ESC, and 6s 25C LiPo pack.
can see this combination fly in my BAe Hawk in
the link below. Although the FlyFly rotor worked
fine, the Midi rotor had less vibration and gave
similar thrust for less current draw. It was more
efficient than the FlyFly rotor.