Batteries are available in different
sizes, weights and capacities C, which refer to their stored
energy expressed either in amps-hour Ah or milliamps-hour mAh.
For example, a battery with a capacity of 500mAh should deliver 500mA
during one hour before it gets totally discharged (flat).
radio control systems are usually powered by rechargeable
There are two main rechargeable battery types available on the
The NiCads (nickel - cadmium) and the NiMH (nickel - metal
Even Lead-Acid batteries are also used as ground power source.
Normally the NiCads stand more "abuse" which means that they may be
charged at higher rate (normally 2 - 4C) and have the ability to deliver
higher current, i.e. discharge rates up to 2C continuous or 8 to 10C
during 4 - 5 minutes and even up to 100C during very short time.
They have some designations such as the Sanyo AE for high capacity and
AR or SCR for quick charge/discharge.
A NiCad cell consists basically in a positive plate foil of nickel metal
with nickel oxide/hydroxide, a negative plate foil of cadmium metal with
cadmium hydroxide and an isolating porous separator film moistened with
an electrolyte of potassium hydroxide (caustic potash).
The two plates are sandwiched between the isolating porous separator
films, rolled up and enclosed in a nickel-plated steel can. A
spring-loaded vent is fitted at the positive terminal end in order to
release the electrolyte and/or gasses, in case overpressure occurs due
The NiMH have higher capacity/weight compared with the NiCads but are
more sensitive to high charge rates (max recommended 1C) and normally it
is not recommended to discharge the NiMH batteries at higher rates than
3 - 5C.
The NiMH self-discharge rate is also about 50% higher than the NiCads.
However, the NiMH are more environment-friendly.
Both battery types lose their stored charge due to internal chemical
action, even when not in use. Normally the NiCads lose around 10%
of its charge in the first 24 hours after been charged and keep losing
it by 10% per month. The rate of self-discharge doubles for a rise
in temperature of 10 degrees C. Some NiCads can discharge
themselves completely in a period of six months.
The best way to keep batteries which are not in use for a long time, is
by having them stored in the refrigerator (not in the freezer).
Just allow the battery to reach the ambient temperature before
Some manufacturers claim that these battery types are able to stand at
least 1000 charges/discharges during their lifetime, assuming they have
been subject to the ideal charging and handling methods. In
practice however, we may expect about 600 - 800 charges/discharges.
A safe method to charge both the NiCads and the NiMHs is by using a
constant charge current (CC) at 1/10 of their capacity (0.1C) during 14
hours. For other charge current values one may use the following
Time (Hours) = 1.4 x Battery Capacity / Charge Current
(assuming a constant charge current is used)
However, low cost CC chargers provide no way of detecting when the
battery is fully charged. The user is then expected to estimate
the charging time based on the constant charging current value and the
battery capacity, according to the formula above. And providing
the NiCads' are discharged to about 1.1V p/cell each time before
recharging, this charging method can be used to achieve a reasonably
long battery life. Since repeatedly recharging an already fully charged
NiCad or one with a large part of its charge remaining will degrade its
Some chargers provide the option to discharge the batteries down to
about 1.1V per cell before starting the charging process. There
are also fast battery chargers on the market charging from 1C up to 4C.
But due to the high charging current level, it is required a reliable
method of stopping the charge once the battery is fully charged,
otherwise overheating and battery damage may occur.
Since the NiMHs' and NiCads' voltage actually starts dropping after they
have reached the fully charged state, the fast chargers use the
so-called Delta Peak detecting method. There are "negative delta V
(-DV)" and "zero delta V (0D)" detectors. Also "change of
temperature (dT/dt)" detectors are commonly used. Some
manufacturers use negative or zero delta V together with change of
temperature detection, in case of one method fails to detect.
Since NiMHs' voltage drop (delta V) after the fully charged state is
lower than the NiCads, a more sensitive delta V charger is required for
the NiMH batteries. Some chargers allow the user to set the value
of the delta peak detection, which may be between 10 - 20mV per cell for
NiCads and 5 - 10mV for NiMHs. A too low value may cause false
peak detection due to electric noise, preventing the batteries from
getting fully charged, whereas a too large value may result in
overcharge, which reduces the batteries' life.
Some fast chargers offer the possibility to automatically change over to
slow charge (trickle-charge, for example at 0.05C) when the fully charge
status is detected.
graph on the right shows the voltage and temperature variation
of a four cell NiCad during charging at 1C constant charge
Notice how the voltage drops after it has reached a top value,
whereas the temperature keeps rising.
The battery is considered fully charged when the temp. rises
about 10°C above the ambient temp. ( 24 + 10 = 34°C )
batteries tend to dissipate heat during all the charging process, while
the NiCads get warm only when they reach the full charge point.
The nominal voltage is 1.2V per cell for both battery types and a
charged cell may have about 1.45 - 1.50V.
It's not possible to know exactly the NiCad's or NiMH's cell charge
status by only measuring it's terminal voltage, as the cell's charge
status is not a linear function of the cell's voltage. A reliable
method to know how much charge is left or whether a cell still has its
nominal capacity, is by discharging it with a known constant current and
measure the time until the cell voltage reaches about 1.1V. For
example, it should take about two hours to discharge a fully charged
500mAh cell by using a constant discharging current of 250mAh.
Battery researchers have in the recent years come to conclusion that
NiCads respond better to a pulsed charging waveform than to a steady DC
current. By applying the charge current in one-second pulses with
brief "rest" periods between them, ions are able to diffuse over the
plate area and the cells are better able to absorb the charge.
This is particularly true at the higher charge rates used by fast
chargers. These chargers have a microprocessor that samples the
"rest" periods between the charging pulses to read the battery terminal
voltage. Another interesting discovery is that the charging
process actually improves even further if during the "rest period"
between charging pulses, the cells are subject to very brief discharging
pulses with an amplitude of about 2.5 times the charging current, but
lasting only about 5ms.
It is claimed
that these short discharge pulses actually dislodge oxygen bubbles from
the plates and help them diffuse during the "rest period". The use
of these brief discharge pulses is known as "burp charging". Tests
done by both US military and NASA have shown that NiCads charged by
using fast chargers employing the burped pulse system tend to last up to
Twice as long as those charged by traditional CC chargers. Many of
the high-end fast pulse chargers for NiCads use a charging method
according to those findings.
A battery pack consists of several cells connected in series, which
inevitably age at different rates and gradually develop individual
different charge status, and since the battery pack as a whole is
charged and discharged repeatedly, these differences may become
accentuated. The result is that some weaker cells can eventually
be discharged well below 1.0 V and even driven into reverse polarity
before the others reach the fully discharged state.
During the recharging process, the weaker cells will be improperly
recharged and tend to suffer increased crystal growth, while the others
will absorb most of the charge and overheat, which dramatically degrades
the whole battery pack performance.
It's therefore advisable checking if the battery cells get different
temperatures during the charging process, specially when high charge
current rates are used.
It's claimed that individual cell differences may level out by slow
charging the battery pack from time to time at 0.1C during 14h or so.
Some few examples of many battery chargers available on the market.
Input Voltage range: 9-15V DC 4-12 cells of 50mAh - 3000mAh
NiCad or NiMH pack can be charged.
TRITON Charger, Discharger
Handles 1-24 NiCd or NiMH cells, 1-4 Li-Ion cells or 6,12, and
24V Lead Acid batteries.
NiCads and NiMHs may be on charge during relatively long time without
the risk of overcharging damage when using a constant current equal or
less than 0.1C. However, it is not advisable to have the batteries
continuously on charge longer than 24h, so one may connect the charger
to a timer in order to cut the charging after about 14 -18h.
rechargeable battery types, such as the Li-Ion (liquid
electrolyte), the flat Lithium-Polymer (solid polymer
electrolyte) and Lithium-Ion-Polymer (gel electrolyte), are now
often used with slow-flyers, indoors and even with bigger
models. The cell shown on right (Kokam) has 3.7V as
nominal voltage, 4.2V max and 3.0V minimum.
These battery types have much higher energy density than both
the NiCads and the NiMHs.
The max charge rate recommended is 1C while the discharge rate should
not be higher than 3 - 4C continuous or 5 - 6C during short time.
The self-discharge rate is claimed to be very low, typically 5% per
year. These batteries cannot be charged with the same chargers
that are designed for NiCads or NiMH.
In order to correctly charge the Li-ion/Lithium-polymer batteries, it
must be taken into account the number of cells in the actual battery
pack, since both the max charging current and voltage have to be set
according to the cells' specifications. Charging these batteries
with a wrong charger may cause them to explode. Also a short
circuited pack may easily catch fire. According to Kokam, the
Lithium-polymer batteries should not be discharged below 2.5V per cell,
otherwise a rapid deterioration will occur.
The basic charging procedure is by limiting the current (from 0.2 C to
max 1C depending on manufacturer) until the battery reaches 4.2 V/cell
and keeping this voltage until the charge current has dropped to 10% of
the capacity C. Since the batteries only have 40 to 70% of full
capacity when 4.2V/cell is reached, it's necessary to continue charging
them until the current drops as described above.
A charge timer should be used to terminate the charge in case the top
voltage and/or termination current never reach their values within a
certain time, which depends on the initial charging current, (e.g. 2
hours at 1C or 10 hours at 0.2C).
Trickle charging is not good for Lithium batteries, as the chemistry
cannot accept an overcharge without causing damage to the cells.
Panasonic's charge curve for their 830mAh cells is shown below.
Note : if a Li-ion battery gets discharged below 2.9V/cell, it needs to
be slow charged at 0.1C until 3.0V/cell is reached before a higher
charging current rate may be used. Also discharging below
2.3V/cell will damage the battery.
According to the manufacturers the Li-ion batteries should be stored
charged to about 30 - 50% of capacity at room temperature. For
prolonged storage periods, store discharged (i.e. 2.5 to 3.0V/cell) at
-20° to 25° C.
Make sure to
set your charger to the correct voltage according to the number of
cells. Failure to do this may result in battery fire!
Before you charge a new Lithium pack, check the voltage of each cell
individually. This is absolutely critical as an unbalanced pack
may explode while charging even if the correct cell count was chosen.
If the voltage difference between cells is greater than 0.1V, charge
each cell individually to 4.2V so that they are all equal.
If after discharge, the pack still is unbalanced you have a faulty cell
that must be replaced.
Do not charge at more than 1C.
the batteries unattended.
If you crash
with Lithium cells there is a risk that they get a latent internal
short-circuit. The cells may still look just fine but, if you
crash in any way remove the battery pack carefully from the model and
place it on a non-flammable place, as these cells may catch fire later
on. (A box with sand is a cheap fire extinguisher).
Lithium batteries when flying in areas with large amounts of dry
vegetation, as a crash may result in a serious forest fire.
These cells have a nominal voltage of 3.2V, can be discharged down to 2V
and charged to 4.2V. The recommended discharge rate is 5 to 6C
continuous for a long life or higher discharge rates for a shorter life.
lead - acid batteries have much lower energy/weight ratio than
all those previously mentioned. Which means that the lead
- acid batteries are heavier for the same capacity.
They are not suitable to be used airborne, but since they are
rather cheap, they are often used on the flying fields as ground
power supply for engine starters and/or to charge the smaller
There are various versions of lead acid batteries.
The Gel-Cell, the Absorbed Glass Mat (AGM) and the Wet Cell. The
Gel-Cell and the AGM batteries cost about twice as much as the Wet Cell.
store very well and do not tend to sulfate or degrade as easily as the
Wet Cell. Lead acid batteries get "sulfated" when the soft lead
sulfate normally formed on the positive and negative plates' surfaces
re-crystallizes into hard lead sulfate when the batteries are left
uncharged during long time. This reduces the battery's capacity and
ability to be recharged. Both the Gel-Cell and AGM are the safest
lead acid batteries one can use. However, Gel-Cell and some AGM
batteries may require a special charging rate.
There are sealed (maintenance free) and serviceable non-sealed Wet Cell
batteries. Non-sealed batteries are recommended in hot climates since
distilled water can be added through the filler caps when the
electrolyte evaporates due to the high environment temperature.
The lead acid batteries have a self -discharge rate of about 1% to 25% a
month. They will discharge faster at higher temperature. For
example, a battery stored at 35°C (95°F) will self-discharge twice as
fast than one stored at 24°C (75°F).
Lead acid batteries left uncharged during long time will become fully
discharged and sulfated. The best way to prevent sulfation is by
periodically recharging the battery when it drops below 80% of its
charge. It is possible to determine a non-sealed battery's charge
status by measuring the concentration of the sulfuric acid of the
battery electrolyte ("battery acid") with a hydrometer.
The lead- acid batteries have normally 3 or 6 cells connected in series.
Each cell has a nominal voltage of 2V resulting in a nominal pack
voltage of 6V and 12V respectively.
They are usually charged with a constant voltage of 2.4 - 2.5V per cell
having the charging current limited to 1/10C. It is not recommended
charging these batteries with a charging current exceeding 1/3C. A
lead -acid battery pack is considered fully charged when the charging
current falls below 10mA and/or the cell voltage reaches 2.4 - 2.5V.
Should a lead - acid battery be continuously left on charge (when used
as power backup); the charging voltage should not exceed 2.25 - 2.30V
per cell. It is also advisable to charge these batteries in a
well-ventilated area/room, since it produces hydrogen-oxygen gases that
can be explosive and also the electrolyte contains sulfuric acid that
can cause severe burns.
Lead - acid batteries' life span is about 4 - 6 years depending on the