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Radio
Control Model World - Nov '95
by
Stan Yeo
INTRODUCTION
Over
the years there have been a number of articles on Nickel Cadmium
batteries. Most have been quite informative, some have been very
technical and some posed more questions than they answered. The
purpose of this article is to discuss Nickel Cadmium batteries,
in what I hope is a not too scientific way, their capacity, storage,
charging and how to evaluate their serviceability.
CAPACITY
& DISCHARGE CHARACTERISTICS
The
rated or nominal capacity of a Nickel Cadmium battery is the maximum
current a fully charged cell can deliver continuously for a 5 hour
period, i.e. a 500mA hr cell is capable of delivering 100 mA for
5 hours before it is fully discharged. Unfortunately the full capacity
can only be realised under certain conditions as the ambient temperature
and the discharge current have an effect upon the available capacity.
The higher the discharge current the lower the capacity. Discharging
a cell at 2 x its rated capacity reduces it by 10% whilst discharging
it at 1/2 the rated capacity will yield at least 10% more. This
is an important point to consider when selecting the capacity of
the batteries to fit in your model. Another important point to consider
is battery voltage under load and its ability to deliver high currents.
The higher the current demand the lower the terminal voltage. This
means that under high current demand situations it is possible for
the battery voltage to fall below that which is required to operate
the circuit. In an airborne pack this would result in a receiver
glitch or temporary loss of control. On the bench a typical three
function model with all three servos moving at the same time draws
a current of 300 - 400 mA. More if the controls are not very free.
In the air this current would increase significantly due to the
aerodynamic loads on the control surfaces, consequently it is advisable
to use at least a 500 mA hr battery pack where possible.
The
temperature at which the batteries are being discharged also has
a significant impact upon the available capacity. If the temperature
is too high or too low then the capacity can be greatly reduced.
Fortunately the optimum discharge temperature of 30 deg. C is sufficiently
close to average UK temperatures to have little impact upon the
capacity of our batteries but on a bitterly cold winter's day it
is worth remembering that the capacity of our cells could be reduced
by up to 20%.
Graphs:
Capacity v Discharge Temperature and Voltage v Discharge Current
STORAGE
Nickel
Cadmium batteries self discharge during storage. Basically the higher
the temperature at they are stored the quicker they self discharge.
If the batteries were kept in a freezer at -20 deg. C then after
three months they would still be holding 85% of the charge whilst
those kept at room temperature would only hold 20% of their charge.
Although Ever Ready and Varta technical information do not mention
the affect of humidity on charge retention during storage common
sense dictates that the batteries should be stored in a relatively
dry atmosphere as recent research by Roger Todd (RCMW June 1994)
suggests that it is the ingress of moisture that is responsible
for the dreaded black wire corrosion.
Storing
your models in a dry garage or shed is probably the best place to
keep them, particularly in winter. It is not so ideal in the summer
though as the temperature in un-insulated buildings can soar to
extreme levels should the sun decide shine for more than a short
time. At 30 deg. C Nicads can loose 25% of their charge in less
than a week consequently in warmer weather it is advisable to give
your equipment a top-up charge just before you go flying.
Graph:
Capacity Loss v Storage temperature
CHARGING
There
are four basic methods of charging Nicads based on the charging
current. These are:
1.
Standard charge
2.
Accelerated charge
3.
Fast charge
4.
Trickle charge
Only
with the trickle charge can the batteries be left on charge indefinitely
without the risk of shortening their life.
1.
Standard Charge
The
standard charge is carried out at a current equivalent to 10% of
the nominal capacity of the battery, i.e. for a 500 mA hr battery
the charging current would be 50 mA. A full charge takes 14 hours.
The extra 4 hours representing energy lost during the charge cycle.
Contrary to popular belief it is not safe to leave batteries on
standard charge for more than the recommended time. Over charging
will reduce the life of the batteries (Ref. Varta technical handbook).
For new cells, cells that have been in storage for a long time or
deeply discharged cells a 24 hour charge is recommended to re-condition
them as a normal 14 hour charge is insufficient to bring them up
to full capacity. Standard charging is the most common method of
charging Nicads as it is the only method that fully charges the
batteries. As the charging current is increased so the charge efficiency
is reduced. Standard chargers are the type normally supplied with
radio control outfits fitted with re-chargeable cells.
2.
Accelerated Charge
Accelerated
charges are carried with a charge current of between 10 and 30%
of the nominal battery capacity. This, as previously mentioned is
not the normal charging method but should you wish to use it then
the charge time can be calculated as follows
Charge
Time = Battery Capacity x 1.4
Charge
Current
When
charging Nicads there is a constant danger of overcharging them
consequently with the Accelerated and the Fast Charge it is recommend
that the chargers are fitted with automatic timers and the charge
starts with the batteries in the fully discharged state i.e. with
a cell voltage of 1 to 1.1 volts. This is necessary because unless
the batteries are fully discharged there is no way of knowing what
percentage of charge remains and therefore no way of calculating
the charge time. A 10% overcharge can result in permanent damage
to the batteries.
3.
Fast Charging
Fast
charging is carried out at currents corresponding to 80 - 120% of
nominal capacity. Normal practice is to charge at the nominal capacity
current i.e. at 500 mA for a 500 mA hr battery pack for 60 to 75
minutes. With fast charging it is even more imperative that the
charge is tightly monitored as the difference between a full charge
and an overcharge is only a few minutes. Better quality Fast Chargers
will discharge the batteries before charging them for the predetermined
time or monitor the cell voltage and switch off automatically when
it detects that the battery is fully charged.
There
are other methods of fast charging being developed, one is Pulse
charging. Here the battery is given a high current pulse for a few
seconds followed by a stabilisation period at the standard charge
rate. This is followed by a short high current discharge before
the cycle is repeated. The main purpose of pulse charging is to
increase the charge efficiency of the high current charge without
damaging the batteries. There are a number of pulse charging methods
and with the help of a friend we have developed our own. Our method
is to charge the battery at the fast charge rate for a nominal time
of 20 seconds and then discharge the battery at 25% of this rate,
down to a predetermined voltage level. When the battery reaches
this level the charge cycle is then repeated. If the battery is
only partially charged the time taken to reach this predetermined
level is minimal so the duty cycle of the charger is close to 100%
but as the battery becomes fully charged so the discharge time increases.
Whilst this technique can increase the overall charge time for fully
discharged batteries it does have two main advantages over other
methods of fast charging.
1.
The batteries do not have to be discharged prior to charging saving
valuable charging/flying time.
2.
The charger does not require a timer or sensitive sensing circuitry
to prevent overcharging.
In
addition, by monitoring the discharge cycle time and comparing this
with the charge cycle time it is possible to make a reasoned assessment
as to the state of charge of the batteries. The discharge to charge
ratio can also be used to detect problems with the cells themselves
or the wiring harness as any fault will show up as a shortened discharge
cycle. One further bonus is the discharge cycle performs a similar
function to that of the discharge during cycling. Nicad cycling
is discussed later.
Trickle
Charging
Trickle
charging is the only charging method where the batteries can be
left on charge indefinitely without fear of overcharging. The purpose
of trickle charging is to keep a battery fitted to electrical equipment
fully charged, ready for emergency use or, as in the case of computers,
to supply power to the memory circuits when the computer is switched
off in order to retain start-up information. Trickle charging is
carried out at currents equivalent to 0.03 - 0.05 x nominal cell
capacity i.e. 15 - 25 mA for a 500 mA hr cell.
BATTERY
CHECKERS AND CYCLERS
Battery
Checkers measure the voltage of a battery pack under load and are
very useful for checking the serviceability of a battery pack in
the field. Because of the very flat voltage discharge curve characteristics
of Nickel Cadmium batteries Checkers are not particularly reliable
as indicators of battery charge state nevertheless they haved saved
many a model from certain disaster. Checkers however can identify
battery packs or switch harnesses that are in a poor condition i.e.
cells that are in need of cycling / replacement or leads that are
suffering from the dreaded black wire corrosion. Any resistance
in the battery or leads will reduce the voltage that is being measured
and the Checker will give a low reading that needs investigating.
Batteries
that are stored in a semi or fully charged state oxidise internally
and it is this oxidisation process that reduces the available capacity
of the battery. This is the so called memory effect of Nicads.
Discharging the cells at currents of 0.2 - 0.4 times the nominal
capacity of the cell breaks down these oxides so that when the cell
is recharged capacity is hopefully restored back to normal. This
is why periodic cycling is so necessary. Over the life of a Nickel
Cadmium battery there is a steady deterioration in performance and
whilst a cell can fail at any time this deterioration can be monitored
by regular cycling (say every three months) and recording of the
results. In the Varta technical handbook cycling is referred to
as conditioning.
The
normal procedure adopted to cycle a battery pack is to fully charge
it at the standard rate. Then discharge it under controlled conditions
i.e. use a Cycler, down to 1.1 volts per cell. The time taken for
the discharge is recorded and compared with previous results to
see what if any deterioration has taken place since the last cycling
operation. If there are no previous results cycle a known good pack
of the same capacity and use that as a standard. If there has been
a deterioration repeat the cycling operation and check the discharge
time again. Should the battery pack fail to recover to at least
80% of its original new capacity investigate the cause (it could
be the switch harness etc.) and take the necessary remedial action
i.e. change the leads or replace the complete battery pack.
CHARGE
EFFICIENCY
Charge
efficiency, as well as available battery capacity, is very much
dependant on the ambient temperature and the currents involved.
The higher the temperature the lower the charge efficiency likewise
for the current. This can be overcome to some extent by extending
the charge time i.e. from 14 to 16 hours for a standard charge,
but at higher than standard charge currents this is not always possible
due to other factors coming into play. The standard charge is the
most efficient charging method and the one that is recommended,
unless time is a criteria, as it provides the maximum available
capacity after a full charg
Graph:
Charge Efficiency v Charge Temperature.
SUMMARY
Well
I hope you have found this article informative and I have not baffled
you with too much science. Most of the information contained in
it was extracted from the Ever Ready and Varta technical handbooks
on Nickel Cadmium cells. Nickel Cadmium batteries are very reliable
but as with most things a little care will improve reliability and
prolong active life or at least warn of impending disaster!
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