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EDITORIAL.
Welcome to our February edition of DC
Supply.
Battery management is a
science!
Getting the very best performance from any
battery pack depends upon applying the
correct charging regime, limiting the
discharge to a safe level and making sure
the
batteries are operating a full capacity in
the working environment. This month, we look
at
the principles of battery management and how they should
be applied in the most
common
applications.
We are
frequently asked how a battery charger can be built using simple
electronics
that are
both inexpensive and reliable. In our second feature, we'll show you
how you can construct a quality charging system for small
lead-acid cells up to 60AH using thyristor
control
in a
constant potential charging regime.
As
always, we welcome any comments you may have and hope you enjoy this
months issue!
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Editor: Alan Fidler.
Alan is the owner and manager of CBC
Design, a leading battery management company
based in the UK. He has worked in the
industry for over eighteen years and has designed
charging equipment and battery monitors for some of the world
largest companies.
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ARTICLE: Battery Management. Author: Alan Fidler.
Battery
management is essential if an installation is to perform to the
requirements
specified in the original design. Failure
to properly maintain the cells will, at the very
least
cause problems and at worst, may lead to a loss of life. Imagine how
catastrophic
it would
be if the battery used in a special care baby unit (Scbu)
incubator failed!
The
first principle in battery care it to charge the cells at regular
intervals in accordance
with the
battery manufacturers instructions. In some cases, this may involve
scheduled
charges
or the connection of a permanent constant potential system that
recharges the
battery
continuously. Visit the following page for a
demonstration of the three principal
Once the
charger has been selected, the next point to consider is the
anticipated duty
of the
installation. Lead-acid batteries should not be discharged
beyond 1.55 volts per cell
in any situation. In some cases 1.75
volts per cell is the minimum discharge voltage.
A method
of limiting the end of discharge voltage may be
required using a low volts
disconnect circuit. Nicad batteries are
more tolerant to excessive discharge than lead acid cells are but
similar considerations should be applied for
maximum reliability.
The
third consideration is voltage and current monitoring as discussed
in last months issue.
It is
imperative that the installation function within prescribed limits
for reliable operation
and systems in remote locations may
require signal telemetry or similar to warn end
users
that a
potential problem has occurred. A number of alarms may be required
to achieve this,
a High
Volts Monitor for over-charge warning, a Low Volts Monitor to
indicate that the battery
is
discharging or a Charge Failure Monitor to warn end users that the
batteries are off charge.
All
industrial battery charger manufacturers apply the basic
elements discussed above
in order
to make sure the end customers dc supply is a reliable one.
Sadly, dispite
the best
efforts of these companies to create the perfect system, end users
sometimes
ignore
the maintenance requirements laid down by the manufacturers
and wonder why
their
batteries fail 2 or 3 years before they should.
Batteries have a fixed life from several
years to over a decade. It is therefore
essential
that the
batteries are discharged at regular intervals to confirm that they
are operating
to the
manufacturers specification. This is particularly important as the
batteries age.
The
tests would traditional be applied at a current level that is
similar to the normal
application in which the cells function.
Sadly, this obvious necessity is often ignored
until it
is too late.
Battery
management then, encompasses the following
points:
1.
Selection of a battery with an appropriate voltage and current
rating.
2.
Selection of an appropriate battery type, i.e. leisure or
traction.
3.
Selection of an appropriate charging system to suit the battery and
working ambient.
4.
Installation of appropriate discharge voltage limiting
circuits.
5. Choosing appropriate voltage and
current monitors for early fault detection.
6. Battery discharge testing on a
regular basis to confirm battery performance.
Battery
manufacturers work hard to produce reliable cells. Battery
charger
manufacturers take pride in building a
suitable charger. The rest is up to the
end
user!
Remember: Look after your batteries and
your batteries will look after you!
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CLASSIFIED ADS:
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product in more detail!
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ARTICLE: Designing your own Charger. Author: Alan
Fidler.
Building
your own battery charger may seem straight forward. After all, the
average charger
for
vehicle batteries consist of nothing more that a mains transformer,
a full wave bridge
rectifier and a simple moving coil ammeter
to indicate charging current.
Whilst
CBC Design acknowledge the popularity of these cheap and cheerful
chargers, we
are in
the business of battery management and chargers of this nature have
no place in
a
properly constructed system that can maintain the batteries
indefinitely.
To build
a charging system that can be used safely and permanently in some
cases, a
slightly
more sophisticated system is required and this
is precisely what we want to help
you
achieve.
PARTS
REQUIRED:
At the
heart of this type of battery charger is the mains
transformer. To calculate the
transformer rating we multiply the charging
current by 1.5 to derive an RMS rating.
We will
be constructing a charger suitable for batteries up to 60AH so a 6A
charger
will
require a transformer with a 9A rms rating. The transformer
secondary voltage
should
be approx 17 volts for a 12V charger or 33 volts for a 24V model.
The input
voltage
will be 110V or 230VAC depending upon your mains
supply.
Next we
need to select a suitable full wave bridge rectifier. Since the
charger has an
operating voltage of 12 or 24V, a 60V
rectifier rated at 10A will be adequate.
The
rectifier will need to be mounted to a metal surface so that it
stays cool when
delivering full current and because of its
mounting position must have an isolated base.
The main
conducting element will be a Thyristor. We will require a 10A type
with a
voltage
rating of 60VDC or above. Most thyristors of this rating are housed
in a transistor package called a TO220 can. Since the housing
includes a live metal base and tab, it
will
need to be mounted on a heat sink but isolated from it using a TO220
insulating kit.
The
heasink will need to have a rating of at least 4oC per watt or
better to maintain
a
temperature of less than 24oC above ambient.
The last
item we need is a suitable controller to switch the Thyristor.
They are generally
called series controllers and are
available from several companies for about £20.00
or
so in
ratings of 5 or 10ADC at 12 or 24V.
TRANSFORMER
CONNECTIONS:
The
transformer will have 4 connections, two primary (Live &
Neutral) and a secondary
output
of 0v & 17v or 0v & 33v depending upon the nominal charger
voltage of 12 or 24VDC.
Connect
the two secondary cable to the terminals marked AC or ~. There are
two
terminations, one for each secondary
connection. Make sure you use 10A rated cable
for all
the transformer connections.
Connect
the primary connections to the incoming mains supply. One of the
primaries can
be
connected to Neutral. The second should be connected to Live via a
fuse and switch. We
recommend fitting a fuse and rated
according to the following calculations:
Transformer secondary voltage multiplied by
the secondary current = VA1.
Divide
VA1 by the mains supply voltage and multiply by
1.4.
The
results indicate the nearest type "T" fuse value
required.
Clearly,
the live supply switch must be rated to withstand the input current
drawn by the
transformer and have a surge rating of at
least 10 times the fuse value due to transformer
inrush
when the mains supply is connected.
THYRISTOR & CONTROLLER
CONNECTIONS:
The
anode of the Thyristor must be connected via 10A cable to the
positive rectifier connection
whilst
the cathode is connected to charger output positive known as B+
using similar cable.
Connect
the controller negative terminal to rectifier
negative.
Connect
a 10A rated cable from rectifier negative to charger output
negative.
Connect
the controller battery positive cable to B+.
Connect
the controller supply to rectifier positive.
Connect
the grey controller cable to charger output negative known as
B-.
The blue
cable is connected to the Thyristor gate
terminal.
Please
check the connections with the manufacturer since they vary slightly
from one
company
to another.
AMMETER:
Most
chargers incorporate a moving coil ammeter to indicate the charging
current. Connect
the
meter is series with the supply from the Thyristor output to the
battery positive supply
(+ side
of meter to Thyristor) or between the battery negative terminal and
rectifier negative
(+ side
of meter to battery).
You now
have a fully functional constant potential charger that can be used
to recharge
lead
acid batteries at 12 or 24V. Connect the positive charger output to
battery positive and the charger output negative to battery
negative. Switch on the mains supply to energise the
charger.
If you have any questions regarding the
controller, contact the manufacturer.
We hope
you enjoy building your own fully automatic battery charger and
enjoy many years
of
service from it.
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CLASSIFIED ADS:
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and inexpensive to
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Designed to protect batteries in industry, cars, trucks, boats and
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Built in warning sounder!
order your alarm NOW!
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COMPETITION:
Subscribe to our ezine and you
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Simply send your email
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subject line.
______________________________________________________________________
READERS QUESTIONS.
Questions from Stephanie Willis!
Question 1.
What
does CC and CP stand for?
CC
stands for Constant Current, CP stands for Constant Potential. They
refer
to charging methods applied to nicad (CC) or
lead-acid (CP) batteries.
Question 2.
How many
batteries can be connected in series?.
As many
as you like up to a few hundred volts or so
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