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EDITORIAL.
Welcome to our June issue
of DC Supply.
Once
again we must apologise for the late circulation of this
months
issue.
We will try to be on time next month!
In this
issue we look at dual charging systems and DC-DC
Converters.
Many
industrial applications are critical and failure of the supply is
not
an
option. Dual charging systems provide protection against single
point
failure
and are widely used in applications of this
nature.
Loads
connected to a battery are often voltage sensitive. DC-DC
Converters
regulate
the DC supply in step-up or step-down mode and are highly
efficient
too. If
you are looking for a stable supply regardless of fluctuating
battery
supplies, DC-DC converters may provide the
solution.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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 nineteen years and has designed
charging equipment and battery monitors for some of the world
largest companies.
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ARTICLE: Dual
Charging Systems: Alan Fidler.
Many
industrial applications are so important that a DC supply failure is
completely
out of
the question. The supply must be permanently available and steps
must be
taken to
ensure that the system continues to operate in the event of a
failure.
In most
critical applications, a back up is provided so that the system
continues
to work should the main
charging system fail. Systems of this nature
are
commonly
referred to as dual charging systems or DCP (dual charging
panels).
They
normally consist of two charging systems, two battery banks and a
single
distribution panel sourced from the two
separate DC supplies via isolating diodes.
The above is by far the most effective
method but there are applications which
deviate
from the it to some degree. Charger 1
maintains the battery bank
whilst
charger 2 sits in standby mode, ready to go but
inhibited whilst charger
1 is
functional. Budget installations of this nature can be
very effective but take
no
account of battery failure and are therefore less
reliable than our first version.
Of
course the battery installations must be isolated from each other in
order
to
prevent a failure from effecting the whole installation. This is
achieved using
two
power diodes, one from battery bank 1 and the other from bank 2
with the
diode
cathodes connected together to supply the load. A connection
from load
negative
to each battery bank negative is required so the negative returns
are
commoned
whilst the positive supplies are isolated as
shown.
True
isolation can be provided using DC-DC converters which incorporate
switch
mode
technology and high frequency isolating transformers but the cost is
often
prohibitive in high power applications
dispite the obvious advantages.
The
batteries and chargers need to be monitored continuously
in critical applications
and because they are
isolated two sets of alarm monitors are required, one
for
each
charger/battery bank. As a minimum each set should
include:-
Mains
Failure Alarm (one per charger)
Low
Voltage Alarm (one per battery bank)
Charge
Failure Alarm (one per charger)
Low
Volts Disconnect (one per battery bank)
The
purpose of the mains failure alarm is self explanatory as is the Low
Voltage
Alarm
and Charge Failure Alarm. The purpose of the Low Volts Disconnect
alarm
may not
be so obvious. In practice, a discharge
limit needs to be applied to the
battery
banks to prevent cell failure. This is particularly important in
lead-acid
installations. The Low Volts Disconnect
device performs this role by disconnecting
the load
if the battery voltage falls below a pre-determined minimum,
normally 1.75
volts
per cell.
Other
alarm monitors such as High Voltage or Earth Leakage Monitors can
be
fitted
as required although neither are as important as the
others.
Since
the alarms include volt free relay contacts for remote switching or
indications,
it is
often advantageous to incorporate a common alarm relay. This
provides a
single
set of changeover contacts on each charger/battery bank which change
state
if a
fault develops anywhere in the system.
The
exact configuration depends upon a number of factors, the importance
of the
supply
integrity, space and cost. A 3.6KW charger and 8.5KW battery bank
can
cost
anything up to £9,000 at 110VDC so the specification must be
read
carefully to design the most
appropriate system at the best possible price.
If the
most important consideration is the integrity of the supply which
can in
failure cases have life threatening
consequences, costs are some way down on
the
list of
priorities. The system must include protection against single point
failure
regardless of the extra costs
involved. Adhering to the specification is vital
and
deviating from it to reduce costs has
risks.
If you
need reliability, a dual charging system is the best
option. Scrutinise the
application, design a system that considers
all possible scenarios and contact
suppliers who can advise you on the
best course of action.
Remember: Look after your batteries and
your batteries will look after you!
______________________________________________________________________
______________________________________________________________________
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______________________________________________________________________
ARTICLE: DC-DC
Converters. Author: Alan Fidler.
DC-DC
converters have been widely used in industrial applications for many
years
due to
their efficiency, compact size and reliable performance.
Unfortunately, the
term
DC-DC converter is often used inappropriately and there are a number
of step-
down
regulators on the market which fall into this
category.
For the
purposes of this article, a DC-DC converter is a switched mode
device
capable
of supplying a fixed voltage at a variable current level
regardless of input
supply
variations. The input to the converter can be higher or lower than
the output
supply
so a 12V battery operating over a 10-14VDC range could supply the
input
to a 12V
converter with a fixed output voltage of 12VDC +/-1% for
example.
A device
that requires a supply a volt or too higher than its output is a
regulator
and
should not be confused with a true converter dispite the industries
tendency
to do
just that. A true converter can in theory at least, recharge one
battery of a
fixed
voltage from another battery of the same voltage,
regulators cannot.
A DC-DC
converter regulates the supply to a high frequency transformer
using
pulse
width modulation (see September 2001 Issue). The transformer output
is
filtered
and monitored by the voltage control network which in turn controls
the
pulse
width resulting in a stabilised output supply.
Most
converters require an input to output power ratio of about
110% so a converter
with a
10A 12V output will draw approx 11A from a 12V
input supply, 13.2A from a
10V
supply or 9.4A from a 14V supply. As you can
see, the overall input power is
the same
regardless of input voltage, the input current simply rises or falls
as the
voltage
supply varies.
DC-DC
Converters are particularly suited to voltage sensitive loads or for
critical
supply
applications. A good example would be a boat where a starting
battery
could be
kept permanently charged via a secondary battery pack such as
the
cabin
batteries. Audio or video equipment may also require a stable 12V
input
for
reliable performance and converters are an ideal solution here
too.
Sadly,
like all items switch mode, true DC-DC converters are expensive
and you
can
expect to pay anything between £0.50 and £1.50 per watt depending
upon
the
application, required rating and choice of supplier. On the positive
side, they
are very
reliable, compact and highly efficient so running costs are
minimal.
If you
require a stabilised supply from a variable but nominally identical
input supply
voltage, a DC -DC converter may be an
ideal solution.
______________________________________________________________________
______________________________________________________________________
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NOW!
______________________________________________________________________
COMPETITION:
Subscribe to our ezine and you
will be automatically entered into our competition where you can win
a FREE Nicad "AA" battery pack.
Simply send your email
address (No free email) to cbc_design@btconnect.com with the words
"SUBSCRIBE" in the
subject line.
______________________________________________________________________
READERS QUESTIONS.
Questions from Colin
Macer!
Question 1.
Can batteries of differing capacities be
connected in parrallell?
No!. Batteries can only be
connected in series or parrallel if their
characteristics are
identical.
Question 2.
What
will happen if batteries of different capacities are
connected?.
The
larger battery will discharge into the smaller one and the
charger
will
either undercharge the larger battery or overcharge the smaller
one.
In a
worst case senario, the smaller battery could get very hot
which
may lead
to an explosion.
______________________________________________________________________
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