Welcome to our June issue
of DC Supply.
again we must apologise for the late circulation of this
We will try to be on time next month!
issue we look at dual charging systems and DC-DC
industrial applications are critical and failure of the supply is
option. Dual charging systems provide protection against single
and are widely used in applications of this
connected to a battery are often voltage sensitive. DC-DC
the DC supply in step-up or step-down mode and are highly
you are looking for a stable supply regardless of fluctuating
supplies, DC-DC converters may provide the
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
Charging Systems: Alan Fidler.
industrial applications are so important that a DC supply failure is
the question. The supply must be permanently available and steps
ensure that the system continues to operate in the event of a
critical applications, a back up is provided so that the system
to work should the main
charging system fail. Systems of this nature
referred to as dual charging systems or DCP (dual charging
normally consist of two charging systems, two battery banks and a
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
from the it to some degree. Charger 1
maintains the battery bank
charger 2 sits in standby mode, ready to go but
inhibited whilst charger
functional. Budget installations of this nature can be
very effective but take
account of battery failure and are therefore less
reliable than our first version.
course the battery installations must be isolated from each other in
prevent a failure from effecting the whole installation. This is
power diodes, one from battery bank 1 and the other from bank 2
cathodes connected together to supply the load. A connection
to each battery bank negative is required so the negative returns
whilst the positive supplies are isolated as
isolation can be provided using DC-DC converters which incorporate
technology and high frequency isolating transformers but the cost is
prohibitive in high power applications
dispite the obvious advantages.
batteries and chargers need to be monitored continuously
in critical applications
and because they are
isolated two sets of alarm monitors are required, one
charger/battery bank. As a minimum each set should
Failure Alarm (one per charger)
Voltage Alarm (one per battery bank)
Failure Alarm (one per charger)
Volts Disconnect (one per battery bank)
purpose of the mains failure alarm is self explanatory as is the Low
and Charge Failure Alarm. The purpose of the Low Volts Disconnect
be so obvious. In practice, a discharge
limit needs to be applied to the
banks to prevent cell failure. This is particularly important in
installations. The Low Volts Disconnect
device performs this role by disconnecting
if the battery voltage falls below a pre-determined minimum,
alarm monitors such as High Voltage or Earth Leakage Monitors can
as required although neither are as important as the
the alarms include volt free relay contacts for remote switching or
often advantageous to incorporate a common alarm relay. This
set of changeover contacts on each charger/battery bank which change
fault develops anywhere in the system.
exact configuration depends upon a number of factors, the importance
integrity, space and cost. A 3.6KW charger and 8.5KW battery bank
anything up to £9,000 at 110VDC so the specification must be
carefully to design the most
appropriate system at the best possible price.
most important consideration is the integrity of the supply which
failure cases have life threatening
consequences, costs are some way down on
priorities. The system must include protection against single point
regardless of the extra costs
involved. Adhering to the specification is vital
deviating from it to reduce costs has
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!
A new concept in battery
Current discharge characteristics for easy battery
operating range from 20-130VDC for true
Available in low and high power
the guesswork out of battery capacity factory acceptance
Converters. Author: Alan Fidler.
converters have been widely used in industrial applications for many
their efficiency, compact size and reliable performance.
DC-DC converter is often used inappropriately and there are a number
regulators on the market which fall into this
purposes of this article, a DC-DC converter is a switched mode
of supplying a fixed voltage at a variable current level
regardless of input
variations. The input to the converter can be higher or lower than
so a 12V battery operating over a 10-14VDC range could supply the
to a 12V
converter with a fixed output voltage of 12VDC +/-1% for
that requires a supply a volt or too higher than its output is a
should not be confused with a true converter dispite the industries
just that. A true converter can in theory at least, recharge one
battery of a
voltage from another battery of the same voltage,
converter regulates the supply to a high frequency transformer
width modulation (see September 2001 Issue). The transformer output
and monitored by the voltage control network which in turn controls
width resulting in a stabilised output supply.
converters require an input to output power ratio of about
110% so a converter
10A 12V output will draw approx 11A from a 12V
input supply, 13.2A from a
supply or 9.4A from a 14V supply. As you can
see, the overall input power is
regardless of input voltage, the input current simply rises or falls
Converters are particularly suited to voltage sensitive loads or for
applications. A good example would be a boat where a starting
kept permanently charged via a secondary battery pack such as
batteries. Audio or video equipment may also require a stable 12V
reliable performance and converters are an ideal solution here
like all items switch mode, true DC-DC converters are expensive
expect to pay anything between £0.50 and £1.50 per watt depending
application, required rating and choice of supplier. On the positive
reliable, compact and highly efficient so running costs are
require a stabilised supply from a variable but nominally identical
voltage, a DC -DC converter may be an
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Questions from Colin
Can batteries of differing capacities be
connected in parrallell?
No!. Batteries can only be
connected in series or parrallel if their
will happen if batteries of different capacities are
larger battery will discharge into the smaller one and the
either undercharge the larger battery or overcharge the smaller
worst case senario, the smaller battery could get very hot
to an explosion.
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