DC Supply! Monthly Electrical Ezine

CBC Design (tm) - June 2002 Issue. ISSN 1475-3464
Email: cbc_design@btconnect.com
http://www.cbcdesign.co.uk

"...Maintaining a reliable DC supply."

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IN THIS MONTHS ISSUE

- Editorial
Dual Charging Systems. (Article)
DC-DC Converters. (Article)
- Competition - Win FREE battery pack!.
- Readers Questions
- Subscriber Ads

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EDITORIAL. 

We
lcome 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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COMPETITION:

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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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