DC Supply! Monthly Electrical Ezine

CBC Design (tm) - February 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
Battery Management. (Article)
Designing your own Charger. (Article)
- Competition - Win FREE battery pack!.
- Readers Questions
- Subscriber Ads

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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!
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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
charging methods commonly employed : http://www.cbcdesign.co.uk/regimes/index.html
 
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:

*** NEW! 12 and 24V DC 60 Watt lamp dimmers ***

Dims single or multiple lamp assemblies with a maximum rating of 60 watts!
Lamps can be adjusted from a dim glow to bright light!
Regular use reduces battery power consumption and increase lamp life!
Fitted with RFI Suppression!

Go to http://www.cbcdesign.co.uk/dimmer.html and take a look at the
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:

*** NEW! 12 and 24V DC Low Battery Alarm ***
Protect your vehicle batteries from deep discharge with a low battery alarm.
Easy to Fit, Easy to Use and inexpensive to boot!
Designed to protect batteries in industry, cars, trucks, boats and mobile homes!
Built in warning sounder!
Goto http://www.cbcdesign.co.uk/battalarm.html and download a data sheet or
order your alarm NOW!
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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.

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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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