DC Supply! Monthly Electrical Ezine

CBC Design (tm) - May 2002 Issue. ISSN 1475-3464
Email: cbc_design@btconnect.com

"...Maintaining a reliable DC supply."



- Editorial
Switch Tripping Battery Chargers. (Article)
Standby or Cycling. (Article)
- Competition - Win FREE battery pack!.
- Readers Questions
- Subscriber Ads



lcome to the May issue of DC Supply.
We apologise for the late circulation of this particular issue and hope
you will bear with us through this rather busy period. Summer is often
the industries most active period.
This month we look at Switch Tripping Battery Chargers. Widely used
in industrial applications, they are the life blood of many installations
and therefore extremely important. But what is switch tripping?
Does your battery installation fall into the standby or cycling criteria?
Charging batteries in the correct manner for a given duty is vital. Get it
wrong and the cells could boil dry or discharge. Either way, they won't
do the job they were commissioned to do and you'll end up out of pocket.
Please feel free to submit articles or questions. Dispite our industrial guise,
we are more than happy to answer questions from any of our readers.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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.

ARTICLE: Switch Tripping Battery Chargers: Alan Fidler.
Switch tripping battery chargers are the most popular system used in industrial
applications worldwide. They are more often desiged as low power chargers maintaining
sealed lead acid batteries configured for short duration high current supplies.
But what is switch tripping?
Switch Tripping is a term used to describe loads of a pulsed nature. Relays,
contactors and solenoids would be typical examples of components that may
be employed in an application of this type.
More often than not, any standing loads on chargers of this nature are small
in comparison to the switched loads which may have a short duration high current
requirement. Batteries must be sized accordingly and since sealed lead acid cells
are able to deliver a lot of current for their size, they are often utilised. Nicads work
just as well though, of course.
In some cases, the performance of a switch tripping system is critical and money
or worst still, lives could be in jeopardy where failure occurs. For absolute safety,
two or more systems are sometimes used to eradicate single point failure. 
Chargers of this type may be referred to as a dual redundant charging systems.
A ripple free supply is the preferred power source in the majority of installations
and this has a direct bearing on unit cost. A switch tripping charging system is
generally more expensive than a conventional battery charger, even an automatic
one. A number of alarms are normally included to provide an advanced warning
should a mains failure or charger failure occur.
Of course, there are other factors to consider too. The end discharge voltage must
be calculated in order to select the most appropriate charging regime. A standby
system is configured differently to a cycling system for a very good reason, depth
of discharge determines which of the two is the most appropriate in terms of current
in. We will be looking at this aspect of battery charging in more detail in our next article.
Switch tripping battery charging equipment is available from most manufacturers in
open chassis or enclosed versions. Additionally, many will custom build the system to
your exact specification. It is simply a question of determining the total load, the
autonomy (back up time), the operating temperature, the duty cycle (how much time
is available to recharge the batteries) and the end discharge voltage.
Batteries are often supplied with the charger as a package. The batteries may be
located inside an enclosed system in its coolest point, the lower half, with the
charging equipment located in the top half. Any other arrangement wouldn't work
reliably due to temperature fluctuations within the cubicle. Indeed, We have
witnessed a number of failures  over the years due to batteries being located next to
a heat sink or transformer. Don't do it if you value your cells and where possible,
use a metal barrier to segregate the charger from them.
Lastly, when placing an order for a switch tripping charging system, don't be afraid
to ask questions. Reputable suppliers will be only too pleased to help you and besides,
if you want a system that works well, communication between you and your selected
vendor is vital.


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ARTICLE: Standby or Cycling. Author: Alan Fidler. 
Trying to decide if an application falls into the standby or cycling category is very
important if you want reliability. If the wrong charging regime's is used, the
battery will never perform to specification and disappointment and cost will follow.
It is not unusual for an application to fall into both of the above at different times
of course and this identifies a further problem, what type of charger is suitable in
this situation?
Firstly, let me explain briefly the differences between the two operating duties!
Standby is a term used to describe a battery installation that spends most of its life
in a full charged state with minimal loading. A typical example would be an emergency generator battery which may not be used for years at a time apart from an occasional
test run, electrical supplies are so reliable these days.
Cycling is a term used to describe a charger and battery system connected to loads
of a more frequent nature, weekly or even daily in many cases. The battery works much
harder compared to the standby version and requires a different charging regime to
fully recover to 100% of its rating every time.
A reliable battery charger must be correctly configured for the application in which
it has to function. A standby charger can be a simple constant potential system with
a taper charge characteristic and little else in the way of complicated control.
A cycling charger will require a fixed period at a raised charging voltage to approximate constant current mode at the beginning of the cycle. This is followed by a taper charge
after a fixed time period has elapsed or when a particular battery voltage is reached.
There are slight variations on this but by and large, this is how it works.
Low Voltage Alarms are often employed to differentiate between a partial discharge
and a full one so that the appropriate charging regime can commence. This configuration
is always employed where the duty flips between standby and cycling.
Of course, the charger must be designed to recharge the batteries as recommended
by the manufacturer. Every single one of them will factor temperature into their equations
and we must do the same. If the installation has to operate in a room with wildly fluctuating
ambient temperatures, the charger must include battery temperature compensation. This
is more important than almost anything else in my opinion.
You can probably categorise every battery application you see into the standby or cycling
category if you follow the guidelines above. Standby is precisely that, the system is ready
to provide emergency power should it prove necessary but probably won't have too. Cycling
is charge and discharge, charge and discharge in an endless cycle. Correctly categorising
an application is the first step to success.
Remember: Look after your batteries and your batteries will look after you!


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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.



Questions from Roger Burrows
Question 1.
What is a shunt?
A shunt is resistive device used to generate a small signal voltage
to display current on a moving coil ammeter. Charging current is
passed through the shunt and the signal voltage is generated. 

Question 2.
What is a current transformer?.
Current transformer are used to measure current in ac circuits.
They are isolated devices manufactured on an iron ring core with
a central hole through which an ac cable is passed.
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