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EDITORIAL.
Welcome 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.
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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: 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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______________________________________________________________________
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
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subject line.
______________________________________________________________________
READERS QUESTIONS.
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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