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EDITORIAL.
Well, spring is just around the corner and some of us will be looking
forward to warm
days
and a more relaxed way of life. The batteries many of us stored away for
the
winter,
on a temperature controlled constant potential battery charger
hopefully, are
ready
to be re-installed in our boats and vehicles.
Unfortunately,
there will be a few batteries here and there that have been on charge
all
winter but cannot deliver any current. They are victims of a phenomenon
known as
sulphation,
largely caused by in appropriate charging potential or deep discharge
and an
often deadly condition, this is the subject of our first article.
Ok so your
batteries died. Could it have been avoided, possibly but it is difficult to
be
sure
unless the history of the cells is well known. The best thing to do is
replace the cells,
look
after them for as long as possible, charging them correctly, and apply appropriate
protection. Have a look at our second article and learn how to avoid
problems next time
around.
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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 eighteen years and
has designed charging equipment and battery monitors for some of the world
largest companies.
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ARTICLE: Sulphation in Lead Acid Cells. Author: Alan Fidler.
Those
of us who work with batteries on a daily basis have undoubtedly come across
sulphated cells
at one time or another. The symptoms are always the same. The
battery
reaches the float voltage on a charge cycle very quickly drawing hardly any
current
to do so. On discharge, the battery has no capacity and the voltage drops
very
quickly in seconds or minutes rather than hours.
Of
course the problem may not manifest itself until a load is applied for the
first time
since
the battery was put into storage or discharged the last time. The
charger may
have
maintained the float voltage but the battery still won't work because the
cells
have been
left in a fairly inactive state and sulphation has begun.
So what
is sulphation and how can it be prevented?
The
electrolyte inside a lead acid battery is usually in the form of a liquid
sulphuric
acid. When
batteries are left in a discharged condition or are simply not used, the
acid is
absorbed by the lead plates and a substance called lead sulphate is
formed.
The sulphate forms an insulating layer on the plates which increases the
internal
resistance of the battery and prevents it from drawing or delivering
current.
It does
not kill the battery completely though if it is discovered early enough.
A
typical example of a sulphated battery is a vehicle type which struggles to
turn the
car
over in winter but recharges quickly once the engine starts. The battery
generally
behaves
for the rest of the day only to repeat the performance the following
morning.
Unfortunately,
sulphation is the biggest cause of premature cell failure in most
industrial
and automotive applications. There is a school of thought that suggests
that a
periodic exposure to a higher potential of 2.45 volts per cell can help to
prevent
sulphation or break it down where it has arisen. however we already
know
that a high charge rate can cause internal heating of the battery which
buckles
the
plates and if this happens, the battery is permanently damaged.
The
higher charging voltage certainly helps in some cases, but it should only
be used
where
cells are displaying the symptoms already. We would NEVER recommend
applying
a
higher than normal charging voltage to a healthy battery. If you recharge
the battery
in the
correct way for the application in which it is used, sulphation will not be
a problem
to
begin with.
We have
visited websites during our research into this subject which state that
vehicle
batteries benefit from a charge rate of 14.5 volts instead of 14.1. We
agree
wholeheartedly
since the application is cyclic in nature but there are other
applications,
typically
stand-by, where this potential would damage the cells permanently. As
always,
our
advice is to check the battery manufacturers data and follow their advice
first.
In
reality, the actual charge voltage depends upon the application in which
the cells
are
used. Traction and automotive batteries require a different charging
voltage
to
batteries used in standby applications. Often, an installation for standby
use has
no
higher charging voltage or boost level at all but sulphation never
occurs because the batteries are discharged every month or so, religiously,
then immediately recharged.
This
sort of regime prevents sulphation even though the charging voltage never
exceeds
2.25
volts per cell at 20oC.
One
thing is certain. There is a point when a battery become so severely
sulphated
that it
cannot be recovered, ever. Batteries have a finite life and will eventually
fail
anyway
dispite your best efforts. We managed to squeeze 4.5 years out of a battery
designed
with a three year life in a vehicle in the UK which is about as good as it
gets.
Most
battery manufacturers recommend constant potential charging methods with
an
equalising charging voltage on cycled cells to get the best performance
from their
batteries.
Furthermore, they are very keen that users should use temperature
compensated chargers in applications where the ambient temperature
varies. This is by far the best way to prevent sulphation in the first
place. Charging the batteries correctly at all times as well as making them
do some work occasionally will do more to prevent failure than anything
else.
One
interesting point is that cells which are partially sulphated are actually
useful in
the
testing of a typical constant potential battery charger. The float voltage
can be
set with
the minimum of charging current whilst a suitable load allows the current
limit
to be set. Healthy batteries take time to charge up from a partial self
discharge
and
this can be somewhat frustrating so, sulphation can be beneficial
to some anyway!
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ARTICLE: Protecting Sealed Lead-Acid Batteries. Author: Alan
Fidler.
Unlike
Nicad batteries which, as far as abuse is concerned are more forgiving,
lead acid batteries demand particular conditions to work at their best.
There is a distinct voltage band within which a lead acid cell will
operate. Push the cells above or below it for any length of time
and they will be permanently damaged.
Lead
acid batteries need to be protected then, particularly from deep discharge.
If a typical
battery
is allowed to fall much below 1.55 volts per cell and is left in this state
for some
time,
it will never recover, no matter how long it is left on charge. It has been
damaged
permanently
and must be taken out of service. A dreadful waste I am sure you'll agree.
The
first problem, Overcharge, stems from a charger failure or incorrect
float voltage setting.
Over
charging a battery does two things, it causes the battery to gas and pushes
the
internal
temperature of the cells above recommended levels. This usually results in
buckled plates, lost electrolyte and in the case of most lead acid
batteries, a damaged pack.
Clearly
the battery must be protected if you want total peace of mind. If the
application
in
which the battery is employed isn't a critical one, it may just be
inconvenient when the
battery
fails but in some applications, failure could lead to loss of life.
There
is though a third condition which we covered in our first article,
that of sulphation
which can
also cause problems and it must be prevented for as long as possible or
detected
at an
early stage if reliability of the cells and the installation as a whole is
important.
So to
summarise, we have three conditions that must be avoided if we want the
best from
our
batteries, high voltage, low voltage and cell sulphation. Ignoring any one
of these three
has
disastrous consequences.
A high
voltage alarm is by far the best way to protect the cells from over
charge. It can be configured to energise a siren or led and disconnect
the charging supply to prevent battery
damage.
We also recommend fitting a fuse in series with the charger output positive
lead
to
protect the battery in the event of a charger rectifier diode short. Sadly,
diodes have a
tendency
to go short circuit when failing rather than blowing open circuit and this
results
in
unrestricted current flow and a high charging voltage.
Deep
discharge is prevented by using a Low Volts Disconnect device. A typical
LVD includes
a low
volts monitor and a heavy duty relay or contactor which disconnects the
load when the
battery
reaches a pre-determined minimum. A siren or led can be energised by the
alarm to
warn
the user that the battery is approaching its minimum limit.
Sulphation
can be avoided by charging the battery correctly and discharging the cells
on a
monthly
basis to keep them active. Wet lead acid cells must be topped up with
distilled
water. Maintenance free cells require nothing other than the above to keep
them
in
tip-top condition.
That's
really all there is to it. Protect the batteries from these three
conditions and you will
enjoy
many years of service from them. Neglect any one of the above and you will
find your
self
having to purchase new cells prematurely.
Remember:
Look after your batteries and your batteries will look after you!
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______________________________________________________________________
COMPETITION:
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______________________________________________________________________
READERS QUESTIONS.
Questions from Roger Burrows!
Question
1.
What
happens if a Sealed Lead Acid Battery is overcharged?
Sealed
lead-acid batteries are usually fitted with special vents
which
release some of the pressure that build up in the battery
through
heat. The battery is permanently damaged of course.
Question 2.
How can
a battery be protected from overcharge?.
By
fitting a fuse in series with the charger output in case of
failure
and by using a High Volts Monitor or Alarm to detect the
high
charge condition so action can be taken before the battery
is
damaged.
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