CIE-SEP09-PG28-29:Layout 1 07/09/2009 11:58 Page 28
Power Management
Intelligent charging
How do you go about maximising the battery life of a
gelling agent and is largely absorbed on the plates, where it will crystallise,
and held by insulating separators which also causes a permanent loss of
Sealed Lead Acid battery? Intelligent charging could be
between the plates, see Figure 1. capacity.
the answer, according to Tony Morgan
When an SLA battery is being It is also important to remember
discharged; the lead (Pb) on the that SLA batteries have a self
negative plate and the lead dioxide discharge rate of approximately 5%
(PbO2) on the positive plate are per month. This is less than most
C
harging Sealed Lead Acid (SLA) intelligent charger is not only cost converted to lead sulphate (PbSO4). other forms of rechargeable batteries,
batteries does not, on the face effective, it is also better for the At the same time the sulphuric acid but has to be considered.
of it, seem a particularly environment. (H2SO4) is converted to water (H2O). Manufacturers recommend
difficult process, but the hard part in Before looking at the different In a normal charge, the chemical recharging when the battery reaches
charging an SLA battery is charging techniques it is important to reaction is reversed. The lead sulphate about 70% of its capacity
maximising the battery life. Simple understand the battery chemistry and and water are electro-chemically (approximately 2.1 volts per cell).
constant current/constant voltage what happens during normal charge converted to lead, lead dioxide and They use this to calculate the
chargers will do the job for a while, and discharge cycles. sulphuric acid. During a full charge maximum life of the battery, but this
but the battery life expectancy quoted Typically the positive plates in an cycle any gasses produced need to be is very difficult to implement in a real
by the manufacturer will be greatly SLA battery are made from lead re-combined in a so called ‘oxygen world application.
reduced by using non-intelligent dioxide and the negative plates from cycle’. Oxygen is generated at the
chargers like this. Maximising the life a sponge lead. The electrolyte is positive plates during the latter stages So let us look at different charging
of your SLA battery by using an usually sulphuric acid mixed with a of the charge cycle, this reacts with techniques: -
and partially discharges in the sponge
lead of the negative plates. As
Constant Voltage Charging:
charging continues, the oxygen
This method is the most commonly
produced also re-combines with the
used for SLA batteries as the
hydrogen being produced on the
individual cells tend to share the
negative plate forming water. With
voltage and equalize the charge
correct and accurate cell voltage
between them. It is important to limit
control all gasses produced during the
the initial charging current to prevent
charge cycle will be re-combined
damage to the battery. However, with
completely into the negative plates
a single fixed voltage, it is impossible
and returned to water in the
to properly balance the requirements
electrolyte.
of a fast charge cycle against the
If an SLA battery is over-charged,
danger of overcharge.
the excess cell voltage will result in
the conversion of electrolyte into
Constant Current Charging:
large amounts of hydrogen and
This method can be used for a single
oxygen gasses which cannot be
2V cell but is not recommended for
recombined by the normal processes.
charging a number of series
A pressure-release valve will open and
connected cells, a battery, at the same
vent the excess gas, resulting in the
time. This is because some cells will
loss of electrolyte and a loss of
reach full charge before others and it
capacity.
is very difficult to determine when
If the battery is undercharged; the
the battery has reached a fully
low cell voltage will cause the charge
charged state. If the charge is
current to diminish to zero well
continued at the same rate, for any
before full capacity is reached. This
extended period of time, severe
will allow some of the lead sulphate
overcharge may occur to some cells,
produced during discharge to remain
resulting in damage to the battery.
Taper Current Charging:
Figure 1: Typical SLA Battery Construction
28 September 2009 Components in Electronics www.cieonline.co.uk
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