Charging
Sealed lead acid batteries require current limited constant voltage charging.
Cells should not be charged in less than four or five hours for optimum life
(C/4 rate). Accurate charge voltage control is essential.
The type of charger is determined by the following application parameters:
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Cyclic or standby operation
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Operating temperature
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Charge voltage stability
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Initial current limit
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Permissible overcharge
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Charge time
Manufacturers of SLA batteries recommend constant voltage current limited
charging from a basic single voltage taper charge or multi stage constant
voltage current limited source to realise effective charge management.
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Standby Application
Where the SLA application is for standby use and the battery is seldom required
to deliver power a single stage current limited float charger preset to
2.25V/cell at normal temperatures is adequate. At this potential the battery
will not overcharge and may remain connected indefinitely without incurring
damage.
Figure 1: Basic Taper Charging Profile
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Standby and Cyclic Application
If a single voltage (one step) charger set at 2.45V/cell remains connected to a
battery, the high residual value of trickle current causes overcharge which
cannot be absorbed into cells the excess energy is dissipated as heat resulting
in reduced operating life and performance (see Figure 1).
The reduction in cell voltage to the float value of 2.25V minimises the
overcharge current to a safe sustainable level although the penalty is
increased charge time.
Optimum charge is obtained by combining the cyclic and standby modes in an
automatic charger with re-forming mode for initial recovery.
When a battery is used in a cyclic manner a charging target potential of
2.45V/cell at normal temperatures is acceptable but should be terminated before
the onset of overcharge.
Overcharge is prevented by selecting a multistage managed charger.
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The charge voltage recommended by most battery manufacturers is 2.45V/cell with
a maximum in-rush current of 0.25C Amps. This limits the safe charge time to
approximately 5 hours to reach 90% capacity. Exceeding the voltage or current
limit is detrimental to battery life. Some cell manufacturers permit a larger
in-rush current but the additional cost of the enhanced charger may not be
acceptable.
If the battery is fully or deeply discharged, a three-step charger initially
generates a small forming current to restore the battery to 1.75V/cell (10.5V
in a 12V battery) before enabling the bulk current to the maximum design limit.
The battery voltage rises towards the limit of 2.45V/cell (14.7V) and the
current starts to decay. When the battery voltage limit is achieved, the
charger voltage reduces to 2.25V/cell (13.5V) and the current continues to
fall. (see Figure 2.)
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| Figure 2: Multistage Managed Charging Profile
Full capacity is recovered when the trickle current falls to 0.01C Amps and
continues to decay until the natural maintenance level of approximately 0.001C
Amps is reached. The battery can remain in this state continuously.
The Lawtronics current limited/constant voltage three-step charger offers a
further improvement in charge time.
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Charge Time
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An approximate calculation of charge time may be derived from:-
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| | Battery Capacity (in Ah) |
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Hours = |
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+2
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Current Limit (A)
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Step 1
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Forming current until deep discharged battery rises to approximately 1.75V/cell.
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Step 2
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Bulk charge to 2.45V/cell (approximately 80% returned capacity).
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Step 3
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Trickle charge at 2.25V/cell indefinitely (90% capacity after 5-6 hours).
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