Handbook of Electrical Installation Practice - BeKnowledge

Fig. 6.9 Circuit and waveform of a typical taper charger.
Standby Power Supplies 143
to the trickle charge after a boost charge, so that the electrolyte will rapidly be boiled
off.
A far better approach to battery charging is to use a constant-voltage charger. In
this type of charger, the voltage applied to the battery is controlled automatically
and kept constant, independent of any mains or load fluctuations. Constant-voltage
charging is recommended for both sealed lead-acid and nickel-cadmium cells.
Constant-voltage chargers use a power electronic regulator, with a currentlimiting
circuit to protect the components in the event of an excessive overload. The
circuit of a constant-voltage charger with series regulator is shown in Fig. 6.10.
From a practical viewpoint, the main benefit of the constant-voltage charger is its
ability to compensate automatically for variations in load and mains voltage. The
battery is kept is a healthy state and maintenance intervals are extended. There is
no need to incorporate a battery load test circuit in order to check that the battery
is fully charged.
Another benefit is that the size of the charger does not depend on the size of the
battery installation. So extra battery capacity may be added to meet changing
requirements without necessarily incurring the cost of a replacement charger (as
would be the case with a taper charger).The constant-voltage charger automatically
compensates for the increased charge requirements of some batteries as they age.
Meters and alarms
Some meters and alarms are desirable on any battery charger, although it is not
uncommon for users to request far more complex metering than is strictly justified.
For most users, alarms are more useful than meters.
A charge-fail alarm is probably the most valuable alarm because there are conditions
where the charger can be working satisfactorily within current limits, but the
battery is discharging. A high-voltage alarm, which indicates if the charger is left in