Civil Engineering Reference
In-Depth Information
the capacity is achieved. The charge current is then cut back, in steps, until
the battery is fully charged. At this time, the charge current is further reduced
to trickle-charge rate, keeping it fully charged until the next load demand
comes on the battery. This method, therefore, needs at least three charge
rates in the charge regulator design.
10.8.2
Single Charge Rate
This method uses a simple low cost regulator which is either on or off. The
regulator is designed for only one charge rate. When the battery is fully
charged, as measured by its terminal voltage, the charger is turned off by a
relay. When the battery voltage drops below a preset value, the charger is
again connected in full force. Since the charging is not gentle in this method,
full charge is difficult to achieve and maintain. An alternate version of this
charging method is the multiple pulse charging. Full current charges the
battery up to the high preset voltage just below the gassing threshold. At
this time, the charger is shut off for a short time to allow the battery chemicals
to mix and voltage to fall. When the voltage falls below the low preset
threshold, the charger is reconnected, again passing full current to the battery.
10.8.3
Unregulated Charging
This least cost method can be used in photovoltaic power systems. It uses
no charge regulator. The battery is charged directly from a solar module
dedicated just for charging. The charging module is properly designed for
safe operation for the given number of cells in the battery. For example, in
12 volts lead-acid battery, the maximum photovoltaic module voltage is kept
below 15 V, thus, making it difficult to overcharge. When the battery is fully
charged, the array is fully shunted to ground by a shorting switch (transis-
tor). The shunt transistor switch is open when the battery voltage drops
below certain value. The isolation diode blocks the battery powering the
array or the shunt at night, a discussed in Section 8.8.
10.9
Battery Management
Drawing the electrical power from the battery when needed, and charging
it back when access power is available, requires a well-controlled charge and
discharge process. Otherwise, the battery performance could suffer, the life
shortened and the maintenance increased. Some common performance prob-
lems are as follows:
•
low charge efficiency resulting in low state of charge.
•
loss of capacity to hold the rated Ah charge.
