Environmental Engineering Reference
In-Depth Information
Table 4.8
Dependence of the Open Circuit Voltage and the Charge Density
on the State of Charge of a 12-V Lead-Acid battery
State of charge (SOC)
100%
75%
50%
25%
0%
Voltage in V
12.7
12.4
12.2
12.0
11.9
Acid density in kg/l
1.265
1.225
1.190
1.115
1.120
reaches the end of discharge voltage after 100 hours. A rechargeable battery
with a capacity of
C
100
= 100 A h has a nominal discharge current of
I
100
=
C
100
/100 hours = 1 A. If the battery is discharged in 10 hours with a current
of 8 A, the capacity
C
10
is reduced to less than 80 per cent of
C
100
. The usable
capacity decreases to about 50 per cent at a temperature of 0°C and a
discharge time of 5 hours. The lifetime of the battery, i.e. the number of cycles
achievable, decreases with increasing temperature and discharge depth. The
recommended maximum depth of discharge is normally about 80 per cent,
whereas depths of discharge of more than 50 per cent (filling level below 50
per cent) should be avoided if possible.
Table 4.8 shows the open circuit voltage and acid density of a 12-V
lead-acid battery. The nominal acid density for 100% charge varies between
1.22 kg/litre and 1.28 kg/litre depending on battery design and operational
area. Higher acid densities improve the operating behaviour at low
temperatures, whereas lower acid densities reduce corrosion. The given voltages
are only valid for open circuit operation, i.e. the battery has not been charged
or discharged recently. A temperature compensation of -25 mV/°C can be
expected. When charging or discharging the battery, the voltage is above or
below the open circuit voltage. The voltage difference compared with the open
circuit voltage depends on the current. Figure 4.43 shows the voltage over the
discharge time. Starting at the open circuit voltage of 12.7 V for a fully charged
battery, the voltage falls depending on the discharge current. If the initial charge
is lower, the voltage drop is higher.
The battery's state of health also influences the voltage slightly. The voltage
is also an indicator of the
state of charge (SOC)
of a rechargeable battery (see
Table 4.9).
The rechargeable battery should be protected against deep discharge or
overcharging. If the battery is totally empty, crystalline lead sulphate is
created. This type of lead sulphate is difficult to reconvert and some material
will remain in the crystalline form. This damages the battery permanently.
Therefore, deep discharge should be avoided in any case. This can be
achieved in most cases by switching off the load at about 30 per cent of the
remaining capacity. At common operating conditions, this is equivalent to a
battery voltage of about 11.4 V. Lower voltages for ending discharge can be
chosen for higher discharge currents above
I
10
. In addition, if the battery is
not used for a long time, damage as a result of deep self-discharge is possible.
The battery should be recharged from time to time to minimize the risk of
damage.
