Environmental Engineering Reference
In-Depth Information
be effective when the switch is closed and that happens for values of V higher than
a specific value depending on constructive characteristics of the battery.
5.3.1.2 Battery Capacity
The battery capacity represents the electric charge that a battery can supply.
Instead of using the Coulomb, which is the SI unit for the electric capacity but is
an inconveniently small unit, the Amphour (Ah) is commonly used to refer to the
capacity of electric vehicle batteries. Batteries are characterized by a nominal
value of capacity, determined with predefined procedures. However, the real
capacity of a battery depends on the current values drawn out from it. This
changing in the expected capacity is caused by uncompleted or unwanted reactions
inside the cell. This effect occurs in all the types of batteries, but it is particularly
accentuated for lead-acid batteries. Figure
5.13
shows typical discharge curves for
different discharge rates (I
1
, I
2
,…, I
6
), where the black line shows the available
capacity as a function of the discharge time.
The capacity of batteries used to supply electric vehicles is generally referred to
a 5 h discharge, and in that case, the capacity is represented with the symbol C
5
.
Moreover, the value of battery capacity is often used to represent a current value,
Fig. 5.12 Charging equiva-
lent circuit for a battery
R
1
I
Batt
S
+
-
Power
Supply
E
o
V
R
2
Fig. 5.13 Typical lead-acid
battery discharge voltage as a
function of time
2,1
1,9
I
6
I
4
I
5
I
3
I
2
I
1
100
1,7
60
1,5
1,3
20
0,1
1
10
Discharge time [h]
