Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 71

Fig. 4.24 Battery equivalent circuit

Several circuit models were proposed for simulation purposes, different in effort
and accuracy. One of them is discussed here shortly [Sal92]. Figure 4.24 shows the
equivalent circuit. The model components represent:

C1 battery main capacitance

R5 self-discharge resistance

C2 overvoltage capacitance

R3, R4 internal resistance for charge and discharge, respectively

R1, R2 overvoltage resistance for charge and discharge, respectively

The generally nonlinear parameters are determined from tests. For simulation the
properties are modelled by functions, e.g. by U C1 ( Q C1 ) for C1 or I R3 ( U R3 ) for R3.
For practical use the resistance values are described in sums as R1+R3 and R2+R4.
Temperature variation may be taken into account by a temperature compensation
algorithm.

4.4.2.2 Other Secondary Batteries

Electro-chemical storage devices apart from the lead-acid accumulator are known
for either higher energy densities, longer life cycle or aptitude for quick charge. On
the other hand they are more costly. In wind energy systems they have not found
much application, but a few of them which are of general interest are mentioned be-
low for comparison. Table 4.1 lists the main properties, with lead-acid cells included
for completeness.
NiCd-batteries are widely used in appliances and notebooks. Metals are nickel
and cadmium, the electrolyte being caustic potash dilution. They are problematic
due to the detrimental effect of cadmium for the environment. Besides, they show
the memory-effect, i.e. an irreversible decrease of capacity occurring when the bat-
tery is not fully charged in suitable intervals.
NiMH-batteries are less problematic regarding environmental impact. Metals are
nickel, titan, vanadium, circonium and chrome, the electrolyte is caustic potash
dilution. Compared with NiCd-batteries they have the same cell voltage of 1,2 V,
but a larger energy density, and no memory-effect is observed. On the other hand
their temperature operating range is more limited.
NaS-batteries feature a higher energy density; however the high operation tem-
perature and the chemically active component natrium were responsible for the fact