B. Extracting Model Parameters
The model can accurately represent the behaviour of many
battery types, provided the parameters are well determined.
The main feature of this battery model is that the parameters
can easily be deduced from a manufacturer’s discharge curve.
Fig. 2 shows a typical discharge characteristic, for a 1.2V 6.5
Ah Nickel-Metal-Hydrid cell. The discharge curve is for a constant
current of 1.3A (0.2 C rate1).
1) Internal resistance approximation: Internal resistance is
very important in order to adequately represent the voltage
drop caused by a current variation in the battery. The internal
impedance is generally specified in the manufacturer’s
datasheet. Tests have determined that the internal impedance
provided by the manufacturer doesn’t allow, for the proposed
model, to accurately represent the potential difference caused
by the variation of the current. For example, the resistance
described on the datasheet of the Panasonic HHR650D3 NiMH
battery is 2mΩ but it was found that the resistance that best fits
the three current curves (Fig. 32) is 4.6mΩ.