PhD Thesis - Cranfield University

Chapter 3
Battery
Voc
Ri
Vb i
Test circuit to measure Ri DC
S2
Ib i
S1
R1
R2
Vb 0 Vb 3
Vb 1
Vb 2
71
Ib 1
S1=1
Ib 2
S1,S2=1
S1,S2=0
Voltage and current traces during an Ri measurement experiment
Figure 3.6 Tests to measure the DC internal resistance of a battery.
The exponential decay and rise of the voltage traces shown in the Figure 3.6 is due to the
lead capacitance of the measuring instrumentation. This capacitance does not influence the
DC resistance measurement, which is calculated based on voltage measurement points taken
when the battery voltage trace is parallel to the horizontal axis.
3.14 Determining Battery Operating Constraints
Maximum Discharging Power
The maximum discharging power of a battery system is limited by the maximum allowable
discharging current, minimum terminal voltage and temperature. Theoretically, the
maximum current that the battery can deliver occurs when the battery is at full charge and
the internal resistance is at its minimum value. However, in practice, specification of the
maximum discharging current (Ibattmax) is normally limited by mandatory safe operating
conditions of the battery as well as the current handling capacity of the electrodes itself. The
limitation in discharging power due to the current constraint can be expressed as,
2
dis max batt
[ SoC ] batt
[ SoC ]
Pb = I max ( Voc ) − I max ( Ri )
( 3-28)