PhD Thesis - Cranfield University

Chapter 3
Maximum Charging Power
Similar to the maximum discharging power, the battery system is accordingly specified with a
maximum charging power that also depends on the charging current and charging voltage.
With all battery systems, the charge acceptance rate is always less than the charge release rate
[5]. Therefore, for and efficient charging processes, the maximum charging current is
significantly less than maximum discharging current. The maximum charging power
constrained by the maximum charging current can be expressed as,
2
chg max batt
[ SoC]
batt
[ SoC]
Pb = I max ( Voc ) + I max ( Ri )
( 3-30)
The maximum charging power is also limited by the maximum charging voltage that can be
safely applied across the battery electrodes. This voltage value and the sensitivity of
exceeding this value vary for different battery technology. Lead acid batteries for example
can handle an over voltage with less resultant damage compared to lithium ion batteries. The
charging power limit due to maximum battery voltage can be expressed as,
⎡Vb max−
Voc[
SoC ] ⎤
Pb chg max = Vb max⎢
⎥
( 3-31)
⎢⎣
Ri[
SoC]
⎥⎦
where Vbmax is the maximum voltage that can be applied to the battery.
With the same experimentally obtained values of Voc[SoC] and Ri[SoC] ,a value for Vbmax of
14.4V and the maximum charging current Ibattmax set to100A, the maximum charging
power using (3-30) and (3-31) in shown in Figure 3.8. As can be seen, the maximum
charging power due to the charging current limit is fairly constant across the SoC range.
Instead, as the battery approaches full charge (100% SoC), the limiting factor of the charging
power is the maximum battery voltage. Again, the power management system has to limit
the battery charging power accordingly to ensure that the charging power is below both
traces on the graph.
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