Handbook of Energy Storage for Transmission or ... - W2agz.com

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Figure 15 Comparison of physical packages of 12-V, 83A-hour, lead-acid cranking type battery
(Delco 1150) and 42-V, 220 kJ, pulse type electrochemical capacitor (ESMA 30EC402)
Voltage responses during charge and discharge are significantly different for capacitors
and batteries. Figure 16 shows these characteristics for a 576 V system designed for a
250 kW, 30-second discharge and a 4 kW, one-hour recharge. Note that the capacitor can
recharge faster than shown; the recharge time for the capacitor is limited by the size of
the charger, which is rated at 4 kW.
Figure 16 Comparison of Capacitor and Battery discharge and charge characteristics
It is informative to compare the charge/discharge cycle efficiencies of a lead acid battery
with a typical electrochemical capacitor. Both exhibit the same type of losses due to
ohmic heating during charge and discharge, so-called IR losses. Equations for capacitor
discharge efficiency are listed in Table 1. The equivalent series resistance, Rs, of a
capacitor is constant and independent of its state of charge (i.e., capacitor voltage) while
the equivalent series resistance of a battery increases as the battery discharges, becoming
highest when the battery is fully discharged. Thus, a capacitor with the same initial
equivalent series resistance as a battery, and thus, the same power performance as the
battery, will have higher energy efficiency during charge/discharge cycling. Stated
differently, the battery will dissipate more energy during charge and discharge than a
capacitor of the same peak power rating. Note, these efficiencies should not be confused
with standby or self-discharge losses, which both technologies experience at low levels
and increase with temperature.
Electrochemical Capacitors 37