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ACEP 12-03 | March 2012 Charge and Discharge Parameters for Test #50 Fig. 12 shows results for test no. 50 conducted for about 40 hours continuously (between 3/24/2011 10:06:53 and 3/26/2011 01:27:21). The battery was on charge for about 8 hours, when it attained a peak voltage of 58.361 V. It was then left to discharge by feeding power back to the grid for another 19.4 hours, after which charging was resumed at the minimum set value for the state of charge (SOC) at 52% – this was arbitrarily set to monitor other parameters. It can be observed that under normal conditions the battery operates as per the specifications detailed in Table II. A Note on Ambient and System Temperatures: Suitability for Cold Climate Operation Fig. 12(c) shows the progression in the system temperature over the duration the operation. The experiments are carried out in a controlled facility, thus there is little deviation in the ambient temperature. It is seen that, as expected, the system temperature tends to rise with time, but more so with each successive discharge of the battery. The system has a self-regulatory power conditioning system that stabilizes this situation to ensure the limits of electrolyte operating temperature are respected. By Arrhenius Law, the rate at which the chemical reaction proceeds increases exponentially as temperature rises, allowing more instantaneous power to be extracted from the battery — higher temperatures improve ion mobility reducing the cell’s internal impedance and increasing its capacity. However, thermal management is necessary so that both charge capacity and cycle life can be optimized since high temperatures may also initiate irreversible chemical reactions that can cause permanent damage or complete failure of the battery. Typically, the temperature of the electrolyte is managed internally by the VRB-ESS. There are two issues worth mention: at high temperatures (say above 40 degC which is NOT an Alaskan concern) and at low temperatures say below 10 degC which is an Alaskan concern. At low temperatures the electrolyte must be warmed, and this is accomplished automatically if the system is operating (cycling). The reason for this is not so much a case of freezing since the electrolyte does not freeze until below about -20 degC, but is more a case of viscosity. It is hard to pump the thicker electrolyte given the pump designs in use. Generally the capacity of the system is not affected by the cold. If it is extremely cold then the system has to be designed for it. Similarly, in very hot climates the system must be so designed as to cool the electrolyte to below 40 degC (105 degF). ADVANCED ENERGY STORAGE RESEARCH 19 | P a g ePDF Image | Advanced Battery Storage Systems Testing at ACEP VRB ESS
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