Baterias Energia Solar

Proceedings of SPIE, 4393, pp, 230-240 (2001)

Solar Battery Recharge Options for Unattended Ground Sensors
Paul E. Sims, AstroPower, Inc.

ABSTRACT
The operational lifetime of an Unattended Ground Sensor (UGS) depends on the power consumption of the package and the space allocated for batteries. Solar cells have the potential of dramatically increasing operational lifetimes of UGSinstruments by providing supplemental power, but in this application solar cells are subject to a number of non-traditional constraints. There are UGS applications where the solar array will need to be covert or have a high shock resistance. It is also possible that a UGS-solar array will be placed in a shaded area or randomly oriented with respect to the path of the sun. This paper will first surveyconventional approaches towards solar battery charging and then discuss non-conventional approaches applicable to randomly oriented and covert UGS solar cell arrays. Keywords: Battery-Recharge, Solar-Cell, Unattended-Ground-Sensors

1. BATTERIES
Batteries are electrochemical energy storage devices. Primary batteries are designed for a one-way chemical reaction and are not rechargeable. An exampleis the lithium sulfur dioxide (LiSO2) primary battery, the standard chemistry used in military applications. Secondary batteries are designed to accommodate a reversible chemical reaction and can typically be recharged more than 500 times. Table 1 displays some of the metrics of the most common rechargeable battery chemistries compared to the LiSO2 battery. A cursory examination of Table 1 mightlead one to the conclusion that a Libased battery is the best choice for UGS applications. However, there are a number of other considerations and the proper choice of battery technology is not obvious without a detailed examination of the UGS operational profile. For instance, in a pulse-discharge scenario, a Li battery would perform poorly while a NiCd would perform well due to the largedifferences in the internal resistance of these battery types. Table 1. Battery Technology Comparison1 Capacity Energy Weight Battery Chemistry Nominal (Wh/L) compared compared Voltage to LiSO2 to LiSO2 (V) LiSO2 (Primary) 3.0 415 1.00 1.00 Sealed Lead Acid 2.0 90 2.04 0.30 Sealed NiCd 1.2 80-105 1.80 0.39 NiMH 1.2 175 1.73 0.75 Li-ion 3.6 200 1.32 0.78 Li-polymer 3.2 350 1.45 0.89 Battery Metrics Voltage.The instantaneous battery voltage depends on the state of charge and the temperature. The nominal voltage of a cell is merely a representative value. An important parameter is the End of Discharge Voltage (EODV). This is the voltage where the battery is completely discharged. Electronics powered by batteries must be designed to operate down to the EODV in order to utilize the rated capacity of abattery. Capacity. The capacity of a battery is the amount of energy it can store when it is fully charged. This is expressed in Watt Hours (Wh) or, when divided by the nominal terminal voltage, in Amp Hours (Ah). Under ideal conditions, this is also the amount of energy that can be extracted from a battery. A simple rule of thumb is that a 1 Wh battery can, in the best case, supply a 1 W electricload for one hour. Factors that can reduce the amount of energy that can be extracted from a battery include temperature variations, very high or very low current discharge levels, voltage mismatch and self-discharge. C rate. The C rate is the amount of current a battery can supply in one hour, i.e. a 2 Ah battery has a C/1 rate of 2 A, C/10 rate of 200 mA, etc.

Proceedings of SPIE, 4393, pp,230-240 (2001)
Battery performance estimates using Ragone Diagrams A Ragone diagram2 plots specific power vs. specific energy and can be used to compare many different power storage technologies. Diagonal lines in a Ragone diagram have the units of time so that required weight for constant discharge situations can be compared for different energy storage devices. An example of the use of a...