CHAPTER 9 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL 162 that a rechargeable battery can last on a single discharge) would not be available. Ground controllers must actively manage battery states of charge during high beta operations by reducing the current used to charge batteries and occasionally deactivating batteries to prevent them from overcharging. As a result, some of the power that is generated is wasted and cannot be used by the EPS. Annually, batteries must be reconditioned by taking each battery off-line and completely discharging it to remove any memory buildup and allowing the state of charge (SOC) software calculations to be updated. Additionally, the high inclination of the sun with respect to the orbit of the ISS can cause parts of the ISS structure to cast shadows across the arrays. This can reduce power-generation capability, cause changes in nominal heater control cycles (i.e., some items getting warmer or colder than normal), and potentially cause thermal stresses on hardware (see Longeron Shadowing, discussed below). ISS Program management has Groundrules and Constraints (see Chapter 1) that limit scheduling of dynamic events at high beta, due to these environmental effects of high beta and the solar array constraints often associated with dynamic operations. For example, visiting vehicles are generally not allowed to dock at these times of the year. The SPARTAN team develops a weekly solar array plan that takes into consideration any solar array positioning requirements and determines a time-phased power availability for each power channel. This availability is then adjusted for any Primary Power System maintenance that is planned, such as battery reconditioning. At the same time, the SPARTAN team gathers inputs from control centers around the globe to develop a usage profile that includes the standard systems power requirement (i.e., the overhead to keep the ISS operating), dynamic events loading, and payload requirements. The power availability is then compared to the load profile to determine whether the power system will be balanced. The strictest definition of energy balance would have the Primary Power System batteries discharge and recharge match on each orbit. From a planning perspective, the operations team plans for the batteries to fully recharge each orbit and is limited to a maximum discharge down to 65% SOC to prevent excessive wear on the battery hardware and maintain contingency reserve power in case of a failure preventing a battery from recharging. Furthermore, this protects the vehicle from significant failures. For example, if a system failure that will take a few hours to fix occurs when the batteries only have 30% to 40% SOC, there is a higher chance the system will lose all power before a recovery. If the power availability is greater than or equal to the load profile, the system is considered to be in positive energy balance (i.e. generating more electricity than is being used). If the power availability is less than the load profile, the system is in negative energy balance. Positive energy balance will allow the batteries to recharge to the same point they started on the previous orbit—hopefully fully charged. Negative energy balance will prevent the batteries from fully charging each orbit and will cause the batteries to discharge further each orbit until they deplete their usable energy. Negative energy balance on the ISS is often caused by dynamic events that create the need for solar array feathering, thus reducing the power availability. If, during weekly planning, the initial comparison of power availability to the load profile shows the ISS will be in negative energy balance, the operations team will work through multiple options to correct the situation. Usually, the team will develop a manual powerdown plan. Through this plan, ground controllers will deactivate noncritical equipment for the duration of the energy- negative timeframe, usually on the order of 2 to 8 hours. If a suitable powerdown cannot be found, it may be necessary to postpone an activity (e.g., dynamic event, payload, etc.) until power is available to support it. It may possible for the ISS to continue in negative energy balance during rare, high-priority, short-duration events. This would potentially cause additional wear of battery hardware and eat into the power available for contingencies. These cases are weighed against the risks of replanning the high- priority event (e.g., delaying the docking or spacewalk). All these operations are carefully defined in the flight rules, which detail under what conditions the batteries can be discharged below the nominal limits. The flight director will weigh these considerations when determining what level will be tolerated.