Satellite EPS systems are designed to operate on the power produced from the Solar Arrays with a battery system to store and provide power during eclipse periods and emergencies. During initial launch and commissioning the batteries are charged and discharged in the process of supplementing the power generated from the Solar Arrays. Once in a stable GEO orbit the batteries are only used to provide power during eclipses and emergencies. In LEO orbits the batteries are used every orbit to provide power as the satellite passes behind the Earth and charging is started as soon as it re-enters Sun light.

To achieve the high reliability and exceptional performance requirements over the extended life time of the battery, strict standards are maintained in the assembly process. Individual battery cells are rigorously tested. The maximum charge capacity, peak voltage, internal resistance, discharge rate and end of discharge voltages are closely matched between all cells to insure that the cells are as close to identical as possible. Independent cell voltage sensors are installed on each cell to provide the ability to monitor their performance. Monitors are also installed to measure the total battery current providing a negative reading when the battery is discharging and a positive reading when the battery is being charged. Temperature sensors are strategically located on the completed battery to sample the temperature at significant locations. Excess heat is generated during the discharge and charging of the battery and extreme low temperatures are experienced during eclipse periods. Management of the recommended operational temperature range is achieved by mounting the battery on a thermally conductive surface with the ability to radiate excess heat into cold space. To counteract extreme cold conditions supplemental heaters are selected and installed to insure that the battery can be maintained above the minimum operational temperature requirements during these periods.

The three most commonly used battery types are Nickel-cadmium, Nickel hydrogen, and Lithium-ion batteries. The batteries are designed and manufactured with the storage capacity required to power the satellite during eclipse with additional margin to account for potential emergencies. To provide added protection, the system power requirements are normally supported by two batteries to protect against failure of a single battery.

Satellite operations staff closely monitor battery and cell voltages, temperatures and currents in addition to load current and voltage during eclipse periods and battery charging. This data is collected and used to calculate the proper charge to recharge ratio based on the battery type, charging current and time required to complete charging.

A simplified battery design example would be, if the satellite uses 3000 watts of power (a small satellite) at a voltage of 36 volts using NiCad batteries with a cell voltage of 1.2 volts. The number of cells in the battery would be (36/1.2) 30 cells. The current draw form the batteries would be approximately 83.3 amps. The longest eclipse is 70 minutes, this converted to amp hours is 1.167 hours, with the resulting current draw of (83.3 x 1.167) 97.25 amps from the batteries. Applying a margin of 25% to this shows that the battery would have to be rated for over 121.6 amp/hours of capacity or if two batteries are employed, a minimum of 60.8 amp/hours each to adequately power the satellite. To minimize cost or increase the margin this value or the next larger standard battery capacity value can be selected for use.