Lithium-ion batteries (Li-ion) are increasingly being used on satellites as a replaced for Ni-Cad and Ni-Hyd batteries. They continue to build a heritage based on high reliability. When properly operated and maintained they provide reliable reserve power. Their design life meets the 15 years life expectancy required in on orbit operations of GEO satellites.

The most significant differences in these batteries are related to their operation. The deterioration of the state of charge capacity is attributed to the chemical breakdown of the Lithium component in the battery over time. To slow this process, during periods when the batteries are not being discharged and charged (storage) between eclipse seasons, the batteries are maintained at a lower temperature and at a reduced state of charge. Typically at 50 % state of charge and the temperature is reduced by 10 degrees C, this varies based on the manufactures recommendations. Automated command sequences are stored in the flight computer and triggered by battery telemetry monitors. The flight computer maintains the battery state of charge at the recommended level and during emergencies will turn off units in a predefined sequence to reduce the power load and extend the time on batteries.

Eclipse operations have also been automated requiring additional preparations. Initially the batteries are warmed up by changing the heater set-points to bringing them up to the normal operation temperature. The battery discharge for each eclipse is calculated and then the batteries are charged to a higher state of charge to account for the expected discharge. At the start of eclipse season the periods are only a few minutes, at the center or longest eclipse is 70 minutes then gradually decrease back to a few minutes in duration. Plotting the battery charge and the battery discharge over the eclipse season shows curves that resemble a football and is loosely referred to as the football curve. These charging values are entered into a table in the flight computer and the charging profile is enabled allowing charging to be completed autonomously. Implementation of a day counter allows the flight computer to progress through the charging schedule.

Over charging of Lithium-ion batteries will lead them to catastrophic failure. The battery is comprised of modules vs cells on other types of batteries. Independent charge/discharge circuits are included in each module and to protect the battery, bypass relays are installed to isolate week or failed modules from the circuit. If a module is bypassed the module is completely discharged forcing it to permanently fail. Once a module has been bypassed it can no longer be used. Each module operates at 4 to 4.5 volts one design for a 36 volt battery has 9 modules. With all modules functioning, each module is charged to 4.0 volts and with one module in bypass the remaining 8 modules are charged to 4.5 volts to compensate.

Prolonged short discharge and recharge cycles do not have a significant affect on the capacity of these batteries (noted at this time).

Manual battery recondition has been replaced by automated sequences with ground commanded table values.

Due to technological advancements in battery chemistry and design, Nickel hydrogen batteries batteries are starting to be replaced by more efficient Lithium-ion batteries as they prove their reliability.

Additional information about Lithium-ion batteries is available on Wikipedia