Nickel hydrogen battery (NiH2 or Ni-H2) are used extensively on satellites. These batteries have replaced the use of NiCad batteries in almost all cases and continue to build a heritage based on high reliability. When properly operated and maintained they have provided reliable reserve power. Their design life exceeds the 15 years life expectancy required in on orbit operations of GEO satellites.

One of the most significant differences in these batteries is the power density to weight ratio allowing them to store more energy while reducing weight, These batteries are pressure vessels and the state of charge can be derived from the pressure and temperature of each cell. Another unique feature is that modules are available that contain 2 cells in one vessel. The design, testing and assembly of cells into a completed battery utilize similar process standards with minor changes. In addition to voltage, current and temperature, pressure monitoring sensors are included on the cells.

When monitoring cell voltages during eclipse or discharging, the voltage drop profile follows that similar to NiCad battery cells. The pressures will drop on a liner slope. Nearing the depletion of capacity the voltage will drop by as much as 0.1 to 0.2 volts over a few minutes resembling the initial rate at the start of discharge and below 1.0 volts dramatically drop off. These battery cells can be discharged safely below the 1.0 volt limitation placed on NiCad battery cells.

During charging, cell pressure and temperatures have to be closely monitored to ensure maximum charge and to prevent over pressure conditions that could lead to venting or bursting the pressure vessels. After discharging the batteries pressure and temperature are used in the determination of when the full state of charge is reached. Initially the battery will become endothermic and cool as it charges resulting in the battery heaters cycling on and off to maintain temperature. Once the battery state of charge nears 80% the battery will become exothermic and the temperature will start to rise. At this point the temperature rise rate should be monitored along with voltage and pressure. Monitoring the battery and cell voltages, the voltage will increase until they reach full charge and then slightly decrease before charging is complete. The pressure will rise and as it approaches full charge the ability to store energy at the high charge rate diminishes and the excess energy starts to be converted to heat account for the temperature increase and an increase in the rate pressure increases.

Prolonged short discharge and recharge cycles do not have a significant affect on the capacity of Nickel hydrogen batteries.

Battery recondition is not required and in most cases is preformed to obtain measurement and verification of battery aging. The open circuit stand is beneficial in minimizing the difference between the highest and lowest cell voltages, known as cell spreading and allows cells to reach a chemical balance and independent cell voltages to equalize.

Self discharge will occur in Nickel hydrogen batteries during storage periods due to the internal resistance of the battery. To overcome this affect a supplemental charge current at low level is applied to the battery known as a trickle charge.

Due to technological advancements in battery chemistry and design, driven by power storage density Nickel hydrogen batteries batteries are starting to be replaced by more efficient Lithium-ion batteries on GEO satellites.

Additional information about Nickel hydrogen batteries is available on Wikipedia